Astronomy Picture of the Day Search Results for "Titan"

APOD: 2024 August 15 - Late Night Vallentuna
Explanation: Bright Mars and even brighter Jupiter are in close conjunction just above the pine trees in this post-midnight skyscape from Vallentuna, Sweden. Taken on August 12 during a geomagnetic storm, the snapshot records the glow of aurora borealis or northern lights, beaming from the left side of the frame. Of course on that date Perseid meteors rained through planet Earth's skies, grains of dust from the shower's parent, periodic comet Swift-Tuttle. The meteor streak at the upper right is a Perseid plowing through the atmosphere at about 60 kilometers per second. Also well-known in Earth's night sky, the bright Pleiades star cluster shines below the Perseid meteor streak. In Greek myth, the Pleiades were seven daughters of the astronomical titan Atlas and sea-nymph Pleione. The Pleiades and their parents' names are given to the cluster's nine brightest stars.

APOD: 2023 March 31 - Seeing Titan
Explanation: Shrouded in a thick atmosphere, Saturn's largest moon Titan really is hard to see. Small particles suspended in the upper atmosphere cause an almost impenetrable haze, strongly scattering light at visible wavelengths and hiding Titan's surface features from prying eyes. But Titan's surface is better imaged at infrared wavelengths where scattering is weaker and atmospheric absorption is reduced. Arrayed around this visible light image (center) of Titan are some of the clearest global infrared views of the tantalizing moon so far. In false color, the six panels present a consistent processing of 13 years of infrared image data from the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft orbiting Saturn from 2004 to 2017. They offer a stunning comparison with Cassini's visible light view. NASA's revolutionary rotorcraft mission to Titan is due to launch in 2027.

APOD: 2023 January 5 - Messier 45: The Daughters of Atlas and Pleione
Explanation: Hurtling through a cosmic dust cloud a mere 400 light-years away, the lovely Pleiades or Seven Sisters open star cluster is well-known for its striking blue reflection nebulae. It lies in the night sky toward the constellation Taurus and the Orion Arm of our Milky Way galaxy. The sister stars are not related to the dusty cloud though. They just happen to be passing through the same region of space. Known since antiquity as a compact grouping of stars, Galileo first sketched the star cluster viewed through his telescope with stars too faint to be seen by eye. Charles Messier recorded the position of the cluster as the 45th entry in his famous catalog of things which are not comets. In Greek myth, the Pleiades were seven daughters of the astronomical titan Atlas and sea-nymph Pleione. Their parents names are included in the cluster's nine brightest stars. This well-processed, color-calibrated telescopic image features pin-point stars and detailed filaments of interstellar dust captured in over 9 hours of exposure. It spans more than 20 light-years across the Pleiades star cluster.

APOD: 2022 December 30 - Mars and the Star Clusters
Explanation: At this year's end Mars still shines brightly in planet Earth's night as it wanders through the head-strong constellation Taurus. Its bright yellowish hue dominates this starry field of view that includes Taurus' alpha star Aldebaran and the Hyades and Pleiades star clusters. While red giant Aldebaran appears to anchor the V-shape of the Hyades at the left of the frame, Aldebaran is not a member of the Hyades star cluster. The Hyades cluster is 151 light-years away making it the nearest established open star cluster, but Aldebaran lies at less than half that distance, along the same line-of-sight. At the right, some 400 light-years distant is the open star cluster cataloged as Messier 45, also known as the Pleiades or Seven Sisters. In Greek myth, the Pleiades were daughters of the astronomical titan Atlas and sea-nymph Pleione.

APOD: 2022 May 28 - RCW 86: Historical Supernova Remnant
Explanation: In 185 AD, Chinese astronomers recorded the appearance of a new star in the Nanmen asterism. That part of the sky is identified with Alpha and Beta Centauri on modern star charts. The new star was visible for months and is thought to be the earliest recorded supernova. This deep image shows emission nebula RCW 86, understood to be the remnant of that stellar explosion. The narrowband data trace gas ionized by the still expanding shock wave. Space-based images indicate an abundance of the element iron and lack of a neutron star or pulsar in the remnant, suggesting that the original supernova was Type Ia. Unlike the core collapse supernova explosion of a massive star, a Type Ia supernova is a thermonuclear detonation on a a white dwarf star that accretes material from a companion in a binary star system. Near the plane of our Milky Way galaxy and larger than a full moon on the sky this supernova remnant is too faint to be seen by eye though. RCW 86 is some 8,000 light-years distant and around 100 light-years across.

APOD: 2022 May 27 - Titan: Moon over Saturn
Explanation: Like Earth's moon, Saturn's largest moon Titan is locked in synchronous rotation. This mosaic of images recorded by the Cassini spacecraft in May of 2012 shows its anti-Saturn side, the side always facing away from the ringed gas giant. The only moon in the solar system with a dense atmosphere, Titan is the only solar system world besides Earth known to have standing bodies of liquid on its surface and an earthlike cycle of liquid rain and evaporation. Its high altitude layer of atmospheric haze is evident in the Cassini view of the 5,000 kilometer diameter moon over Saturn's rings and cloud tops. Near center is the dark dune-filled region known as Shangri-La. The Cassini-delivered Huygens probe rests below and left of center, after the most distant landing for a spacecraft from Earth.

APOD: 2022 April 9 - Mars-Saturn Conjunction
Explanation: Fainter stars in the zodiacal constellation Capricornus are scattered near the plane of the ecliptic in this field of view. The two brightest ones at center aren't stars at all though, but the planets Mars and Saturn. Taken on the morning of April 4, the telescopic snapshot captured their tantalizing close conjunction in a predawn sky, the pair of planets separated by only about 1/3 of a degree. That's easily less than the apparent width of a Full Moon. Can you tell which planet is which? If you guessed Mars is the redder one , you'd be right. Above Mars, slightly fainter Saturn still shines with a paler yellowish tinge in reflected sunlight. Even at the low magnification, Saturn's largest and brightest moon Titan can be spotted hugging the planet very closely on the left.

APOD: 2022 March 27 - Titan Seas Reflect Sunlight
Explanation: Why would the surface of Titan light up with a blinding flash? The reason: a sunglint from liquid seas. Saturn's moon Titan has numerous smooth lakes of methane that, when the angle is right, reflect sunlight as if they were mirrors. Pictured here in false-color, the robotic Cassini spacecraft that orbited Saturn from 2004 to 2017 imaged the cloud-covered Titan in 2014 in different bands of cloud-piercing infrared light. This specular reflection was so bright it saturated one of Cassini's infrared cameras. Although the sunglint was annoying -- it was also useful. The reflecting regions confirm that northern Titan houses a wide and complex array of seas with a geometry that indicates periods of significant evaporation. During its numerous passes of our Solar System's most mysterious moon, Cassini has revealed Titan to be a world with active weather -- including times when it rains a liquefied version of natural gas.

APOD: 2021 July 6 - Saturn and Six Moons
Explanation: How many moons does Saturn have? So far 82 have been confirmed, the smallest being only a fraction of a kilometer across. Six of its largest satellites can be seen here in a composite image with 13 short exposure of the bright planet, and 13 long exposures of the brightest of its faint moons, taken over two weeks last month. Larger than Earth's Moon and even slightly larger than Mercury,Saturn's largest moon Titan has a diameter of 5,150 kilometers and was captured making nearly a complete orbit around its ringed parent planet. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, in contrast with several newly discovered moons announced in 2019. The trail on the far right belongs to Iapetus, Saturn's third largest moon. The radius of painted Iapetus' orbit is so large that only a portion of it was captured here. Saturn leads Jupiter across the night sky this month, rising soon after sunset toward the southeast, and remaining visible until dawn.

APOD: 2021 April 20 - Ingenuity: First Flight over Mars
Explanation: What's the best way to explore Mars? Perhaps there is no single best way, but a newly demonstrated method shows tremendous promise: flight. Powered flight has the promise to search vast regions and scout out particularly interesting areas for more detailed investigation. Yesterday, for the first time, powered flight was demonstrated on Mars by a small helicopter named Ingenuity. In the featured video, Ingenuity is first imaged by the Perseverance rover sitting quietly on the Martian surface. After a few seconds, Ingenuity's long rotors begin to spin, and a few seconds after that -- history is made as Ingenuity actually takes off, hovers for a few seconds, and then lands safely. More tests of Ingenuity's unprecedented ability are planned over the next few months. Flight may help humanity better explore not only Mars, but Saturn's moon Titan over the next few decades.

APOD: 2021 April 4 - In, Through, and Beyond Saturn's Rings
Explanation: Four moons are visible on the featured image -- can you find them all? First -- and farthest in the background -- is Titan, the largest moon of Saturn and one of the larger moons in the Solar System. The dark feature across the top of this perpetually cloudy world is the north polar hood. The next most obvious moon is bright Dione, visible in the foreground, complete with craters and long ice cliffs. Jutting in from the left are several of Saturn's expansive rings, including Saturn's A ring featuring the dark Encke Gap. On the far right, just outside the rings, is Pandora, a moon only 80-kilometers across that helps shepherd Saturn's F ring. The fourth moon? If you look closely inside Saturn's rings, in the Encke Gap, you will find a speck that is actually Pan. Although one of Saturn's smallest moons at 35-kilometers across, Pan is massive enough to help keep the Encke gap relatively free of ring particles. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 and was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2021 March 2 - Ingenuity: A Mini Helicopter Now on Mars
Explanation: What if you could fly around Mars? NASA may have achieved that capability last month with the landing of Perseverance, a rover which included a small flight-worthy companion called Ingenuity, nicknamed Ginny. Even though Ginny is small -- a toaster-sized helicopter with four long legs and two even-longer (1.2-meter) rotors, she is the first of her kind -- there has never been anything like her before. After being deployed, possibly in April, the car-sized Perseverance ("Percy") will back away to give Ginny ample room to attempt her unprecedented first flight. In the featured artistic illustration, Ginny's long rotors are depicted giving her the lift she needs to fly into the thin Martian atmosphere and explore the area near Perseverance. Although Ingenuity herself will not fly very far, she is a prototype for all future airborne Solar-System robots that may fly far across not only Mars, but Titan.

APOD: 2021 January 9 - Titan: Moon over Saturn
Explanation: Like Earth's moon, Saturn's largest moon Titan is locked in synchronous rotation. This mosaic of images recorded by the Cassini spacecraft in May of 2012 shows its anti-Saturn side, the side always facing away from the ringed gas giant. The only moon in the solar system with a dense atmosphere, Titan is the only solar system world besides Earth known to have standing bodies of liquid on its surface and an earthlike cycle of liquid rain and evaporation. Its high altitude layer of atmospheric haze is evident in the Cassini view of the 5,000 kilometer diameter moon over Saturn's rings and cloud tops. Near center is the dark dune-filled region known as Shangri-La. The Cassini-delivered Huygens probe rests below and left of center, after the most distant landing for a spacecraft from Earth.

APOD: 2020 December 23 - Jupiter Meets Saturn: A Red Spotted Great Conjunction
Explanation: It was time for their close-up. Two days ago Jupiter and Saturn passed a tenth of a degree from each other in what is known a Great Conjunction. Although the two planets pass each other on the sky every 20 years, this was the closest pass in nearly four centuries. Taken early in day of the Great Conjunction, the featured multiple-exposure combination captures not only both giant planets in a single frame, but also Jupiter's four largest moons (left to right) Callisto, Ganymede, Io, and Europa -- and Saturn's largest moon Titan. If you look very closely, the clear Chilescope image even captures Jupiter's Great Red Spot. The now-separating planets can still be seen remarkably close -- within about a degree -- as they set just after the Sun, toward the west, each night for the remainder of the year.

APOD: 2020 August 20 - Seeing Titan
Explanation: Shrouded in a thick atmosphere, Saturn's largest moon Titan really is hard to see. Small particles suspended in the upper atmosphere cause an almost impenetrable haze, strongly scattering light at visible wavelengths and hiding Titan's surface features from prying eyes. But Titan's surface is better imaged at infrared wavelengths where scattering is weaker and atmospheric absorption is reduced. Arrayed around this visible light image (center) of Titan are some of the clearest global infrared views of the tantalizing moon so far. In false color, the six panels present a consistent processing of 13 years of infrared image data from the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft. They offer a stunning comparison with Cassini's visible light view.

APOD: 2020 June 20 - Northern Summer on Titan
Explanation: Today's solstice brings summer to planet Earth's northern hemisphere. But the northern summer solstice arrived for ringed planet Saturn over three years ago on May 24, 2017. Orbiting the gas giant, Saturn's moon Titan experiences the Saturnian seasons that are about 7 Earth-years long. Larger than inner planet Mercury, Titan was captured in this Cassini spacecraft image about two weeks after its northern summer began. The near-infrared view finds bright methane clouds drifting through Titan's dense, hazy atmosphere as seen from a distance of about 507,000 kilometers. Below the clouds, dark hydrocarbon lakes sprawl near its fully illuminated north pole.

APOD: 2020 May 19 - Posters of the Solar System
Explanation: Would you like a NASA astronomy-exploration poster? You are just one page-print away. Any of the panels you see on the featured image can appear on your wall. Moreover, this NASA page has, typically, several more posters of each of the Solar System objects depicted. These posters highlight many of the places humanity, through NASA, has explored in the past 50 years, including our Sun, and planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Moons of Jupiter that have been posterized include Europa, Ganymede, Callisto, and Io, while moons of Saturn that can be framed include Enceladus and Titan. Images of Pluto, Ceres, comets and asteroids are also presented, while six deep space scenes -- well beyond our Solar System -- can also be prominently displayed. If you lack wall space or blank poster sheets don't despair -- you can still print many of these out as trading cards.

APOD: 2020 April 19 - Cassini Approaches Saturn
Explanation: What would it look like to approach Saturn in a spaceship? One doesn't have to just imagine -- the Cassini spacecraft did just this in 2004, recording thousands of images along the way, and hundreds of thousands more since entering orbit. Some of Cassini's early images have been digitally tweaked, cropped, and compiled into the featured inspiring video which is part of a larger developing IMAX movie project named In Saturn's Rings. In the concluding sequence, Saturn looms increasingly large on approach as cloudy Titan swoops below. With Saturn whirling around in the background, Cassini is next depicted flying over Mimas, with large Herschel Crater clearly visible. Saturn's majestic rings then take over the show as Cassini crosses Saturn's thin ring plane. Dark shadows of the ring appear on Saturn itself. Finally, the enigmatic ice-geyser moon Enceladus appears in the distance and then is approached just as the video clip ends. After more than a decade of exploration and discovery, the Cassini spacecraft ran low on fuel in 2017 was directed to enter Saturn's atmosphere, where it surely melted.

APOD: 2019 November 7 - Messier 45: The Daughters of Atlas and Pleione
Explanation: Hurtling through a cosmic dust cloud a mere 400 light-years away, the lovely Pleiades or Seven Sisters open star cluster is well-known for its striking blue reflection nebulae. It lies in the night sky toward the constellation Taurus and the Orion Arm of our Milky Way Galaxy. The sister stars and cosmic dust cloud are not related though, they just happen to be passing through the same region of space. Known since antiquity as a compact grouping of stars, Galileo first sketched the star cluster viewed through his telescope with stars too faint to be seen by eye. Charles Messier recorded the position of the cluster as the 45th entry in his famous catalog of things which are not comets. In Greek myth, the Pleiades were seven daughters of the astronomical titan Atlas and sea-nymph Pleione. Their parents names are included in the cluster's nine brightest stars. This deep and wide telescopic image spans over 20 light-years across the Pleides star cluster.

APOD: 2019 October 5 - Jupiter and the Moons
Explanation: After sunset on October 3, some of the Solar System's largest moons stood low along the western horizon with the largest planet. Just after nightfall, a pairing of the Moon approaching first quarter phase and Jupiter was captured in this telephoto field of view. A blend of short and long exposures, it reveals the familiar face of our fair planet's own large natural satellite in stark sunlight and faint earthshine. At lower right are the ruling gas giant and its four Galilean moons. Left to right, the tiny pinpricks of light are Ganymede, [Jupiter], Io, Europa, and Callisto. Our own natural satellite appears to loom large because it's close, but Ganymede, Io, and Callisto are actually larger than Earth's Moon. Water world Europa is only slightly smaller. Of the Solar System's six largest planetary satellites, only Saturn's moon Titan, is missing from this scene. But be sure to check for large moons in your sky tonight.

APOD: 2019 July 3 - Robotic Dragonfly Selected to Fly Across Titan
Explanation: If you could fly across Titan, what would you see? To find out and to better explore this exotic moon of Saturn, NASA recently green-lighted Dragonfly, a mission to Titan with plans to deploy a helicopter-like drone. Saturn's moon Titan is one of the largest moons in the Solar System and the only moon known to have a thick atmosphere and changing hydrocarbon lakes. After development, building, testing, and launch, Dragonfly is currently scheduled to reach Titan in 2034. The featured animated video envisions Dragonfly arriving at Titan, beginning its airborne exploration, landing to establishing a radio link back to Earth, and then continuing on to another trans-Titanian flight. It is hoped that Dragonfly will not only help humanity better understanding Titan's weather, chemistry, and changing landscape, but also bolster humanity's understanding of how life first developed on our young Earth.

APOD: 2019 May 23 - Moons Near Jupiter
Explanation: On May 20, a nearly Full Moon and Jupiter shared this telephoto field of view. Captured when a passing cloud bank dimmed the moonlight, the single exposure reveals the familiar face of our fair planet's own large natural satellite, along with bright Jupiter (lower right) and some of its Galilean moons. Lined up left to right the tiny pinpricks of light near Jupiter are Ganymede, Europa, [Jupiter] and Callisto. (That's not just dust on your screen ...) Closer and brighter, our own natural satellite appears to loom large. But Ganymede, and Callisto are physically larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites, Saturn's moon Titan is missing from the scene and a fourth Galilean moon, Io, is hidden by our ruling gas giant.

APOD: 2019 May 5 - Saturn, Titan, Rings, and Haze
Explanation: This is not a solar eclipse. Pictured here is a busy vista of moons and rings taken at Saturn. The large circular object in the center of the image is Titan, the largest moon of Saturn and one of the most intriguing objects in the entire Solar System. The dark spot in the center is the main solid part of the moon. The bright surrounding ring is atmospheric haze above Titan, gas that is scattering sunlight to a camera operating onboard the robotic Cassini spacecraft. Cutting horizontally across the image are the rings of Saturn, seen nearly edge on. At the lower right of Titan is Enceladus, a small moon of Saturn. Since the image was taken pointing nearly at the Sun, the surfaces of Titan and Enceladus appear in silhouette, and the rings of Saturn appear similar to a photographic negative. Now if you look really really closely at Enceladus, you can see a hint of icy jets shooting out toward the bottom of the image. It is these jets that inspired future proposals to land on Enceladus, burrow into the ice, and search for signs of extraterrestrial life.

APOD: 2018 September 29 - 55 Nights with Saturn
Explanation: For 55 consecutive nights Mediterranean skies were at least partly clear this summer, from the 1st of July to the 24th of August 2018. An exposure from each night was incorporated in this composited telephoto and telescopic image to follow bright planet Saturn as it wandered through the generous evening skies. Through August, the outer planet's seasonal apparent retrograde motion slowed and drifted to the right, framed by a starry background. That brought it near the line-of-sight to the central Milky Way, and the beautiful Lagoon (M8) and Trifid (M20) nebulae. Of course Saturn's largest moon Titan was also along for the ride. Swinging around the gas giant in a 16 day long orbit, Titan's resulting wave-like motion is easier to spot when the almost-too-bright Saturn is digitally edited from the scene.

APOD: 2018 August 18 - Seeing Titan
Explanation: Shrouded in a thick atmosphere, Saturn's largest moon Titan really is hard to see. Small particles suspended in the upper atmosphere cause an almost impenetrable haze, strongly scattering light at visible wavelengths and hiding Titan's surface features from prying eyes. But Titan's surface is better imaged at infrared wavelengths where scattering is weaker and atmospheric absorption is reduced. Arrayed around this centered visible light image of Titan are some of the clearest global infrared views of the tantalizing moon so far. In false color, the six panels present a consistent processing of 13 years of infrared image data from the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft. They offer a stunning comparison with Cassini's visible light view.

APOD: 2018 May 26 - Titan: Moon over Saturn
Explanation: Like Earth's moon, Saturn's largest moon Titan is locked in synchronous rotation. This mosaic of images recorded by the Cassini spacecraft in May of 2012 shows its anti-Saturn side, the side always facing away from the ringed gas giant. The only moon in the solar system with a dense atmosphere, Titan is the only solar system world besides Earth known to have standing bodies of liquid on its surface and an earthlike cycle of liquid rain and evaporation. Its high altitude layer of atmospheric haze is evident in the Cassini view of the 5,000 kilometer diameter moon over Saturn's rings and cloud tops. Near center is the dark dune-filled region known as Shangri-La. The Cassini-delivered Huygens probe rests below and left of center, after the most distant landing for a spacecraft from Earth.

APOD: 2018 April 12 - M22 and the Wanderers
Explanation: Wandering through the constellation Sagittarius, bright planets Mars and Saturn appeared together in early morning skies over the last weeks. They are captured in this 3 degree wide field-of-view from March 31 in a close celestial triangle with large globular star cluster Messier 22. Of course M22 (bottom left) is about 10,000 light-years distant, a massive ball of over 100,000 stars much older than our Sun. Pale yellow and shining by reflected sunlight, Saturn (on top) is about 82 light-minutes away. Look carefully and you can spot large moon Titan as a pinpoint of light at about the 5 o'clock position in the glare of Saturn's overexposed disk. Slightly brighter and redder Mars is 9 light-minutes distant. While both planets are moving on toward upcoming oppositions, by July Mars will become much brighter still, with good telescopic views near its 2018 opposition. Then it will be a mere 3.2 light-minutes from planet Earth.

APOD: 2017 September 11 - Cassini Approaches Saturn
Explanation: What would it look like to approach Saturn in a spaceship? One doesn't have to just imagine -- the Cassini spacecraft did just this in 2004, recording thousands of images along the way, and hundreds of thousands more since entering orbit. Some of Cassini's early images have been digitally tweaked, cropped, and compiled into the featured inspiring video which is part of a larger developing IMAX movie project named In Saturn's Rings. In the concluding sequence, Saturn looms increasingly large on approach as cloudy Titan swoops below. With Saturn whirling around in the background, Cassini is next depicted flying over Mimas, with large Herschel Crater clearly visible. Saturn's majestic rings then take over the show as Cassini crosses Saturn's thin ring plane. Dark shadows of the ring appear on Saturn itself. Finally, the enigmatic ice-geyser moon Enceladus appears in the distance and then is approached just as the video clip ends. The Cassini spacecraft itself, low on fuel, is scheduled to end on Friday when it will be directed to approach so close to Saturn that it falls in and melts.

APOD: 2017 June 22 - Northern Summer on Titan
Explanation: While yesterday's solstice brought summer to planet Earth's northern hemisphere, a northern summer solstice arrived for ringed planet Saturn nearly a month ago on May 24. Following the Saturnian seasons, its large moon Titan was captured in this Cassini spacecraft image from June 9. The near-infrared view finds bright methane clouds drifting through Titan's northern summer skies as seen from a distance of about 507,000 kilometers. Below Titan's clouds, dark hydrocarbon lakes sprawl near the large moon's now illuminated north pole.

APOD: 2017 June 18 - Views from Cassini at Saturn
Explanation: What has the Cassini orbiter seen at Saturn? The featured music video shows some of the early highlights. In the first time-lapse sequence (00:07), a vertical line appears that is really Saturn's thin rings seen nearly edge-on. Soon some of Saturn's moon shoot past. The next sequence (00:11) features Saturn's unusually wavy F-ring that is constrained by the two shepherd moons that are also continually perturbing it. Soon much of Saturn's extensive ring system flashes by, sometimes juxtaposed to the grandeur of the immense planet itself. Cloud patterns on Titan (00:39) and Saturn (00:41) are highlighted. Clips from flybys of several of Saturn's moon are then shown, including Phoebe, Mimas, Epimetheus, and Iapetus. In other sequences, moons of Saturn appear to pass each other as they orbit Saturn. Background star fields seen by Cassini are sometimes intruded upon by bright passing moons. The robotic Cassini spacecraft has been revolutionizing humanity's knowledge of Saturn and its moons since 2004. In September, Cassini's mission will be brought to a dramatic conclusion as the spacecraft will be directed to dive into ringed giant.

APOD: 2017 April 22 - Between the Rings
Explanation: On April 12, as the Sun was blocked by the disk of Saturn the Cassini spacecraft camera looked toward the inner Solar System and the gas giant's backlit rings. At the top of the mosaicked view is the A ring with its broader Encke and narrower Keeler gaps visible. At the bottom is the F ring, bright due to the viewing geometry. The point of light between the rings is Earth, 1.4 billion kilometers in the distance. Look carefully and you can even spot Earth's large moon, a pinprick of light to the planet's left. Today Cassini makes its final close approach to Saturn's own large moon Titan, using Titan's gravity to swing into the spacecraft's Grand Finale, the final set of orbits that will bring Cassini just inside Saturn's rings.

APOD: 2017 April 13 - Moons and Jupiter
Explanation: On April 10, a Full Moon and Jupiter shared this telephoto field of view. Both were near opposition, opposite the Sun in Earth's night sky. Captured when a passing cloud bank dimmmed the bright moonlight, the single exposure reveals the familiar face of our fair planet's own large natural satellite, along with a line up of the ruling gas giant's four Galilean moons. Labeled top to bottom, the tiny pinpricks of light above bright Jupiter are Callisto, Europa, Ganymede, and Io. Closer and brighter, our own natural satellite appears to loom large. But Callisto, Ganymede, and Io are physically larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites, only Saturn's moon Titan is missing from the scene.

APOD: 2017 March 13 - Saturn's Moon Pan from Cassini
Explanation: Why does Saturn's moon Pan look so odd? Images taken last week from the robotic Cassini spacecraft orbiting Saturn have resolved the moon in unprecedented detail. The surprising images reveal a moon that looks something like a walnut with a slab through its middle. Other visible features on Pan include rolling terrain, long ridges, and a few craters. Spanning 30-kilometer across, Pan orbits inside the 300-kilometer wide Encke Gap of Saturn's expansive A-ring, a gap known since the late 1800s. Next month, Cassini will be directed to pass near Saturn's massive moon Titan so it can be pulled into a final series of orbits that will take it, on occasion, completely inside Saturn's rings and prepare it to dive into Saturn's atmosphere.

APOD: 2017 January 25 - Cassini's Grand Finale Tour at Saturn
Explanation: Cassini is being prepared to dive into Saturn. The robotic spacecraft that has been orbiting and exploring Saturn for over a decade will end its mission in September with a spectacular atmospheric plunge. Pictured here is a diagram of Cassini's remaining orbits, each taking about one week. Cassini is scheduled to complete a few months of orbits that will take it just outside Saturn's outermost ring F. Then, in April, Titan will give Cassini a gravitational pull into Proximal orbits, the last of which, on September 15, will impact Saturn and cause the spacecraft to implode and melt. Cassini's Grand Finale orbits are designed to record data and first-ever views from inside the rings -- between the rings and planet -- as well as some small moons interspersed in the rings. Cassini's demise is designed to protect any life that may occur around Saturn or its moons from contamination by Cassini itself.

APOD: 2016 December 12 - Over Saturn's Turbulent North Pole
Explanation: The Cassini spacecraft's Grand Finale at Saturn has begun. The Grand Finale will allow Cassini to explore Saturn and some of Saturn's moons and rings in unprecedented detail. The first phase started two weeks ago when a close flyby of Titan changed Cassini's orbit into one that passes near Saturn's poles and just outside of Saturn's outermost F-ring. Featured here is an image taken during the first of Cassini's 20 week-long F-ring orbits around Saturn. Visible are the central polar vortex on the upper left, a hexagonal cloud boundary through the image center, and numerous light-colored turbulent storm systems. In 2017 April, Cassini will again use the gravity of Titan to begin a new series of 22 Proximal orbits -- trajectories that will take Cassini inside of Saturn's rings for the first time. Cassini's new science adventure is scheduled to end on 2017 September 17, though, when the robotic spacecraft will be directed into a dramatic mission-ending dive into Saturn's atmosphere.

APOD: 2016 November 24 - Ring Scan
Explanation: Scroll right and you can cruise along the icy rings of Saturn. This high resolution scan is a mosaic of images presented in natural color. The images were recorded in May 2007 over about 2.5 hours as the Cassini spacecraft passed above the unlit side of the rings. To help track your progress, major rings and gaps are labeled along with the distance from the center of the gas giant in kilometers. The alphabetical designation of Saturn's rings is historically based on their order of discovery; rings A and B are the bright rings separated by the Cassini division. In order of increasing distance from Saturn, the seven main rings run D,C,B,A,F,G,E. (Faint, outer rings G and E are not imaged here.) Four days from now, on November 29, Cassini will make a close flyby of Saturn's moon Titan and use the large moon's gravity to nudge the spacecraft into a series of 20 daring, elliptical, ring-grazing orbits. Diving through the ring plane just 11,000 kilometers outside the F ring (far right) Cassini's first ring-graze will be on December 4.

APOD: 2016 June 22 - Cirrus over Paris
Explanation: What's that over Paris? Cirrus. Typically, cirrus clouds appear white or gray when reflecting sunlight, can appear dark at sunset (or sunrise) against a better lit sky. Cirrus are among the highest types of clouds and are usually thin enough to see stars through. Cirrus clouds may form from moisture released above storm clouds and so may herald the arrival of a significant change in weather. Conversely, cirrus clouds have also been seen on Mars, Jupiter, Saturn, Titan, Uranus, and Neptune. The featured image was taken two days ago from a window in District 15, Paris, France, Earth. The brightly lit object on the lower right is, of course, the Eiffel Tower.

APOD: 2016 April 10 - Cassini Approaches Saturn
Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, became close enough in 2002 to resolve many rings and moons of its destination planet: Saturn. At that time, Cassini snapped several images during an engineering test. Several of those images were combined into the contrast-enhanced color composite featured here. Saturn's rings and cloud-tops are visible toward the image bottom, while Titan, its largest moon, is visible as the speck toward the top. When arriving at Saturn in July 2004, the Cassini orbiter began to circle and study the Saturnian system. A highlight was when Cassini launched the Huygens probe that made an unprecedented landing on Titan in 2005, sending back detailed pictures. Now nearing the end of its mission, Cassini is scheduled to embark on a Grand Finale phase in late 2016 where it will repeatedly dive between the giant planet and its innermost rings.

APOD: 2016 March 7 - Mystery Feature Now Disappears in Titan Lake
Explanation: What is that changing object in a cold hydrocarbon sea of Titan? Radar images from the robotic Cassini spacecraft orbiting Saturn have been recording the surface of the cloud-engulfed moon Titan for years. When imaging the flat -- and hence radar dark -- surface of the methane and ethane lake called Ligeia Mare, an object appeared in 2013 July just was not there in 2007. Subsequent observations in 2014 August found the object remained -- but had changed. In an image released last week, the mystery object seems to have disappeared in 2015 January. The featured false-color image shows how the 20-km long object has come, changed, and gone. Current origin speculative explanations include waves, bubbling foam and floating solids, but still no one is sure. Future observations, in particular Cassini's final close flyby of Titan in 2017 April, may either resolve the enigma or open up more speculation.

APOD: 2016 March 3 - Moons and Jupiter
Explanation: Some of the Solar System's largest moons rose together on February 23. On that night, a twilight pairing of a waning gibbous Moon and Jupiter was captured in this sharp telescopic field of view. The composite of short and long exposures reveals the familiar face of our fair planet's own large natural satellite, along with a line up of the ruling gas giant's four Galilean moons. Left to right, the tiny pinpricks of light are Callisto, Io, Ganymede, [Jupiter], and Europa. Closer and brighter, our own natural satellite appears to loom large. But Callisto, Io, and Ganymede are actually larger than Earth's Moon, while water world Europa is only slightly smaller. In fact, of the Solar System's six largest planetary satellites, only Saturn's moon Titan is missing from the scene. (Editor's note: Composite corrected for orientation and field of view posted on March 7.)

APOD: 2016 January 8 - Prometheus and the F Ring
Explanation: In Greek myth Prometheus was a Titan, known for bringing fire from Mount Olympus. But in modern times the name is given to a small moon of Saturn, orbiting just inside Saturn's F ring. In a complex interaction, the tiny potato-shaped moon interacts with the icy ring particles creating structures along the F ring still not fully understood. One of the highest resolution views of Prometheus, this image of its pocked surface posing with the thin F ring in the background was taken during the Cassini spacecraft's close approach on December 6, 2015. Elongated Prometheus is roughly 150 kilometers (90 miles) long by 70 kilometers wide.

APOD: 2015 May 15 - Jupiter, Ganymede, Great Red Spot
Explanation: In this sharp snapshot, the Solar System's largest moon Ganymede poses next to Jupiter, the largest planet. Captured on March 10 with a small telescope from our fair planet Earth, the scene also includes Jupiter's Great Red Spot, the Solar System's largest storm. In fact, Ganymede is about 5,260 kilometers in diameter. That beats out all three of its other fellow Galilean satellites, along with Saturn's Moon Titan at 5,150 kilometers and Earth's own Moon at 3,480 kilometers. Though its been shrinking lately, the Great Red Spot's diameter is still around 16,500 kilometers. Jupiter, the Solar System's ruling gas giant, is about 143,000 kilometers in diameter at its equator. That's nearly 10 percent the diameter of the Sun.

APOD: 2015 February 2 - Titan Seas Reflect Sunlight
Explanation: Why would the surface of Titan light up with a blinding flash? The reason: a sunglint from liquid seas. Saturn's moon Titan has numerous smooth lakes of methane that, when the angle is right, reflect sunlight as if they were mirrors. Pictured here in false-color, the robotic Cassini spacecraft orbiting Saturn imaged the cloud-covered Titan last summer in different bands of cloud-piercing infrared light. This specular reflection was so bright it saturated one of Cassini's infrared cameras. Although the sunglint was annoying -- it was also useful. The reflecting regions confirm that northern Titan houses a wide and complex array of seas with a geometry that indicates periods of significant evaporation. During its numerous passes of our Solar System's most mysterious moon, Cassini has revealed Titan to be a world with active weather -- including times when it rains a liquefied version of natural gas.

APOD: 2015 January 16 - Huygens Lands on Titan
Explanation: Delivered by Saturn-bound Cassini, ESA's Huygens probe touched down on the ringed planet's largest moon Titan, ten years ago on January 14, 2005. These panels show fisheye images made during its slow descent by parachute through Titan's dense atmosphere. Taken by the probe's descent imager/spectral radiometer instrument they range in altitude from 6 kilometers (upper left) to 0.2 kilometers (lower right) above the moon's surprisingly Earth-like surface of dark channels, floodplains, and bright ridges. But at temperatures near -290 degrees F (-180 degrees C), the liquids flowing across Titan's surface are methane and ethane, hydrocarbons rather than water. After making the most distant landing for a spacecraft from Earth, Huygens transmitted data for more than an hour. The Huygens data and a decade of exploration by Cassini have shown Titan to be a tantalizing world hosting a complex chemistry of organic compounds, dynamic landforms, lakes, seas, and a possible subsurface ocean of liquid water.

APOD: 2014 November 24 - Soaring over Titan
Explanation: What would it look like to fly over Titan? Radar images from NASA's robotic Cassini satellite in orbit around Saturn have been digitally compiled to simulate such a flight. Cassini has swooped past Saturn's cloudiest moon several times since it arrived at the ringed planet in 2004. The virtual flight featured here shows numerous lakes colored black and mountainous terrain colored tan. Surface regions without detailed vertical information appear more flat, while sufficiently mapped regions have their heights digitally stretched. Among the basins visualized is Kraken Mare, Titan's largest lake which spans over 1,000 kilometers long. Titan's lakes are different from Earth's lakes in that they are composed of hydrocarbons with similarities to liquid natural gas. How Titan's lakes were created and why they survive continues to be a topic of research.

APOD: 2014 November 2 - Titan Beyond the Rings
Explanation: When orbiting Saturn, be sure to watch for breathtaking superpositions of moons and rings. One such picturesque vista was visible recently to the robot Cassini spacecraft now orbiting Saturn. In 2006 April, Cassini captured Saturn's A and F rings stretching in front of cloud-shrouded Titan. Near the rings and appearing just above Titan was Epimetheus, a moon which orbits just outside the F ring. The dark space in the A ring is called the Encke Gap, although several thin knotted ringlets and even the small moon Pan orbit there.

APOD: 2014 October 15 - Mysterious Changing feature on Titan
Explanation: What is that changing object in a cold hydrocarbon sea of Titan? Radar images from the robotic Cassini spacecraft orbiting Saturn have been recording the surface of the cloud-engulfed moon Titan for years. When imaging the flat -- and hence radar dark -- surface of the methane and ethane lake called Ligeia Mare, an object appeared in 2013 just was not there in 2007. Subsequent observations in 2014 found the object remained -- but had changed! The featured image shows how the 20-km long object has appeared and evolved. Current origin speculative explanations include bubbling foam and floating solids, but no one is sure. Future observations may either resolve the enigma or open up more speculation.

APOD: 2014 September 19 - Potentially Habitable Moons
Explanation: For astrobiologists, these may be the four most tantalizing moons in our Solar System. Shown at the same scale, their exploration by interplanetary spacecraft has launched the idea that moons, not just planets, could have environments supporting life. The Galileo mission to Jupiter discovered Europa's global subsurface ocean of liquid water and indications of Ganymede's interior seas. At Saturn, the Cassini probe detected erupting fountains of water ice from Enceladus indicating warmer subsurface water on even that small moon, while finding surface lakes of frigid but still liquid hydrocarbons beneath the dense atmosphere of large moon Titan. Now looking beyond the Solar System, new research suggests that sizable exomoons, could actually outnumber exoplanets in stellar habitable zones. That would make moons the most common type of habitable world in the Universe.

APOD: 2013 December 20 - Titan's Land of Lakes
Explanation: Saturn's large moon Titan would be unique in our solar system, the only world with stable liquid lakes and seas on its surface ... except for planet Earth of course. Centered on the north pole, this colorized map shows Titan's bodies of methane and ethane in blue and black, still liquid at frigid surface temperatures of -180 degrees C (-292 degrees F). The map is based on data from the Cassini spacecraft's radar, taken during flybys between 2004 and 2013. Roughly heart-shaped, the lake above and right of the pole is Ligeia Mare, the second largest known body of liquid on Titan and larger than Lake Superior on Earth. Just below the north pole is Punga Mare. The sprawling sea below and right of Punga is the (hopefully sleeping) Kraken Mare, Titan's largest known sea. Above and left of the pole, the moon's surface is dotted with smaller lakes that range up to 50 kilometers across.

APOD: 2013 July 29 - Saturn, Titan, Rings, and Haze
Explanation: This is not a solar eclipse. Pictured above is a busy vista of moons and rings taken at Saturn. The large circular object in the center of the image is Titan, the largest moon of Saturn and one of the most intriguing objects in the entire Solar System. The dark spot in the center is the main solid part of the moon. The bright surrounding ring is atmospheric haze above Titan, gas that is scattering sunlight to a camera operating onboard the robotic Cassini spacecraft. Cutting horizontally across the image are the rings of Saturn, seen nearly edge on. At the lower right of Titan is Enceladus, a small moon of Saturn. Since the image was taken pointing nearly at the Sun, the surfaces of Titan and Enceladus appear in silhouette, and the rings of Saturn appear similar to a photographic negative. Now if you look really really closely at Enceladus, you can see a hint of icy jets shooting out toward the bottom of the image. It is these jets that inspired future proposals to land on Enceladus, burrow into the ice, and search for signs of extraterrestrial life.

APOD: 2013 January 21 - Huygens: Titan Descent Movie
Explanation: What would it look like to land on Saturn's moon Titan? The European Space Agency's Huygens probe set down on the Solar System's cloudiest moon in 2005, and a time-lapse video of its descent images was created. Huygens separated from the robotic Cassini spacecraft soon after it achieved orbit around Saturn in late 2004 and began approaching Titan. For two hours after arriving, Huygens plummeted toward Titan's surface, recording at first only the shrouded moon's opaque atmosphere. The computerized truck-tire sized probe soon deployed a parachute to slow its decent, pierced the thick clouds, and began transmitting images of a strange surface far below never before seen in visible light. Landing in a dried sea and surviving for 90 minutes, Huygen's return unique images of a strange plain of dark sandy soil strewn with smooth, bright, fist-sized rocks of ice.

APOD: 2012 November 6 - Methone: Smooth Egg Moon of Saturn
Explanation: Why is this moon shaped like a smooth egg? The robotic Cassini spacecraft completed the first flyby ever of Saturn's small moon Methone in May and discovered that the moon has no obvious craters. Craters, usually caused by impacts, have been seen on every moon, asteroid, and comet nucleus ever imaged in detail -- until now. Even the Earth and Titan have craters. The smoothness and egg-like shape of the 3-kilometer diameter moon might be caused by Methone's surface being able to shift -- something that might occur were the moon coated by a deep pile of sub-visual rubble. If so, the most similar objects in our Solar System would include Saturn's moons Telesto, Pandora, Calypso, as well as asteroid Itokawa, all of which show sections that are unusually smooth. Methone is not entirely featureless, though, as some surface sections appear darker than others. Although flybys of Methone are difficult, interest in the nature and history of this unusual moon is sure to continue.

APOD: 2012 July 24 - South Polar Vortex Discovered on Titan
Explanation: What's happening over the south pole of Titan? A vortex of haze appears to be forming, although no one is sure why. The above natural-color image shows the light-colored feature. The vortex was found on images taken last month when the robotic Cassini spacecraft flew by the unusual atmosphere-shrouded moon of Saturn. Cassini was only able to see the southern vortex because its orbit around Saturn was recently boosted out of the plane where the rings and moons move. Clues as to what created the enigmatic feature are accumulating, including that Titan's air appears to be sinking in the center and rising around the edges. Winter, however, is slowly descending on the south of Titan, so that the vortex, if it survives, will be plunged into darkness over the next few years.

APOD: 2012 July 3 - In the Shadow of Saturn's Rings
Explanation: Humanity's robot orbiting Saturn has recorded yet another amazing view. That robot, of course, is the spacecraft Cassini, while the new amazing view includes a bright moon, thin rings, oddly broken clouds, and warped shadows. Titan, Saturn's largest moon, appears above as a featureless tan as it is continually shrouded in thick clouds. The rings of Saturn are seen as a thin line because they are so flat and imaged nearly edge on. Details of Saturn's rings are therefore best visible in the dark ring shadows seen across the giant planet's cloud tops. Since the ring particles orbit in the same plane as Titan, they appear to skewer the foreground moon. In the upper hemisphere of Saturn, the clouds show many details, including dips in long bright bands indicating disturbances in a high altitude jet stream. Recent precise measurements of how much Titan flexes as it orbits Saturn hint that vast oceans of water might exist deep underground.

APOD: 2012 May 21 - A Close Pass of Saturn's Moon Dione
Explanation: What's that past Dione? When making its closest pass yet of Saturn's moon Dione late last year, the robotic Cassini spacecraft snapped this far-ranging picture featuring Dione, Saturn's rings, and the two small moons Epimetheus and Prometheus. The above image captures part of the heavily cratered snow-white surface of the 1,100 kilometer wide Dione, the thinness of Saturn's rings, and the comparative darkness of the smaller moon Epimetheus. The image was taken when Cassini was only about 100,000 kilometers from the large icy moon. Future events in Cassini's continuing exploration of Saturn and its moons include tomorrow's flyby of Titan and imaging the distant Earth passing behind Saturn in June.

APOD: 2012 April 14 - Six Moons of Saturn
Explanation: How many moons does Saturn have? So far 62 have been discovered, the smallest only a fraction of a kilometer across. Six of its largest satellites can be seen here, though, in a sharp Saturnian family portrait taken on March 9. Larger than Earth's Moon and even slightly larger than Mercury, Titan has a diameter of 5,150 kilometers and starts the line-up at the lower left. Continuing to the right across the frame are Mimas, Tethys, [Saturn], Enceladus, Dione, and Rhea at far right. Saturn's first known natural satellite, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens, while most recently the satellite provisionally designated S/2009 S1 was found by the Cassini Imaging Science Team in 2009. Tonight, Saturn reaches opposition in planet Earth's sky, offering the best telescopic views of the ringed planet and moons.

APOD: 2012 January 5 - Ringside with Titan and Dione
Explanation: Orbiting in the plane of Saturn's rings, Saturnian moons have a perpetual ringside view of the gorgeous gas giant planet. Of course, while passing near the ring plane the Cassini spacecraft also shares their stunning perspective. The rings themselves can be seen slicing across the middle of this Cassini snapshot from May of last year. The scene features Titan, largest, and Dione, third largest moon of Saturn. Remarkably thin, the bright rings still cast arcing shadows across the planet's cloud tops at the bottom of the frame. Pale Dione is about 1,100 kilometers across and orbits over 300,000 kilometers from the visible outer edge of the A ring. Dione is seen through Titan's atmospheric haze. At 5,150 kilometers across, Titan is about 2.3 million kilometers from Cassini, while Dione is 3.2 million kilometers away.

APOD: 2011 October 26 - In, Through, and Beyond Saturn's Rings
Explanation: A fourth moon is visible on the above image if you look hard enough. First -- and farthest in the background -- is Titan, the largest moon of Saturn and one of the larger moons in the Solar System. The dark feature across the top of this perpetually cloudy world is the north polar hood. The next most obvious moon is bright Dione, visible in the foreground, complete with craters and long ice cliffs. Jutting in from the left are several of Saturn's expansive rings, including Saturn's A ring featuring the dark Encke Gap. On the far right, just outside the rings, is Pandora, a moon only 80-kilometers across that helps shepherd Saturn's F ring. The fourth moon? If you look closely in the Encke Gap you'll find a speck that is actually Pan. Although one of Saturn's smallest moons at 35-kilometers across, Pan is massive enough to help keep the Encke gap relatively free of ring particles.

APOD: 2011 June 13 - Views from Cassini at Saturn
Explanation: What has the Cassini orbiter seen since arriving at Saturn? The above music video shows some of the highlights. In the first time-lapse sequence (00:07), a vertical line appears that is really Saturn's thin rings seen nearly edge-on. Soon some of Saturn's moon shoot past. The next sequence (00:11) features Saturn's unusually wavy F-ring that is constrained by the two shepherd moons that are also continually perturbing it. Soon much of Saturn's extensive ring system flashes by, sometimes juxtaposed to the grandeur of the immense planet itself. Cloud patterns on Titan (00:39) and Saturn (00:41) are highlighted. Clips from flyby's of several of Saturn's moon are then shown, including Phoebe, Mimas, Epimetheus, and Iapetus. In other sequences, moons of Saturn appear to pass each other as they orbit Saturn. Background star fields seen by Cassini are sometimes intruded upon by bright passing moons. The robotic Cassini spacecraft has been revolutionizing humanity's knowledge of Saturn and its moons since 2004.

APOD: 2011 April 1 - It's Raining on Titan
Explanation: It's been raining on Titan. In fact, it's likely been raining methane on Titan and that's not an April Fools' joke. The almost familiar scene depicted in this artist's vision of the surface of Saturn's largest moon looks across an eroding landscape into a stormy sky. That scenario is consistent with seasonal rain storms temporarily darkening Titan's surface along the moon's equatorial regions, as seen by instruments onboard the Cassini spacecraft. Of course on frigid Titan, with surface temperatures of about -290 degrees F (-180 degrees C), the cycle of evaporation, cloud formation, and rain involves liquid methane instead of water. Lightning could also be possible in Titan's thick, nitrogen-rich atmosphere.

APOD: 2011 March 15 - Cassini Approaches Saturn
Explanation: What would it look like to approach Saturn in a spaceship? One doesn't have to just imagine -- the Cassini spacecraft did just this in 2004, recording thousands of images along the way, and thousands more since entering orbit. Recently, some of these images have been digitally tweaked, cropped, and compiled into the above inspiring video which is part of a larger developing IMAX movie project named Outside In. In the last sequence, Saturn looms increasingly large on approach as cloudy Titan swoops below. With Saturn whirling around in the background, Cassini is next depicted flying over Mimas, with large Herschel Crater clearly visible. Saturn's majestic rings then take over the show as Cassini crosses Saturn's thin ring plane. Dark shadows of the ring appear on Saturn itself. Finally, the enigmatic ice-geyser moon Enceladus appears in the distance and then is approached just as the video clip ends.

APOD: 2011 March 8 - Titan, Rings, and Saturn from Cassini
Explanation: How thin are the rings of Saturn? Brightness measurements from different angles have shown Saturn's rings to be about one kilometer thick, making them many times thinner, in relative proportion, than a razor blade. This thinness sometimes appears in dramatic fashion during an image taken nearly along the ring plane. The robot Cassini spacecraft now orbiting Saturn has now captured another shot that dramatically highlights the ring's thinness. The above image was taken in mid January in infrared and polarized light. Titan looms just over the thin rings, while dark ring shadows on Saturn show the Sun to be above the ring plane. Close inspection of the image will show the smaller moon Enceladus on the far right. Cassini, humanity's first mission to orbit Saturn, currently has operations planned until 2017.

APOD: 2010 August 10 - The Sand Dunes of Titan
Explanation: Why do some sand dunes on Titan appear backwards? Central Titan, it turns out, is covered by sand, some of which appears strange. Images from the Cassini spacecraft currently orbiting Saturn have uncovered long rows of huge sand dunes near Titan's equator that rise as high as 300 meters. Shadows indicate that most dune shapes are created by wind blowing from the west. The problem is, the typical wind at Titan's equator blows from the east. One recent hypothesis that might solve this grainy conundrum posits that the only winds strong enough to move sand and create dunes occur during rare equinoxes and blow strongly from the west. The above images show a radar swath of Titan's equatorial sand dunes at the top, while similar sand dunes that formed in Namibia on Earth at the bottom. Why central Titan is even covered by so much sand is still being investigated.

APOD: 2010 April 20 - Saturn's Moons Dione and Titan from Cassini
Explanation: What would it be like to see a sky with many moons? Such is the sky above Saturn. When appearing close to each other, moons will show a similar phase. A view with two of the more famous moons of Saturn in gibbous phase was captured last month by the robot spacecraft Cassini now orbiting Saturn. Titan, on the left, is among the largest moons in the Solar System and is perpetually shrouded in clouds. In 2005, the Huygens probe landed on Titan and gave humanity its first view of its unusual surface. Dione, on the right, has less than a quarter of Titan's diameter and has no significant atmosphere. The above uncalibrated image was taken on April 10 after Cassini swooped by each moon the previous week.

APOD: 2010 April 5 - Prometheus Remastered
Explanation: What does Saturn's shepherd moon Prometheus really look like? The raw images from the robotic Cassini spacecraft's January flyby of the small moon showed tantalizing clues on grainy images, but now that the Cassini team has digitally remastered these images, many more details have come out. Pictured above, Prometheus more clearly shows its oblong shape as well as numerous craters over its 100-kilometer length. In the above image, the bright part of Prometheus is lit directly by the Sun, while much of the dark part is still discernible through sunlight first reflected off of Saturn. These new surface details, together with the moon's high reflectivity, can now help humanity better understand the history of Prometheus and Saturn's rings. Today, Cassini has a planned targeted flyby of Saturn's largest moon Titan, while on Wednesday, Cassini is scheduled to swoop to within 600 kilometers of Dione.

APOD: 2010 January 27 - Tethys Behind Titan
Explanation: What's that behind Titan? It's another of Saturn's moons: Tethys. The robotic Cassini spacecraft orbiting Saturn captured the heavily cratered Tethys slipping behind Saturn's atmosphere-shrouded Titan late last year. The largest crater on Tethys, Odysseus, is easily visible on the distant moon. Titan shows not only its thick and opaque orange lower atmosphere, but also an unusual upper layer of blue-tinted haze. Tethys, at about 2 million kilometers distant, was twice as far from Cassini as was Titan when the above image was taken. In 2004, Cassini released the Hyugens probe which landed on Titan and provided humanity's first views of the surface of the Solar System's only known lake-bearing moon.

APOD: 2009 May 5 - Titan Beyond the Rings
Explanation: When orbiting Saturn, be sure to watch for breathtaking superpositions of moons and rings. One such picturesque vista was visible recently to the robot Cassini spacecraft now orbiting Saturn. In 2006 April, Cassini captured Saturn's A and F rings stretching in front of cloud-shrouded Titan. Near the rings and appearing just above Titan was Epimetheus, a moon which orbits just outside the F ring. The dark space in the A ring is called the Encke Gap, although several thin knotted ringlets and even the small moon Pan orbit there. Cassini and curious Earthlings await the coming Saturnian equinox this summer when the ring plane will point directly at the Sun. Mysterious spokes and telling shadows are expected to become visible that might give away more clues about the nature of Saturn's ring particles.

APOD: 2009 March 19 - Saturn: Moons in Transit
Explanation: Every 14 to 15 years, Saturn's rings are tilted edge-on to our line of sight. As the bright, beautiful rings seem to grow narrower it becomes increasingly difficult to see them, even with large telescopes. But it does provide the opportunity to watch multiple transits of Saturn's moons. During a transit, a sunlit moon and its shadow glide across the cloudy face of the gas giant. Recorded on February 24, this Hubble image is part of a sequence showing the transit of four of Saturn's moons. From left to right are Enceladus and shadow, Dione and shadow, and Saturn's largest moon Titan. Small moon Mimas is just touching Saturn's disk near the ring plane at the far right. The shadows of Titan and Mimas have both moved off the right side of the disk. Saturn itself has an equatorial diameter of about 120,000 kilometers.

APOD: 2008 October 20 - Moons, Rings, and Unexpected Colors on Saturn
Explanation: Why would Saturn show such strange colors? The robotic Cassini spacecraft currently orbiting Saturn has beamed back images showing that the northern hemisphere our Solar System's most spectacularly ringed planet has changed noticeably since Cassini arrived in 2004, now sporting unusual and unexpected colors. No one is sure why. Although the cause for many of Saturn's colors is unknown, the recent change in colors is thought to be related to the changing seasons. Pictured above, the unusual colors are visible just north of the dark ring shadows. The razor-thin plane of ring particles is visible nearly edge-on across the bottom of the image. The cloudy moon Titan looms large just above the rings, while close observation will reveal three other moons. Cassini arrived at Saturn in 2004, sending back data and images that have not only led to a deeper understanding of the Jovian world's atmosphere, moons, and rings, but also raised new mysteries.

APOD: 2008 July 20 - Crescent Rhea Occults Crescent Saturn
Explanation: Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn. Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few years ago. As striking as the above image is, it is but a single frame from a recently released 60-frame silent movie where Rhea can be seen gliding in front of its parent world. Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center. Although Cassini has now concluded its primary mission, its past successes and opportunistic location have prompted NASA to start a two-year Equinox Mission, further exploring not only Saturn's enigmatic moons Titan and Enceladus, but Saturn herself as her grand rings tilt right at the Sun in August 2009.

APOD: 2008 May 13 - Ancient Craters of Southern Rhea
Explanation: Saturn's ragged moon Rhea has one of the oldest surfaces known. Estimated as changing little in the past billion years, Rhea shows craters so old they no longer appear round – their edges have become compromised by more recent cratering. Like Earth's Moon, Rhea's rotation is locked on Saturn, and the above image shows part of Rhea's surface that always faces Saturn. Rhea's leading surface is more highly cratered than its trailing surface. Rhea is composed mostly of water-ice but is thought to include about 25 percent rock and metal. The above image was taken by the robot Cassini spacecraft now orbiting Saturn. Cassini swooped past Rhea last month and captured the above image from about 350,000 kilometers away. Rhea spans 1,500 kilometers making it Saturn's second largest moon after Titan. Several surface features on Rhea remain unexplained including large light patches like those seen near the image top.

APOD: 2008 March 24 - Saturn and Titan from Cassini
Explanation: Spectacular vistas of Saturn and its moon continue to be recorded by the Cassini spacecraft. Launched from Earth in 1997, robotic Cassini entered orbit around Saturn in 2004 and has revolutionized much of humanity's knowledge of Saturn, its expansive and complex rings, and it many old and battered moons. Soon after reaching Saturn, Cassini released the Huygen's probe which landed on Titan, Saturn's largest moon, and send back unprecedented pictures from below Titan's opaque cloud decks. Recent radar images of Titan from Cassini indicate flat regions that are likely lakes of liquid methane, indicating a complex weather system where it likely rains chemicals similar to gasoline. Pictured above, magnificent Saturn and enigmatic Titan were imaged together in true color by Cassini earlier this year.

APOD: 2007 October 11 - Bright Planets, Crescent Moon
Explanation: Early risers are currently enjoying the sight of dazzling Venus, near the eastern horizon as the morning star. Recorded on October 7, this predawn skyview does feature Venus at the upper right. It also includes a crescent Moon and Saturn (lower left). In fact, holding your fist at arms length would have easily covered both planets and the Moon in this 5 degree wide field. Earthshine, sunlight reflected from planet Earth's dayside, illuminates features on the lunar nightside. A close inspection of Saturn itself reveals a nearby pinpoint of light corresponding to Saturn's large moon Titan. Though the Moon has moved on, the tight triangle formed by Venus, Saturn, and Regulus (top), alpha star in the constellation Leo, will continue to look impressive in early morning skies over the next few days. Early bird astrophotographer Jay Ouellet also described Mars as a "brilliant red diode" in his dark country sky east of Quebec City, Canada.

APOD: 2007 May 30 - Liquid Sea on Saturn's Titan
Explanation: What is this vast dark region on Titan? Quite possible a sea of liquid hydrocarbons. The region was imaged earlier this month when the robotic Cassini spacecraft swooped past Saturn's cloudy moon and illuminated part of it with radar. The dark region in the above image reflected little radar, an effect expected were the dark surface relatively flat, as expected for a liquid. Other indications that the vast dark area is liquid include the coastline-like topology of the brighter regions, which appear to include islands, inlets, and tributary channels. The uninterrupted smoothness of much of the dark sea may indicate that the sea runs deep, with speculation holding a depth estimate of tens of meters. A hydrocarbon sea on Titan holds particular interest for exobiologists as it might be a place where life could develop. In 2005 the Huygens probe landed on Titan and returned the first surface images. Cassini will continue to explore Titan, as 13 more flybys are planned.

APOD: 2007 February 7 - Liquid Lakes on Saturns Titan
Explanation: Why would some regions on Titan reflect very little radar? The leading explanation is that these regions are lakes, possibly composed of liquid methane. The above image is a false-color synthetic radar map of a northern region of Titan taken during a flyby of the cloudy moon by the robotic Cassini spacecraft last July. On this map, which spans about 150 kilometers across, dark regions reflect relatively little of the broadcast radar signal. Images like this show Titan to be only the second body in the Solar System to possess liquids on the surface. Future observations from Cassini during Titan flybys will further test the methane lake hypothesis, as comparative wind effects on the regions are studied.

APOD: 2007 January 31 - Movie: Cassini Crosses Saturn's Ring Plane
Explanation: What would the rings of Saturn look like if you passed right through the ring plane? To find out, NASA aimed cameras from the Cassini spacecraft right at Saturn's rings as the robotic explorer passed from the sunlit side of the rings to the shadowed side. Resulting images from a vantage point outside the rings and most moons, but inside the orbit of Titan, have been gathered together in the above time-lapse movie. The dramatic movie demonstrates that ring particle density and reflectivity makes some parts of the shadowed side nearly the photographic negative of the sunlit side, but nearly empty regions remain continually dark. Visible also are Saturn-orbiting moons Enceladus, Mimas, Janus, Epimetheus, Prometheus, and Pandora. The extreme thinness of Saturn's rings can be appreciated from frames taken near the crossing time.

APOD: 2006 December 31 - A Year of Extraterrestrial Fountains and Flows
Explanation: The past year was extraordinary for the discovery of extraterrestrial fountains and flows -- some offering new potential in the search for liquid water and the origin of life beyond planet Earth.. Increased evidence was uncovered that fountains spurt not only from Saturn's moon Enceladus, but from the dunes of Mars as well. Lakes were found on Saturn's moon Titan, and the residual of a flowing liquid was discovered on the walls of Martian craters. The diverse Solar System fluidity may involve forms of slushy water-ice, methane, or sublimating carbon dioxide. Pictured above, the light-colored path below the image center is hypothesized to have been created sometime in just the past few years by liquid water flowing across the surface of Mars.

APOD: 2006 December 14 - Mountains of Titan
Explanation: Peering through the thick, hazy atmosphere of Saturn's largest moon, an infrared camera onboard the Cassini spacecraft recorded this view of the tallest mountains ever seen on Titan. Captured during a flyby in late October, the high resolution, false-color mosaic shows a mountain range about 150 kilometers long and about 1.5 kilometers high - likened to the Sierra Nevada mountain range of the western United States, planet Earth. Along Titan's mountain ridges lie bright deposits, thought to be methane snow or other organic material. The icy mountains of Titan were probably formed like Earth's mid-ocean ridges, from material welling up to fill gaps created as surface tectonic plates spread apart.

APOD: 2006 August 2 - Methane Rain Possible on Titan
Explanation: Might it rain cold methane on Saturn's Titan? Recent analyses of measurements taken by the Huygen's probe that landed on Titan in 2005 January indicate that the atmosphere is actually saturated with methane at a height of about 8 kilometers. Combined with observations of a damp surface and lakes near the poles, some astrobiologists conclude that at least a methane drizzle is common on parts of Titan. Other astrobiologists reported computer models of the clouded moon that indicate that violent methane storms might even occur, complete with flash floods carving channels in the landscape. The later scenario is depicted in the above drawing of Titan. Lightning, as also depicted above, might well exist on Titan but has not been proven. The findings increase speculation that a wet Titanian surface might be hospitable to unusual forms of life.

APOD: 2006 July 31 - Possible Methane Lakes on Titan
Explanation: Have methane lakes been discovered on Saturn's Titan? That exciting possibility was uncovered from analyses of radar images returned last week by the robotic Cassini spacecraft now orbiting Saturn. The above image is a radar reflection from terrain near Titan's North Pole and spans a region about 200 kilometers across. Evidence that the dark areas might be pools of liquid hydrocarbons includes an extreme smoothness implied by the lack of a return radar signal, and apparently connected tributaries. If true, Titan would be only the second body in our Solar System, after Earth, found to possess liquids on the surface. Future observations from Cassini during Titan flybys might test the methane lake hypothesis, as comparative wind affects on the regions are studied.

APOD: 2006 May 30 - Ancient Craters on Saturn's Rhea
Explanation: Saturn's ragged moon Rhea has one of the oldest surfaces known. Estimated as changing little in the past billion years, Rhea shows craters so old they no longer appear round – their edges have become compromised by more recent cratering. Like Earth's Moon, Rhea's rotation is locked on Saturn, and the above image shows part of Rhea's surface that always faces Saturn. Rhea's leading surface is more highly cratered than its trailing surface. Rhea is composed mostly of water-ice but is thought to have a small rocky core. The above image was taken by the robot Cassini spacecraft now orbiting Saturn. Cassini swooped past Rhea two months ago and captured the above image from about 100,000 kilometers away. Rhea spans 1,500 kilometers making it Saturn's second largest moon after Titan. Several surface features on Rhea remain unexplained including large light patches.

APOD: 2006 May 8 - Descent Panorama of Saturns Titan
Explanation: You're the first spacecraft ever to descend to Titan -- what do you see? Immediately after the Huygen's probe pierced the cloud deck of Saturn's moon Titan last January, it took a unique series of pictures of one of the Solar System's most mysterious moon's. Those pictures have recently been digitally stitched together to create spectacular panoramas and a dramatic descent movie. Pictured above is a panoramic fisheye view Huygen's obtained from about five kilometers above Titan's surface. The digital projection makes the local surface, mostly flat, appear as a ball, but allows one to see in all directions. Huygen's eventual landing site was in the large dark area below, just right of the center. This relatively featureless, dark, sandy basin appears to be surrounded by light colored hills to the right and a landscape fractured by streambeds and canyons above. Recent evidence indicates that Titan's lakebeds and streambeds are usually dry but sometimes filled with a flashflood of liquid methane from rare torrents of methane rain.

APOD: 2006 February 15 - Rotating Titan in Infrared Light
Explanation: Titan is one of the strangest places in our Solar System. The only moon known with thick clouds, this unusual satellite of Saturn shows evidence of evaporating lakes created by methane rain. The clouds that make Titan featureless in visible light have now been pierced several times in infrared light by the robot Cassini spacecraft currently orbiting Saturn. These images have been compiled into the above time-lapse movie. Like Earth's Moon, Titan always shows the same face toward its central planet. It therefore takes Titan about 16 days to complete one rotation. Titan has numerous areas of light terrain with some large areas of dark terrain visible near the equator. Small areas of brightest terrain might arise from ice-volcanoes and have a high amount of reflective frozen water-ice. Titan's surface was imaged for the first time early last year by the Huygens probe, which survived for three hours on a cold and sandy dark region.

APOD: 2006 January 31 - Huygens on Titan Illustrated
Explanation: If you could stand on Titan, what might you see? About one year ago the robotic Huygens probe landed on the enigmatic moon of Saturn and sent back the first ever images from beneath Titan's thick cloud layers. From the images sent back, an artist's impression of Huygens on Titan's surface has been reconstructed. In the foreground of the above image sits the car-sized lander that sent back images for more than 90 minutes before running out of battery power. The parachute that slowed Huygen's re-entry is seen in the background, attached to the lander by strong cords. Strange light smooth stones possibly containing water-ice are visible surrounding the landing craft. Analyses of Huygen's images and data shows that Titan's surface today has intriguing similarities to the surface of the early Earth.

APOD: 2005 December 31 - A Year at Saturn
Explanation: Arriving at Saturn in July of 2004, the Cassini spacecraft has now spent a year and a half exploring the magnificent rings and moons of the distant gas giant. The year 2005 began with Cassini's Huygens probe landing on Saturn's large moon Titan. Cassini's continuing series of close flybys also revealed details and discoveries across the surface of the smog shrouded moon. In fact, with a ringside seat throughout 2005, Cassini's cameras have made spectacular pictures of Titan along with Saturn's other moons and rings almost common place. But often, Saturn itself provided the most dramatic backdrop. In this view, Saturn's moon Dione lies in front of edge-on rings and the gas giant's cloud tops draped with broad ring shadows. Dione is 1,118 kilometers across and lies about 300,000 kilometers from the ring's edge.

APOD: 2005 October 27 - The Last Titan
Explanation: On October 19th, a rocket blasted off from Vandenberg Air Force Base - the last Titan rocket. Carrying a payload for the US National Reconnaissance Office, the successful Titan IV B launch brings to a close the Titan program whose first launch was in 1959. Originally designed as an intercontinental ballistic missile, the Titan rocket ultimately evolved into a heavy lift workhorse, launching defense, commercial, and scientific payloads to Earth orbit and beyond. In fact, many historic space explorations began with Titan launches, including manned Gemini missions, the Viking missions to Mars, the Voyager tours of the outer solar system, and the Cassini spacecraft now orbiting Saturn. Cassini's probe Huygens accomplished the most distant landing on another world, while Voyager 1 is now humanity's most distant spacecraft.

APOD: 2005 September 21 - Shoreline Terrain on Saturns Titan
Explanation: What could have created this unusual terrain on Saturn's moon Titan? The robotic Cassini spacecraft now orbiting Saturn swooped once again, earlier this month, past the Solar System's most enigmatic moon and radar imaged a rich but unusual region that appears to be some sort of shoreline. The choppy, light-colored, high regions on the left appears to be have channels cut by a moving fluid, while the smoother dark regions on the right appear to outline bays. Results from the Huygens probe that landed on Titan earlier this year imply that fluids, possibly liquid methane and not water, might only occupy some of these channels and bays intermittently. The radar image shown above spans about 200 kilometers.

APOD: 2005 July 26 - Hyperion: Sponge Moon of Saturn
Explanation: Why is Saturn's moon Hyperion textured like a sponge? Recent high-resolution images from the robot Cassini spacecraft orbiting Saturn show Hyperion to be an even stranger place than thought before. Previously, it was known that the length of a day on Hyperion is unpredictable. The moon's highly elliptical orbit around Saturn, its highly non-spherical shape, and its locked 4:3 orbital resonance with Titan torque Hyperion around so much it is hard to predict when the Sun will rise next. The newly imaged craters on the unusually coarse surface are surely the result of impacts, but for some reason have dark centers. The low density of Hyperion indicates it might even be a spelunker's paradise, riddled with tremendous caverns.

APOD: 2005 June 10 - Titan's Cryovolcano
Explanation: Investigators suspect the domed feature detailed above is an ice volcano, or cryovolcano, seen in infrared light through the hazy atmosphere on Saturn's moon Titan. Since Titan's surface temperature is around minus 180 degrees Celsius, lava welling up to form the volcanic mound would be icy indeed - possibly a slurry of methane, ammonia, and water ice combined with other ices and hydrocarbons. The circular feature is roughly thirty kilometers in diameter. If its volcanic nature is confirmed, the discovery of cryovolcanism on Titan could explain the origin of methane in Titan's atmosphere. Before the Cassini-Huygens mission to Saturn, a popular explanation for replenishing Titan's concentration of atmospheric methane was the presence of an extensive, methane-rich, hydrocarbon sea. But Cassini's instruments and the Huygens surface probe have failed to find such a global ocean.

APOD: 2005 May 30 - A Great White Spot on Rhea
Explanation: What caused this great white spot on the surface of Saturn's moon Rhea? The spot was first noticed last year by the robot Cassini spacecraft now orbiting Saturn. Cassini's flyby of Rhea in April imaged in the spot in great detail. Astronomers hypothesize that the light-colored spot is the result of a relatively recent impact on the surface of the icy moon. The impact that likely created the crater also splashed light-colored material from the interior onto the darker surface. Rhea spans 1,500 kilometers across and is the second largest moon of Saturn after Titan. Rhea sports several other light colored surface features that are, as yet, not well understood.

APOD: 2005 May 27 - Titan s Odd Spot
Explanation: Titan's odd spot could be a cloud, but if so, it's a persistent one. Peering into the thick, hazy atmosphere of Saturn's largest moon, cameras on board the Cassini spacecraft found a bright spot at the same location during Titan encounters in 2005 and 2004. Seen near Titan's upper edge in this false-color image from the VIMS instrument, the spot is almost 500 kilometers wide, and is brightest at infrared wavelengths. In addition to suggesting the uniquely colored spot is a persistent cloud possibly controlled by surface features, researchers also entertain the idea that the spot is caused by unusual surface material or extremely tall mountains. They also note the bright infrared spot could be hot. Further clues to the odd spot's nature will come during a planned encounter in July 2006 when Cassini's cameras will look at the spot during Titan's night. If it glows at night, it's hot.

APOD: 2005 May 18 - Three Kilometers Above Titan
Explanation: What did the Huygens probe see as it descended toward Saturn's Moon Titan? In January the robot Cassini spacecraft now orbiting Saturn released a probe through the dense cloud decks of one of the Solar System's most mysterious moons. Below the clouds, as it descended, the probe took images of the approaching surface as well as several images from the surface itself. Many of the images have now been digitally merged and scaled into the above perspective from 3,000 meters high. The above stereographic projection shows a 360-degree wide-angle view of the surface of Titan. The bright areas toward the top and left of the image are thought to be relatively high ground laced with drainage channels cut by rivers of methane. The bright shapes on the right are now hypothesized to be ridges of ice gravel. Huygen's landing site, labeled, appears to be on a type of dark dry lakebed, once fed by a large dark flow channel on the left. The Huygen's probe lasted an unexpectedly long three hours on Titan's harsh surface.

APOD: 2005 April 18 - Saturnian Moon and Rings
Explanation: When can a robot produce art? When it glides past the rings of Saturn. As the robot spacecraft Cassini orbiting Saturn crossed outside the famous photogenic ring plane of the expansive planet, the rings were imaged from the outside, nearly edge on, and in the shadow of Saturn. From the upper left, ring features include the A ring, the Cassini gap, the B ring, and the darker C ring that includes the Titan gap and a gap yet unnamed. Last month when the above image was taken, the gliding spacecraft was about one million kilometers from foreground Enceladus, a small Saturnian moon only about 500 kilometers across. Cassini is scheduled to continue its 70 orbit tour of Saturn over the next three years, sending back images of the gas giant, its rings, and its moons that will be studied for decades to come.

APOD: 2005 March 29 - Crescents of Titan and Dione
Explanation: What would it be like to see a sky with many moons? Such is the sky above Saturn. When appearing close to each other, moons will show a similar phase. A view with two of the more famous moons of Saturn in crescent phase was captured last month by the robot spacecraft Cassini now orbiting Saturn. Titan, on the lower left, is among the largest moons in the Solar System and is perpetually shrouded in clouds. Recently, the Huygens probe landed on Titan and gave humanity its first view of its unusual surface. Dione, on the upper right, has less than a quarter of Titan's diameter and has no significant atmosphere. Dione, although appearing smaller, was only half the distance to Titan when the above image was taken.

APOD: 2005 March 25 - Huygens Discovers Luna Saturni
Explanation: In 1655, three hundred fifty years ago on this date, Dutch astronomer Christiaan Huygens discovered Luna Saturni - now known as Saturn's moon Titan. To celebrate, consider this intriguing picture of his telescope lens, all that remains of the instrument he used, designed and constructed in collaboration with his brother, Constantijn Huygens. The lens itself measures 57 millimeters (just over 2 inches) in diameter and is inscribed along the border "X 3 FEBR. MDCLV": its focal length (10 Rhineland feet) and the date of its final polishing, 3 February 1655. It also bears a verse from the Roman poet Ovid, "Admovere Oculis Distantia Sidera Nostris" (They brought the distant stars closer to our eyes). Huygens used the verse as part of an anagram announcing his discovery. The use of an anagram, a practice common in his time, established a date for his discovery but kept its details secret until he wished to reveal them. Decoded and translated, his anagram reads "A moon revolves around Saturn in 16 days and 4 hours.", a good agreement with the modern value for Titan's orbital period.

APOD: 2005 February 15 - Saturns Moon Rhea from Cassini
Explanation: Each moon of Saturn seems to come with its own mystery. Rhea, Saturn's second largest moon behind Titan, shows unusual wisps, visible above as light colored streaks. Higher resolution images of similar wisps on Dione indicate that they might be made of long braided fractures. Rhea is composed mostly of water ice, but likely has a small rocky core. Rhea's rotation and orbit are locked together, just like Earth's Moon, so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than the trailing surface, pictured above. The above image in natural color was taken last month by the Cassini robot spacecraft in orbit around Saturn.

APOD: 2005 January 24 - Riverbeds and Lakebeds Discovered on Saturn's Titan
Explanation: Methane rain, evaporating lakes, flowing rivers, and water ice-volcanoes all likely exist on Saturn's moon Titan, according to preliminary analyses of recent images taken by the successful Huygens lander. A snaking and branching riverbed is identified with the dark channel near the top of the above image, while a dark lakebed is identified across the image bottom. Both the riverbed and lakebed were thought to be dry at the time the image was taken but contained a flowing liquid - likely methane - in the recent past. Titan's surface was found to appear strangely similar to Earth even though it is so cold that methane flows and water freezes into rock-hard ice. Although the Huygens probe has now run out of power, the images it returned will likely be studied for decades to come. The Cassini mothership is scheduled to continue to orbit Saturn and return images for several more years.

APOD: 2005 January 19 - Eight Kilometers Above Titan
Explanation: What are these surface features on Titan? Scroll right to see the panoramic view captured last week by the Huygens probe as it descended toward Saturn's mysterious moon. Scientists are not yet sure what the above image is showing. On the far left, a boundary seems to exist between some sort of smooth dark terrain and a type of choppier terrain in the distance. In the image center and on the left, white areas cover the image that might be a type of ground fog. The Huygens probe landed in the dark area of the far right, finding a portion of Titan's surface that had the consistency of wet sand and a surface temperature of -179 degrees Celsius. Huygen's battery lasted an unexpectedly long three hours as it beamed back images and data to the Saturn-orbiting Cassini mothership and an armada of Earth's most sensitive radio telescopes. Analysis of the Huygens' images will likely continue for years in attempts to better understand this cloud-engulfed moon.

APOD: 2005 January 17 - Titan Landscape
Explanation: This color view from Titan gazes across a suddenly familiar but distant landscape on Saturn's largest moon. The scene was recorded by ESA's Huygens probe after a 2 1/2 hour descent through a thick atmosphere of nitrogen laced with methane. Bathed in an eerie orange light at ground level, rocks strewn about the scene could well be composed of water and hydrocarbons frozen solid at an inhospitable temperature of - 179 degrees C. The light-toned rock below and left of center is only about 15 centimeters across and lies 85 centimeters away. Touching down at 4.5 meters per second (16 kilometers per hour), the saucer-shaped probe is believed to have penetrated 15 centimeters or so into a surface with the consistency of wet sand or clay. Huygen's batteries are now exhausted but the probe transmitted data for more than 90 minutes after landing. Titan's bizarre chemical environment may bear similarities to planet Earth's before life evolved.

APOD: 2005 January 15 - Huygens Images Titan's Surface
Explanation: After a seven year interplanetary voyage on board the the Cassini spacecraft, the European Space Agency's Huygens probe parachuted to a historic landing on Saturn's moon Titan on January 14. Above are two of the first raw images Huygens recorded of the mystery moon's surface - a view from an altitude of 16 kilometers (left), and surface level. The altitude image resolves features as small as about 40 meters. In the dramatic surface level vista, the light toned rock-like object below and left of center is only about 15 centimeters across and lies 85 centimeters from the probe. Remarkably, the views of Titan's surface suggest a similarity to eroded surfaces on Earth and Mars.

APOD: 2005 January 14 - Descent to Titan
Explanation: Today's descent to the surface of Titan by the European Space Agency's Huygens probe was the most distant landing ever by a spacecraft from Earth. At 10:13 UT (5:13am EST, 11:13 CET), Huygens entered the atmosphere of Saturn's large mystery moon at an altitude of 180 kilometers. Radio astronomers reported detecting signals from the probe indicating that that Huygens began to deployed a series of parachutes to control its 2 hour descent through Titan's dense atmosphere. Huygens' anticipated landing point is marked by a yellow dot in this near-infrared image from the Cassini spacecraft ... but it is not known if a solid or liquid surface awaited it. The outermost of the nested octagons is about 1,120 kilometers across. The outlines are labeled by altitude and indicate areas of coverage by Huygens' imaging instruments during the descent.

APOD: 2005 January 13 - Infrared Trifid
Explanation: The Trifid Nebula, aka M20, is easy to find with a small telescope, a well known stop in the nebula rich constellation Sagittarius. But where visible light pictures show the nebula divided into three parts by dark, obscuring dust lanes, this penetrating infrared image reveals filaments of luminous gas and newborn stars. The spectacular false-color view is courtesy of the Spitzer Space Telescope. Astronomers have used the Spitzer infrared image data to count newborn and embryonic stars which otherwise can lie hidden in the natal dust and glowing clouds of this intriguing stellar nursery. As seen here, the Trifid is about 30 light-years across and lies only 5,500 light-years away.

APOD: 2004 December 21 - Titan Disguised
Explanation: Will the Huygens probe land or splash down? In the next few days, the Cassini spacecraft currently orbiting Saturn will release a probe that will descend toward Saturn's largest moon in mid-January. That moon, Titan, has a surface normally hidden from view by thick methane cloud decks. What the car-sized flying-saucer-shaped probe will find is unknown. Once reaching the surface, Huygens may survive for as long as 150 minutes and take as many as 1,100 images. These images will be beamed up to the passing Cassini mothership for subsequent transmission to a waiting Earth. The Huygens probe is depicted above entering Titan's atmosphere and deploying its parachute. Uncovering the most mysterious moon in the Solar System may reveal a surface so strange that images of it may not be immediately understood.

APOD: 2004 December 20 - Titan Surmised
Explanation: What does the surface of Titan look like? Thick clouds have always made Saturn's largest moon so mysterious that seemingly farfetched hypotheses like methane rain and lakes have been seriously considered. Later this week, the Cassini spacecraft orbiting Saturn is scheduled to release its probe named Huygens that will actually attempt to land on the shrouded moon in early January. Sketched above is one educated guess of what Huygens might find. In the above depiction, orange hydrocarbons color a landscape covered with lakes and peaks of frozen methane and ammonia. For illustration purposes, the Huygens probe is drawn parachuting down with an oversized Cassini spacecraft orbiting above. Saturn, likely occluded by the clouds, is depicted looming in the distance. What will Huygens really find? Are the building blocks of life frozen onto the surface of Titan? Will the truth be stranger than we imagined?

APOD: 2004 November 24 - A Radar View of Titan
Explanation: Where are Titan's craters? Throughout our Solar System's five billion-year history, dangerous rocks and chunks of ice have continually slammed into planets and moons - usually creating numerous long lasting impact craters. When the robot spacecraft Cassini swooped past Saturn's moon Titan last month, however, radar images showed few craters. One such image, spanning 75 kilometers across, is shown above. The imaged structures are not yet understood, but may involve some sort of flows. Titan is already known to be an unusual moon, sporting a thick atmosphere, large size, small amounts of organic compounds. Craters are surely created on all surfaces in the Solar System, but might be destroyed later, as on Earth and Jupiter's moon Io. How craters are destroyed on Titan remains a topic of speculation, but might become better understood by consideration of data returned by future flybys of Cassini and by the probe Huygens that will descend toward Titan's surface in December.

APOD: 2004 October 28 - Tantalizing Titan
Explanation: Normally hidden by a thick, hazy atmosphere, tantalizing features on Titan's surface appear in this false-color view. The image was recorded as the Cassini spacecraft approached its first close flyby of Saturn's smog-shrouded moon on October 26. Here, red and green colors represent specific infrared wavelengths absorbed by Titan's atmospheric methane while bright and dark surface areas are revealed in a more penetrating infrared band. Ultraviolet data showing the extensive upper atmosphere and haze layers are seen as blue. Sprawling across the 5,000 kilometer wide moon, the bright continent-sized feature known as Xanadu is near picture center, bordered at the left by contrasting dark terrain. Saturn orbiter Cassini and Titan lander Huygens plan further explorations, but for now the origin and nature of Titan's surface features remain unknown.

APOD: 2004 October 26 - Titan Through the Haze
Explanation: What are these surface features on Titan? This planet-sized moon of Saturn had much of its south polar surface imaged during an initial flyby by the Saturn-orbiting Cassini spacecraft back in early July. The above image mosaic was digitally stitched together from pictures taken at a very specific color of polarized infrared light, a color not absorbed and little scattered by Titan's methane haze. Visible are light and dark regions that are not yet understood. Surface features as small as 10 kilometers are resolved from about 340,000 kilometers away. The white region near Titan's South Pole, left of center, is unusually thick clouds also thought to be composed of methane. Today Cassini will swoop to within 1,500 kilometers above Titan and may return data and images that help humanity better understand this strange world.

APOD: 2004 August 10 - The Double Haze above Titan
Explanation: Most moons have no haze layer at all - why does Titan have two? Images from the Cassini spacecraft that slipped into orbit around Saturn last month confirm that the Solar System's most mysterious moon is surrounded not only by a thick atmosphere but also by two distinct spheres of haze. These layers are visible as purple in the above false-color ultraviolet image. Titan's opaque atmosphere is similar to Earth's atmosphere in that it is composed mostly of nitrogen. As energetic sunlight strikes high level atmospheric nitrogen and methane, trace amounts of organic compounds such as ethane and carbon dioxide appear to form. These and other complex organic molecules likely populate the detached haze layer. In December 2004, Cassini will launch the Huygens probe to land on Titan.

APOD: 2004 July 21 - A Shadow on the Rings of Saturn
Explanation: This picture of Saturn could not have been taken from Earth. No Earth based picture could possibly view the night side of Saturn and the corresponding shadow cast across Saturn's rings. Since Earth is much closer to the Sun than Saturn, only the day side of the planet is visible from the Earth. Rather, this picture was taken by the robot Cassini spacecraft that began orbiting Saturn earlier this month. The dark western limb of Saturn looms large on the image right, while complex concentrations of small ring particles reflect sunlight on the image left. Saturn's enigmatic F ring is visible around the outside, showing mysterious knots. The small moon Epimetheus, only about 100 kilometers across, can also been seen on the far left. Cassini is scheduled to drop a probe toward the largest moon Titan in December.

APOD: 2004 July 6 - Titan from Cassini in Infrared
Explanation: Could the building blocks of life exist under the smog of Titan? What is creating all of the methane? To help answer these questions, the largest and most mysterious moon of Saturn got a quick first look from the Cassini robot spacecraft soon after entering orbit around the giant planet last week. Although thick atmospheric smog prevented detailed surface images in visible light, infrared light was able to provide interesting clues to the nature of Titan's surface. The above images show Titan in three different colors of infrared light, with the most energetic on the left. The leftmost image is the most detailed but shows surface features that are not yet well understood. The smoothness of the middle image is consistent with a large frozen ocean of water ice containing simple hydrocarbons. The darker regions on the rightmost image might indicate areas relatively rich in hydrocarbons. The white spot visible near the South Pole is hypothesized to be a persistent cloud of large particles containing methane. A better understanding of the mysterious surface of Titan will hopefully be forthcoming as scientists study these images and those from a planned 45 flybys over the next four years. In January, Cassini is scheduled to drop the Huygens probe onto Titan's surface.

APOD: 2004 July 3 - Cassini to Venus
Explanation: Saturn Orbiter Cassini with Titan Probe Huygens attached rocketed into early morning skies on October 15, 1997. The mighty Titan 4B Centaur rocket is seen here across the water, arcing away from Launch Complex 40 at Cape Canaveral Air Station. Cassini, a sophisticated robot spacecraft was actually headed toward inner planet Venus, the first way point in its 7 year, 2.2 billion mile interplanetary journey to Saturn. In fact, Cassini swung by Venus during April 1998 and June 1999, Earth in August 1999, and Jupiter in December 2000. During each of these "gravity assist" encounters the six ton spacecraft picked up speed, reaching Saturn only three days ago. Cassini is now orbiting the ringed gas giant, with the Huygens Probe scheduled to separate from the spacecraft in December. The probe's descent to the surface of Saturn's large moon Titan will be the most distant landing ever attempted.

APOD: 2004 May 31 - 24 Million Kilometers to Saturn
Explanation: Next stop: Saturn. The Cassini-Huygens spacecraft is approaching Saturn and will fire its engines to break into orbit around the ringed giant on July 1. The robot spacecraft was launched in 1997 and rounded Jupiter in 2001. As Cassini orbits Saturn over the next four years, it will swoop past many of Saturn's moons for unprecedented close-ups and even drop a probe onto Titan. Pictured above, Cassini imaged Saturn two weeks ago as it closed to only 24 million kilometers out. Visible are complex cloud patterns, thousands of rings, a shadow angle not visible from Earth, and a moon (if you can find it).

APOD: 2004 April 29 - Titan's X-Ray
Explanation: This June's rare and much heralded transit of Venus will feature our currently brilliant evening star in silhouette, as the inner planet glides across the face of the Sun. But on January 5, 2003 an even rarer transit took place. Titan, large moon of ringed gas giant Saturn, crossed in front of the Crab Nebula, a supernova remnant some 7,000 light-years away. During Titan's transit, the orbiting Chandra Observatory's x-ray detectors recorded the shadowing of cosmic x-rays generated by the Crab's amazing pulsar nebula, pictured above, in a situation analogous to a medical x-ray. The resulting image (inset at left) probes the extent of Titan's atmosphere. So, how rare was Titan's transit of the Crab? While Saturn itself passes within a few degrees of the Crab Nebula every 30 years, the next similar transit is reportedly due in 2267. And since the stellar explosion which gave birth to the Crab was seen in 1054, the 2003 Titan transit may have been the first to occur ... ever.

APOD: 2004 March 1 - Cassini Closes in on Saturn
Explanation: Are they gone? They were not originally predicted to even be there. The mystery revolves around strange shadow-like spokes that appeared on Saturn's large B-ring, the large middle ring in the complex system of particles that orbits Saturn. The spokes were discovered 23 years ago by the passing Voyager spacecraft and attributed to very fine dust of unknown origin. The missing spokes were noted in the above image, taken last month, from the robot Cassini spacecraft now approaching Saturn. Launched in 1997, the distance remaining between Cassini and Saturn is now less than half that between the Earth and the Sun. Cassini is expected to enter orbit around the ringed Jovian giant planet in July and drop a probe onto Titan, Saturn's largest moon.

APOD: 2003 December 10 - Cassini Approaches Saturn
Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, is close enough now to resolve many rings and moons of its destination planet: Saturn. The spacecraft has now closed to within a single Earth-Sun separation from the ringed giant. Early last month, Cassini snapped the contrast-enhanced color composite pictured above. Many features of Saturn's rings and cloud-tops now show considerable detail. When arriving at Saturn in July 2004, the Cassini orbiter will begin to circle and study the Saturnian system. Several months later, a probe named Huygens will separate and attempt to land on the surface of Titan.

APOD: 2003 September 18 - Saturn by Three
Explanation: These three views of Saturn were recorded by the Hubble Space Telescope on March 7th of this year, as the southern hemisphere of the solar system's most gorgeous planet reached its maximum 27 degree tilt toward Earth. The images used to construct the false-color pictures were made through a combination of filters covering the electromagnetic spectrum from ultraviolet (top), to visible (middle) and infrared (bottom) wavelengths highlighting different features in the Saturnian atmospheric bands and rings. Well known for its bright ring system and large, mysterious moon Titan, gas giant Saturn is also a planet with a dynamic atmosphere and high-speed winds. In fact, in the 1980s, Voyager spacecraft measured equatorial winds of over 1,000 miles per hour. Giant storm systems, comparable in size to planet Earth itself, have been seen erupting in Saturn's cloud tops.

APOD: 2003 June 8 - Rhea: Saturn's Second Largest Moon
Explanation: Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980. NASA's Cassini spacecraft is currently on route to Saturn and will arrive in 2004.

APOD: 2003 March 9 - Farewell Jupiter
Explanation: Next stop: Saturn. The Cassini spacecraft, launched from Earth in 1997, has now swung past Jupiter and should arrive at Saturn in the year 2004. Pictured to the left is a parting shot from Cassini in 2001 January that would not have been possible from Earth: Jupiter showing a crescent phase. From the Earth and all points sunward of Jupiter, the gas giant will always appear more fully lit than a crescent. Recent analysis of Jupiter images taken from Cassini bolsters indications that clouds well up from below in the dark colored belts, not the light colored zones, as believed previously. After arriving at Saturn, Cassini will decelerate to orbit the ringed world and send a probe to its enigmatic moon Titan.

APOD: 2003 February 16 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun causes it to be visible only for a short time just after sunset or just before sunrise.

APOD: 2002 November 4 - Cassini Approaches Saturn
Explanation: Cassini, a robot spacecraft launched in 1997 by NASA, is close enough now to resolve many rings and moons of its destination planet: Saturn. The spacecraft has closed to about two Earth-Sun separations from the ringed giant. Last month, Cassini snapped several images during an engineering test. These images have been combined into the contrast-enhanced color composite pictured above. Saturn's rings and cloud-tops are visible on the far right, while Titan, its largest moon, is visible as the speck on the lower left. When arriving at Saturn in July 2004, the Cassini orbiter will begin to circle and study the Saturnian system. Several months later, a probe named Huygens will separate and attempt to land on the surface of Titan.

APOD: 2002 September 5 - Voyager Views Titan's Haze
Explanation: Launched in 1977, 25 years ago today, the Voyager 1 spacecraft's historic tour of the outer Solar System took it past Saturn in late 1980. On November 12, 1980, Voyager 1 recorded this view looking across the edge of Titan, Saturn's largest moon, from a distance of about 22,000 kilometers. Seen in false color, the moon's hazy atmosphere appears orange with further layers of blue haze suspended above. Titan's mostly nitrogen atmosphere, denser than planet Earth's, also contains methane and is thought to be laced with more complex hydrogen and carbon compounds. The composition is likened to Earth's atmosphere before life began. Spotted by Voyager, the detached layers of haze hundreds of kilometers above the surface, along with details of Titan's atmospheric chemistry, have intrigued earth-bound researchers who have recently proposed a model that links seasonal variations in the haze, winds, and sunlight. Their model accounts for the early Voyager observations as well as subsequent studies. Further tests of the model are anticipated when the Huygens probe, carried by the saturn-bound Cassini spacecraft, enters Titan's atmosphere in 2005.

APOD: 2001 October 13 - A Portrait of Saturn from Titan
Explanation: This artistic portrait of Saturn depicts how it might look from Titan, Saturn's largest moon. In the foreground sits ESA's Huygens probe, which will be released by NASA's Cassini spacecraft and parachute to Titan's surface. Cassini will reach Saturn in 2004 and release the Huygens probe later that year. Titan is one of only two moons in the Solar System to have an atmosphere. It has been suggested Titan might have gasoline-like lakes and an atmospheric chemistry like that found on early Earth. The Cassini spacecraft was launched in October 1997 and has now traveled beyond Jupiter.

APOD: 2001 August 8 - Farewell Jupiter
Explanation: Next stop: Saturn. The Cassini spacecraft, launched from Earth four years ago, has now swung past Jupiter and should arrive at Saturn in the year 2004. Pictured to the left is a parting shot from Cassini in January that would not have been possible from Earth: Jupiter showing a crescent phase. From the Earth and all points sunward of Jupiter, the gas giant will always appear more fully lit than a crescent. After arriving at Saturn, Cassini will decelerate to orbit the ringed world and send a probe to its enigmatic moon Titan.

APOD: 2000 August 20 - The Surface of Titan
Explanation: If sailing the hydrocarbon seas of Titan, beware of gasoline rain. Such might be a travel advisory issued one future day for adventurers visiting Titan, the largest moon of Saturn. New images of Titan's surface were released last week from the Canada-France Hawaii Telescope featuring the finest details yet resolved. Peering into Titan's thick smog atmosphere with infrared light, complex features interpreted as oceans, glaciers, and rock became visible. The high-resolution infrared image pictured above was made possible using an unblurring technique called adaptive optics. The interplanetary probe Cassini will reach Saturn and Titan in 2004 to better explore this unusual world.

APOD: 2000 February 13 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercury rotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.

APOD: December 5, 1999 - Rhea: Saturns Second Largest Moon
Explanation: Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980.

APOD: August 4, 1999 - The Surface of Titan
Explanation: If sailing the hydrocarbon seas of Titan, beware of gasoline rain. Such might be a travel advisory issued next millennium for adventurers visiting Titan, the largest moon of Saturn. Images of Titan's surface were released last week from the Keck 10-meter telescope featuring the finest details yet resolved. Peering into Titan's thick smog atmosphere with infrared light, complex features interpreted as oceans, glaciers, and rock became visible. The high-resolution infrared image pictured above was made possible using an unblurring technique called speckle interferometry. The interplanetary probe Cassini will reach Saturn and Titan in 2004 to better explore this unusual world.

APOD: February 7, 1999 - Titan: Saturns Smog Moon
Explanation: The largest moon of Saturn is a rare wonder. Titan is the only one of Saturn's moons with an atmosphere, and one of only two moons in the Solar System with this distinction (Neptune's Triton is the other). Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but contains much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere cause some to speculate that Titan may harbor life! Because of its thick cloud cover, however, Titan's actual surface properties remain mysterious. Voyager 1 flew by in 1980 taking the above picture, and more recently much has been learned from observations by the Hubble Space Telescope. The Cassini mission launched in 1997 will map Titan's surface in 2004, helping to solve some of its mysteries.

APOD: October 18, 1998 - Saturns Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moons of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: May 12, 1998 - Callisto Enhanced
Explanation: Callisto is half rock and half ice. This moon of Jupiter is approximately the size of the planet Mercury, making it the third largest moon in the Solar System, after Ganymede and Titan. Callisto's icy surface is billions of years old, lacks any sign of volcanic activity, and is densely covered with rifts and craters. These features are particularly apparent in this contrast-enhanced image taken by the Galileo spacecraft, and released last week. Visible near the image center is Valhalla, one of the largest impact craters in the Solar System, measuring about 4,000 kilometers across. The rings and size of Valhalla make its appearance similar to the Caloris Impact Basin on Mercury.

APOD: October 16, 1997 - Cassini To Venus
Explanation: NASA's Saturn Explorer Cassini with ESA's Titan Probe Huygens attached successfully rocketed into the skies early yesterday morning. The mighty Titan 4B Centaur rocket is seen here across the water gracefully arcing away from Launch Complex 40 at Cape Canaveral Air Station. Cassini, a sophisticated, bus-sized robot spacecraft is now on its way ... to Venus, the first planetary way point in its 7 year, 2.2 billion mile journey to Saturn. The mission profile calls for Cassini to swing by Venus during April 1998 and June 1999, Earth in August 1999, and Jupiter in December 2000. During each of these "gravity assist" encounters the six ton spacecraft will pick up energy needed to reach Saturn in July 2004. Cassini's mission is the most ambitious voyage of interplanetary exploration ever mounted by humanity and the Huygens Probe's planned descent to the surface of Titan will be the most distant landing ever attempted.

APOD: August 29, 1997 - Cassini To Saturn
Explanation: Scheduled for launch in October, the Cassini spacecraft will spend seven years traveling through the Solar System -- its destination, Saturn. On arrival Cassini will begin an ambitious mission of exploration which will include parachuting a probe to the surface of Titan, Saturn's largest moon. This artist's vision offers a dramatic view of Cassini's engine firing during the SOI (Saturn Orbit Insertion) maneuver as it passes above the ring plane. Before the development of the telescope, the gas giant Saturn was the most distant planet known to astronomers. Ten times farther from the Sun it receives only 1 percent of the sunlight that Earth does. Operating in this faint sunlight, the Cassini spacecraft can't use solar arrays so, like other missions to the outer Solar System, it will be powered by radioisotope thermoelectric generators (RTGs).

APOD: May 24, 1997 - Saturn's Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: December 26, 1996 - Carl Sagan 1934-1996
Explanation: Carl Sagan died last Friday at the age of 62. Sagan was the world's most famous astronomer. Among his many activities as a scientist, he contributed to the discovery that the atmosphere of Venus is prohibitively hot and dense, and found evidence that Saturn's moon Titan contains oceans stocked with the building blocks of life. Sagan was an outspoken proponent of the search for extra-terrestrial life, including sending probes to other planets and listening with large radio telescopes for signals from intelligent aliens. Sagan's outstanding ability to explain allowed almost a billion people to better understand the cosmos in which they live.

APOD: September 13, 1996 - Southwest Mercury
Explanation: The planet Mercury resembles a moon. Mercury's old surface is heavily cratered like many moons. Mercury is larger than most moons but smaller than Jupiter's moon Ganymede and Saturn's moon Titan. Mercury is much denser and more massive than any moon, though, because it is made mostly of iron. In fact, the Earth is the only planet more dense. A visitor to Mercury's surface would see some strange sights. Because Mercuryrotates exactly three times every two orbits around the Sun, and because Mercury's orbit is so elliptical, a visitor to Mercury might see the Sun rise, stop in the sky, go back toward the rising horizon, stop again, and then set quickly over the other horizon. From Earth, Mercury's proximity to the Sun cause it to be visible only for a short time just after sunset or just before sunrise.

APOD: August 14, 1996 - Galileo Explores Europa
Explanation: Details of the crazed cracks criss-crossing Europa's frozen surface are apparent in this mosaic of the Galileo spacecraft's latest images of Jupiter's ice-covered moon. Curious white stripes, also seen by Voyager, are clearly visible marking the center of the wide dark fractures. One theory suggests that "dirty geysers" erupting along the cracks deposited darker material followed by a flow of cleaner water ice which produced the stripe. The above image also shows an impact crater about 18.5 miles in diameter surrounded by white ejecta (lower left) and a curving x-pattern at bottom left which suggests fractures between icy plates filled with slush frozen in place. Is there now or was there ever liquid water beneath Europa's surface? These latest results still hold out that possibility -- and so the possibility of life. Europa, along with Mars and Saturn's moon Titan is considered to be one of the few places in our Solar System, beyond Earth, where primitive life forms could have developed. Galileo's close flyby of this tantalizing moon is scheduled for December of this year.

APOD: July 16, 1996 - A Portrait of Saturn from Titan
Explanation: The above artistic portrait of Saturn depicts how it might look from Titan, Saturn's largest moon. In the foreground sits ESA's Huygens probe, which will be released by NASA's Cassini spacecraft. The Cassini mission to Saturn in currently planned for launch in late 1997. Cassini will reach Saturn in 2004 and will release the Huygens probe later that year. Titan is one of only two moons in the Solar System to have an atmosphere, It has been suggested Titan might have gasoline-like lakes, and may even harbor life.

APOD: April 29, 1996 - Saturn's Rings Seen Sideways
Explanation: Saturn's rings are actually very thin. This picture from the Hubble Space Telescope was taken on August 6, 1995 when the rings lined up sideways as seen from Earth. Saturn's largest moon Titan is seen on the left, and Titan's shadow can be seen on Saturn's cloud tops! Titan itself looks a brownish color because of its thick atmosphere. Four other moon's of Saturn can be seen just above the ring plane, which are, from left to right: Mimas, Tethys, Janus, and Enceladus. If you look carefully, you will note that the dark band across the planet is actually the shadow of the rings, and is slightly displaced from the real rings - which are best seen away from the planet. Saturn's rings are not solid - they are composed of ice chunks which range in size from a grain of sand to a house.

APOD: October 21, 1995 - A Glimpse of Titan's Surface
Explanation: The surface of Titan, Saturn's largest moon, is normally hidden from view by its thick, hazy atmosphere. However, for the first time astronomers have been able to see surface features in images like the one above, made at near-infrared wavelengths with the Hubble Space Telescope. At these wavelengths (longer than visible light) Titan's smog like atmosphere begins to be transparent enough to allow glimpses of it's surface. The bright feature seen above is about 2,500 miles across, similar in size to Australia. Astronomers are still trying to work out what the bright and dark areas represent - oceans, continents, craters, or other features. The images represent important information for planning the Cassini mission, scheduled for launch in 1997. The Cassini spacecraft will explore the Saturn system and parachute a probe to Titan's surface.

APOD: October 15, 1995 - Iapetus: Saturn's Disappearing Moon
Explanation: Iapetus has an unusual surface, one half of which is very dark, the other half very light. This caused it's discoverer Cassini to remark that Iapetus could only be seen when on one side of Saturn but not the other. The reason for the difference between hemispheres is presently unknown. Iapetus is the third largest of Saturn's moon behind Titan and Rhea. Iapetus is composed predominantly of water ice.

APOD: October 14, 1995 - Rhea: Saturn's Second Largest Moon
Explanation: Rhea is the second largest moon of Saturn, behind Titan, and the largest without an atmosphere. It is composed mostly of water ice, but has a small rocky core. Rhea's rotation and orbit are locked together (just like Earth's Moon) so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than its trailing surface. The above photograph was taken with the Voyager 1 spacecraft in 1980.

APOD: September 23, 1995 - Titan: Saturn's Smog Moon
Explanation: The largest moon of Saturn is a rare wonder. Titan is the only one of Saturn's moons with an atmosphere, and one of only two moons in the Solar System with this distinction (Neptune's Triton is the other). Titan's thick cloudy atmosphere is mostly nitrogen, like Earth's, but contains much higher percentages of "smog-like" chemicals such as methane and ethane. The smog may be so thick that it actually rains "gasoline-like" liquids. The organic nature of some of the chemicals found in Titan's atmosphere cause some to speculate that Titan may harbor life! Because of its thick cloud cover, however, Titan's actual surface properties remain mysterious. Voyager 1 flew by in 1980 taking the above picture, and recently much has been learned from Hubble Space Telescope observations. The Cassini mission currently scheduled for launch in 1997 will map Titan's surface, helping to solve some of its mysteries.