Astronomy Picture of the Day Search Results for "Europa"

APOD: 2024 March 29 - Galileo's Europa
Explanation: Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been remastered here, with improved calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. The Juno spacecraft currently in Jovian orbit has also made repeated flybys of the water world, returning images along with data exploring Europa's habitability. This October will see the launch of the NASA's Europa Clipper on a voyage of exploration. The spacecraft will make nearly 50 flybys, approaching to within 25 kilometers of Europa's icy surface.

APOD: 2024 January 20 - Falcon Heavy Boostback Burn
Explanation: The December 28 night launch of a Falcon Heavy rocket from Kennedy Space Center in Florida marked the fifth launch for the rocket's reusable side boosters. About 2 minutes 20 seconds into the flight, the two side boosters separated from the rocket's core stage. Starting just after booster separation, this three minute long exposure captures the pair's remarkable boostback burns, maneuvers executed prior to their return to landing zones on planet Earth. While no attempt was made to recover the Falcon Heavy's core stage, both side boosters landed successfully and can be flown again. The four previous flights for these side boosters included last October's launch of NASA's asteroid-bound Psyche mission. Their next planned flight is on the Europa Clipper mission scheduled for launch in October 2024.

APOD: 2023 August 8 – Moon Meets Jupiter
Explanation: What's that below the Moon? Jupiter -- and its largest moons. Many skygazers across planet Earth enjoyed the close conjunction of Earth's Moon passing nearly in front of Jupiter in mid-June. The featured image is a single exposure of the event taken from Morón de la Frontera, Spain. The sunlit lunar crescent on the left is overexposed, while the Moon's night side, on the right, is only faintly illuminated by Earthshine. Lined up diagonally below the Moon, left to right, are Jupiter's bright Galilean satellites: Callisto, Ganymede, Io (hard to see as it is very near to Jupiter), and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. NASA's robotic spacecraft Juno is currently orbiting Jupiter and made a close pass near Io only a week ago. If you look up in the night sky tonight, you will again see two of the brightest objects angularly close together -- because tonight is another Moon-Jupiter conjunction.

APOD: 2023 June 13 – Moons Across Jupiter
Explanation: Jupiter's moons circle Jupiter. The featured video depicts Europa and Io, two of Jupiter's largest moons, crossing in front of the grand planet's Great Red Spot, the largest known storm system in our Solar System. The video was composed from images taken by the robotic Cassini spacecraft as it passed Jupiter in 2000, on its way to Saturn. The two moons visible are volcanic Io, in the distance, and icy Europa. In the time-lapse video, Europa appears to overtake Io, which is odd because Io is closer to Jupiter and moves faster. The explanation is that the motion of the fast Cassini spacecraft changes the camera location significantly during imaging. Jupiter is currently being visited by NASA's robotic Juno spacecraft, while ESA's Jupiter Icy Moons Explorer (JUICE), launched in April, is enroute.

APOD: 2023 May 20 - Galileo's Europa
Explanation: Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been remastered here, with improved calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. What kind of life could thrive in a deep, dark, subsurface ocean? Consider planet Earth's own extreme shrimp.

APOD: 2022 October 25 - Jupiter Rotates as Moons Orbit
Explanation: Jupiter and its moons move like our Sun and its planets. Similarly, Jupiter spins while its moons circle around. Jupiter’s rotation can be observed by tracking circulating dark belts and light zones. The Great Red Spot, the largest storm known, rotates to become visible after about 15 seconds in the 48-second time lapse video. The video is a compilation of shorts taken over several nights last month and combined into a digital recreation of how 24-continuous hours would appear. Jupiter's brightest moons always orbit in the plane of the planet's rotation, even as Earth’s spin makes the whole system appear to tilt. The moons Europa, Ganymede, and Io are all visible, with Europa's shadow appearing as the icy Galilean moon crosses Jupiter's disk. Jupiter remains near opposition this month, meaning that it is unusually bright, near to its closest to the Earth, and visible nearly all night long.

APOD: 2022 October 3 - Jupiter's Europa from Spacecraft Juno
Explanation: What mysteries might be solved by peering into this crystal ball? In this case, the ball is actually a moon of Jupiter, the crystals are ice, and the moon is not only dirty but cracked beyond repair. Nevertheless, speculation is rampant that oceans exist under Europa's fractured ice-plains that could support life. Europa, roughly the size of Earth's Moon, is pictured here in an image taken a few days ago when the Jupiter-orbiting robotic spacecraft Juno passed within 325 kilometers of its streaked and shifting surface. Underground oceans are thought likely because Europa undergoes global flexing due to its changing gravitational attraction with Jupiter during its slightly elliptical orbit, and this flexing heats the interior. Studying Juno's close-up images may further humanity's understanding not only of Europa and the early Solar System but also of the possibility that life exists elsewhere in the universe.

APOD: 2022 July 20 - Jupiter and Ring in Infrared from Webb
Explanation: Why does Jupiter have rings? Jupiter's main ring was discovered in 1979 by NASA's passing Voyager 1 spacecraft, but its origin was then a mystery. Data from NASA's Galileo spacecraft that orbited Jupiter from 1995 to 2003, however, confirmed the hypothesis that this ring was created by meteoroid impacts on small nearby moons. As a small meteoroid strikes tiny Metis, for example, it will bore into the moon, vaporize, and explode dirt and dust off into a Jovian orbit. The featured image of Jupiter in infrared light by the James Webb Space Telescope shows not only Jupiter and its clouds, but this ring as well. Also visible is Jupiter's Great Red Spot (GRS) -- in comparatively light color on the right, Jupiter's large moon Europa -- in the center of diffraction spikes on the left, and Europa's shadow -- next to the GRS. Several features in the image are not yet well understood, including the seemingly separated cloud layer on Jupiter's right limb.

APOD: 2022 July 17 - Europa and Jupiter from Voyager 1
Explanation: What are those spots on Jupiter? Largest and furthest, just right of center, is the Great Red Spot -- a huge storm system that has been raging on Jupiter possibly since Giovanni Cassini's likely notation of it 357 years ago. It is not yet known why this Great Spot is red. The spot toward the lower left is one of Jupiter's largest moons: Europa. Images from Voyager in 1979 bolster the modern hypothesis that Europa has an underground ocean and is therefore a good place to look for extraterrestrial life. But what about the dark spot on the upper right? That is a shadow of another of Jupiter's large moons: Io. Voyager 1 discovered Io to be so volcanic that no impact craters could be found. Sixteen frames from Voyager 1's flyby of Jupiter in 1979 were recently reprocessed and merged to create the featured image. Forty-five years ago this September, Voyager 1 launched from Earth and started one of the greatest explorations of the Solar System ever.

APOD: 2022 February 4 - Moons at Twilight
Explanation: Even though Jupiter was the only planet visible in the evening sky on February 2, it shared the twilight above the western horizon with the Solar System's brightest moons. In a single exposure made just after sunset, the Solar System's ruling gas giant is at the upper right in this telephoto field-of-view from Cancun, Mexico. The snapshot also captures our fair planet's own natural satellite in its young crescent phase. The Moon's disk looms large, its familiar face illuminated mostly by earthshine. But the four points of light lined-up with Jupiter are Jupiter's own large Galilean moons. Top to bottom are Ganymede, [Jupiter], Io, Europa, and Callisto. Ganymede, Io, and Callisto are physically larger than Earth's Moon while water world Europa is only slightly smaller.

APOD: 2022 January 15 - Galileo's Europa
Explanation: Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been remastered here, with improved calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. What kind of life could thrive in a deep, dark, subsurface ocean? Consider planet Earth's own extreme shrimp.

APOD: 2021 October 11 - Juno Flyby of Ganymede and Jupiter
Explanation: What would it be like to fly over the largest moon in the Solar System? In June, the robotic Juno spacecraft flew past Jupiter's huge moon Ganymede and took images that have been digitally constructed into a detailed flyby. As the featured video begins, Juno swoops over the two-toned surface of the 2,000-km wide moon, revealing an icy alien landscape filled with grooves and craters. The grooves are likely caused by shifting surface plates, while the craters are caused by violent impacts. Continuing on in its orbit, Juno then performed its 34th close pass over Jupiter's clouds. The digitally-constructed video shows numerous swirling clouds in the north, colorful planet-circling zones and bands across the middle -- featuring several white-oval clouds from the String of Pearls, and finally more swirling clouds in the south. Next September, Juno is scheduled to make a close pass over another of Jupiter's large moons: Europa.

APOD: 2021 August 21 - Triple Transit and Mutual Events
Explanation: These three panels feature the Solar System's ruling gas giant Jupiter on August 15 as seen from Cebu City, Philippines, planet Earth. On that date the well-timed telescopic views detail some remarkable performances, transits and mutual events, by Jupiter's Galilean moons. In the top panel, Io is just disappearing into Jupiter's shadow at the far right, but the three other large Jovian moons appear against the planet's banded disk. Brighter Europa and darker Ganymede are at the far left, also casting their two shadows on the gas giant's cloud tops. Callisto is below and right near the planet's edge, the three moons in a triple transit across the face of Jupiter. Moving to the middle panel, shadows of Europa and Ganymede are still visible near center but Ganymede has occulted or passed in front of Europa. The bottom panel captures a rare view of Jovian moons in eclipse while transiting Jupiter, Ganymede's shadow falling on Europa itself. From planet Earth's perspective, similar mutual events, when Galilean moons occult and eclipse each other, can be seen every six years or so when Jupiter is near its own equinox.

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 September 2 - Jupiter and the Moons
Explanation: How many moons do you see? Many people would say one, referring to the Earth's Moon, prominent on the lower left. But take a closer look at the object on the upper right. That seeming-star is actually the planet Jupiter, and your closer look might reveal that it is not alone – it is surrounded by some of its largest moons. From left to right these Galilean Moons are Io, Ganymende, Europa and Callisto. These moons orbit the Jovian world just like the planets of our Solar System orbit the Sun, in a line when seen from the side. The featured single shot was captured from Cancun, Mexico last week as Luna, in its orbit around the Earth, glided past the distant planet. Even better views of Jupiter are currently being captured by NASA's Juno spacecraft, now in a looping orbit around the Solar System's largest planet. Earth's Moon will continue to pass nearly in front of both Jupiter and Saturn once a month (moon-th) as the two giant planets approach their own great conjunction in December.

APOD: 2020 June 28 - Europa and Jupiter from Voyager 1
Explanation: What are those spots on Jupiter? Largest and furthest, just right of center, is the Great Red Spot -- a huge storm system that has been raging on Jupiter possibly since Giovanni Cassini's likely notation of it 355 years ago. It is not yet known why this Great Spot is red. The spot toward the lower left is one of Jupiter's largest moons: Europa. Images from Voyager in 1979 bolster the modern hypothesis that Europa has an underground ocean and is therefore a good place to look for extraterrestrial life. But what about the dark spot on the upper right? That is a shadow of another of Jupiter's large moons: Io. Voyager 1 discovered Io to be so volcanic that no impact craters could be found. Sixteen frames from Voyager 1's flyby of Jupiter in 1979 were recently reprocessed and merged to create the featured image. About 43 years ago, Voyager 1 launched from Earth and started one of the greatest explorations of the Solar System ever.

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 27 - Fresh Tiger Stripes on Saturns Enceladus
Explanation: How will humanity first learn of extraterrestrial life? One possibility is to find it under the icy surface of Saturn's moon Enceladus. A reason to think that life may exist there are long features -- dubbed tiger stripes -- that are known to be spewing ice from the moon's icy interior into space. These surface cracks create clouds of fine ice particles over the moon's South Pole and create Saturn's mysterious E-ring. Evidence for this has come from the robot Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. The unusual surface tiger stripes are shown in false-color blue. Why Enceladus is active remains a mystery, as the neighboring moon Mimas, approximately the same size, appears quite dead. A recent analysis of ejected ice grains has yielded evidence that complex organic molecules exist inside Enceladus. These large carbon-rich molecules bolster -- but do not prove -- that oceans under Enceladus' surface could contain life. Another Solar System moon that might contain underground life is Europa.

APOD: 2019 November 29 - Galileo's Europa Remastered
Explanation: Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been remastered here, using improved new calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. What kind of life could thrive in a deep, dark, subsurface ocean? Consider planet Earth's own extreme shrimp.

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 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: 2018 May 3 - Opposite the Setting Sun
Explanation: On April 30, a Full Moon rose opposite the setting Sun. Its yellowish moonglow silhouettes a low tree-lined ridge along Lewis Mountain in this northeastern Alabama skyscape. Sharing the telephoto field-of-view opposite the Sun are Earth's grey shadow, the pinkish Belt of Venus, and bright planet Jupiter. Nearing its own 2018 opposition on May 8, Jupiter is flanked by tiny pinpricks of light, three of its large Galilean moons. Europa lies just below Jupiter, and Ganymede and Callisto are just above. Closer and brighter, our own natural satellite appears to loom large but the Moon is physically a little smaller than Ganymede and Callisto, and slightly larger than water world Europa. Sharp eyes will also spot the trails of two jets across the clear evening sky.

APOD: 2017 September 5 - Europa and Jupiter from Voyager 1
Explanation: What are those spots on Jupiter? Largest and furthest, just right of center, is the Great Red Spot -- a huge storm system that has been raging on Jupiter possibly since Giovanni Cassini's likely notation of it 352 years ago. It is not yet known why this Great Spot is red. The spot toward the lower left is one of Jupiter's largest moons: Europa. Images from Voyager in 1979 bolster the modern hypothesis that Europa has an underground ocean and is therefore a good place to look for extraterrestrial life. But what about the dark spot on the upper right? That is a shadow of another of Jupiter's large moons: Io. Voyager 1 discovered Io to be so volcanic that no impact craters could be found. Sixteen frames from Voyager 1's flyby of Jupiter in 1979 were recently reprocessed and merged to create the featured image. Forty years ago today, Voyager 1 launched from Earth and started one of the greatest explorations of the Solar System ever.

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: 2016 September 27 - Jupiter's Europa from Spacecraft Galileo
Explanation: What mysteries might be solved by peering into this crystal ball? In this case, the ball is actually a moon of Jupiter, the crystals are ice, and the moon is not only dirty but cracked beyond repair. Nevertheless, speculation is rampant that oceans exist under Europa's fractured ice-plains that could support life. This speculation was bolstered again this week by released images from the Hubble Space Telescope indicating that plumes of water vapor sometimes emanate from the ice-crusted moon -- plumes that might bring microscopic sea life to the surface. Europa, roughly the size of Earth's Moon, is pictured here in natural color as photographed in 1996 by the now-defunct Jupiter-orbiting Galileo spacecraft. Future observations by Hubble and planned missions such as the James Webb Space Telescope later this decade and a Europa flyby mission in the 2020s may further humanity's understanding not only of Europa and the early Solar System but also of the possibility that life exists elsewhere in the universe.

APOD: 2016 July 10 - Moon Meets Jupiter
Explanation: What's that next to the Moon? Jupiter -- and its four largest moons. Skygazers around planet Earth enjoyed the close encounter of planets and Moon in 2012 July 15's predawn skies. And while many saw bright Jupiter next to the slender, waning crescent, Europeans also had the opportunity to watch the ruling gas giant pass behind the lunar disk, occulted by the Moon as it slid through the night. Clouds threaten in this telescopic view from Montecassiano, Italy, but the frame still captures Jupiter after it emerged from the occultation along with all four of its large Galilean moons. The sunlit crescent is overexposed with the Moon's night side faintly illuminated by Earthshine. Lined up left to right beyond the dark lunar limb are Callisto, Ganymede, Jupiter, Io, and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller. Last week, NASA's Juno became the second spacecraft ever to orbit Jupiter.

APOD: 2016 June 28 - Juno Mission Trailer
Explanation: What will NASA's Juno spacecraft find when it reaches Jupiter next Monday? Very little, if Juno does not survive Jupiter Orbit Insertion, a complex series of operations in an unknown environment just above Jupiter's cloud tops. If successful, as explained in the featured video, Juno will swoop around Jupiter, passing closer than any previous spacecraft. The goal is to decelerate, enter into a highly elliptical orbit, and begin two years of science operations. Juno's science mission objectives include mapping Jupiter's deep structure, determining how much water is in Jupiter's atmosphere, and exploring Jupiter's powerful magnetic field and how it creates auroras around Jupiter's poles. These lessons hold promise to help humanity better understand the history of our Solar System and the dynamics of our Earth. Juno is powered predominantly by three large solar panels, each measuring a side of small truck. Launched in 2011, Juno's planned mission will take it around the Jovian giant 37 times, after which, to avoid contaminating Europa with microbes, it will be directed to dive into Jupiter's thick atmosphere, where it will break apart and melt.

APOD: 2016 May 19 - The Surface of Europa
Explanation: An enhanced-color view, this image covers a 350 by 750 kilometer swath across the surface of Jupiter's tantalizing moon Europa. The close-up combines high-resolution image data with lower resolution color data from observations made in 1998 by the Galileo spacecraft. Smooth ice plains, long fractures, and jumbled blocks of chaos terrain are thought to hide a deep ocean of salty liquid water beneath. Though the ice-covered alien ocean world is outside the Solar System's habitable zone, new studies show the potential chemistry driving its oxygen and hydrogen production, a key indicator of the energy available for life, could produce amounts comparable in scale to planet Earth. Hydrogen would be generated by chemical reactions of the salty water in contact with the rocky ocean floor. Oxygen and other compounds that react with hydrogen would come from Europa's surface. There water ice molecules would be split apart by the intense flux of high-energy radiation from Jupiter and cycled into the Europan ocean from above.

APOD: 2016 April 1 - Europa: Discover Life Under the Ice
Explanation: Looking for an interplanetary vacation destination? Consider a visit to Europa, one of the Solar System's most tantalizing moons. Ice-covered Europa follows an elliptical path in its 85 hour orbit around our ruling gas giant Jupiter. Heat generated from strong tidal flexing by Jupiter's gravity keeps Europa's salty subsurface ocean liquid all year round. That also means even in the absence of sunlight Europa has energy that could support simple life forms. Unfortunately, it is currently not possible to make reservations at restaurants on Europa, where you might enjoy a dish of the local extreme shrimp. But you can always choose another destination from Visions of the Future.

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: 2015 February 6 - Jupiter Triple-Moon Conjunction
Explanation: Our solar system's ruling giant planet Jupiter and 3 of its 4 large Galilean moons are captured in this single Hubble snapshot from January 24. Crossing in front of Jupiter's banded cloud tops Europa, Callisto, and Io are framed from lower left to upper right in a rare triple-moon conjunction. Distinguishable by colors alone icy Europa is almost white, Callisto's ancient cratered surface looks dark brown, and volcanic Io appears yellowish. The transiting moons and moon shadows can be identified by sliding your cursor over the image, or following this link. Remarkably, two small, inner Jovian moons, Amalthea and Thebe, along with their shadows, can also be found in the sharp Hubble view. The Galilean moons have diameters of 3,000 to 5,000 kilometers or so, comparable in size to Earth's moon. But odd-shaped Amalthea and Thebe are only about 260 and 100 kilometers across respectively.

APOD: 2014 November 27 - Galileo's Europa Remastered
Explanation: Looping through the Jovian system in the late 1990s, the Galileo spacecraft recorded stunning views of Europa and uncovered evidence that the moon's icy surface likely hides a deep, global ocean. Galileo's Europa image data has been newly remastered here, using improved new calibrations to produce a color image approximating what the human eye might see. Europa's long curving fractures hint at the subsurface liquid water. The tidal flexing the large moon experiences in its elliptical orbit around Jupiter supplies the energy to keep the ocean liquid. But more tantalizing is the possibility that even in the absence of sunlight that process could also supply the energy to support life, making Europa one of the best places to look for life beyond Earth. What kind of life could thrive in a deep, dark, subsurface ocean? Consider planet Earth's own extreme shrimp.

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 15 - Gibbous Europa
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA has started preliminary development of the Jupiter Icy Moons Explorer (JUICE), a spacecraft proposed for launch around 2022 that would further explore Jupiter and in particular Europa. Recent observations by the Hubble Space Telescope have uncovered new evidence that Europa, like Saturn's moon Enceladus, has ice venting from its surface.

APOD: 2013 November 2 - Jupiter's Triple Shadow Transit
Explanation: This webcam and telescope image of banded gas giant Jupiter shows the transit of three shadows cast by Jupiter's moons in progress, captured in Belgian skies on October 12 at 0528 UT. Such a three shadow transit is a relatively rare event, even for a large planet with many moons. Visible in the frame are the three Galilean moons responsible, Callisto at the far left edge, Io closest to Jupiter's disk, and Europa below and just left of Io. Of their shadows on the sunlit Jovian cloud tops, Callisto casts the most elongated one near the planet's south polar region at the bottom. Io's shadow is above and right of Jupiter's Great Red Spot. Of course viewed from Jupiter's perspective, these shadow crossings could be seen as solar eclipses, analogous to the Moon's shadow crossing the sunlit face of planet Earth.

APOD: 2013 February 25 - Fly Me to the Moons
Explanation: Sometimes the Moon is a busy direction. Last week, for example, our very Moon passed in front of the planet Jupiter. While capturing this unusual spectacle from New South Wales, Australia, a quick-thinking astrophotographer realized that a nearby plane might itself pass in front of the Moon, and so quickly reset his camera to take a continuous series of short duration shots. As hoped, for a brief instant, that airplane, the Moon, and Jupiter were all visible in a single exposure, which is shown above. But the project was not complete -- a longer exposure was then taken to bring up three of the Jupiter's own moons: Io, Callisto, and Europa (from left to right). Unfortunately, this triple spectacle soon disappeared. Less than a second later, the plane flew away from the Moon. A few seconds after that, the Moon moved to cover all of Jupiter. A few minutes after that, Jupiter reappeared on the other side of the Moon, and even a few minutes after that the Moon moved completely away from Jupiter. Although hard to catch, planes cross in front of the Moon quite frequently, but the Moon won't eclipse Jupiter again for another three years.

APOD: 2012 July 20 - Moon Meets Jupiter
Explanation: Skygazers around planet Earth enjoyed the close encounter of planets and Moon in July 15's predawn skies. And while many saw bright Jupiter next to the slender, waning crescent, Europeans also had the opportunity to watch the ruling gas giant pass behind the lunar disk, occulted by the Moon as it slid through the night. Clouds threaten in this telescopic view from Montecassiano, Italy, but the frame still captures Jupiter after it emerged from the occultation along with all four of its large Galilean moons. The sunlit crescent is overexposed with the Moon's night side faintly illuminated by Earthshine. Lined up left to right beyond the dark lunar limb are Callisto, Ganymede, Jupiter, Io, and Europa. In fact, Callisto, Ganymede, and Io are larger than Earth's Moon, while Europa is only slightly smaller.

APOD: 2012 May 24 - All the Water on Europa
Explanation: How much of Jupiter's moon Europa is made of water? A lot, actually. Based on the Galileo probe data acquired during its exploration of the Jovian system from 1995 to 2003, Europa possesses a deep, global ocean of liquid water beneath a layer of surface ice. The subsurface ocean plus ice layer could range from 80 to 170 kilometers in average depth. Adopting an estimate of 100 kilometers depth, if all the water on Europa were gathered into a ball it would have a radius of 877 kilometers. To scale, this intriguing illustration compares that hypothetical ball of all the water on Europa to the size of Europa itself (left) - and similarly to all the water on planet Earth. With a volume 2-3 times the volume of water in Earth's oceans, the global ocean on Europa holds out a tantalizing destination in the search for extraterrestrial life in our solar system.

APOD: 2012 March 2 - Jupiter Unplugged
Explanation: Five hand drawn sketches of Jupiter were used to create this beautifully detailed flat map of the ruling gas giant's turbulent cloud tops. Made with colored pencils at the eyepiece of a 16 inch diameter telescope, the original drawings are about 5 inches (12.5 cm) in diameter. The drawn planisphere map dimensions are 16x8 inches (40x20 cm). Observing on different dates in November and December of 2011, astronomical artist Fred Burgeot has relied on Jupiter's rotation to cover the planet's complete circumference. Digital animator Pascal Chauvet has also translated Burgeot's drawings into an intriguing video (vimeo), synthesizing a telescopic view of the rotating planet with a tilt and phase appropriate for the observing dates. The video includes the Galilean moons moving along their orbits, beginning with Ganymede and Io casting shadows as they glide in front of Jupiter, followed by Europa and Callisto passing behind the planet's banded disk.

APOD: 2011 January 30 - Gibbous Europa
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, NASA and ESA have started preliminary development of the Europa Jupiter System Mission, a spacecraft proposed for launch around 2020 that would further explore Jupiter and in particular Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

APOD: 2010 October 8 - Two Planet Opposition
Explanation: In late September, two planets were opposite the Sun in Earth's sky, Jupiter and Uranus. Consequently closest to Earth, at a distance of only 33 light-minutes and 2.65 light-hours respectively, both were good targets for telescopic observers. Recorded on September 27, this well-planned composite of consecutive multiple exposures captured both gas giants in their remarkable celestial line-up accompanied by their brighter moons. The faint greenish disk of distant planet Uranus is near the upper left corner. Of the tilted planet's 5 larger moons, two can be spotted just above and left of the planet's disk. Both discovered by 18th century British astronomer Sir William Herschel and later named for characters in Shakespeare's A Midsummer Night's Dream, Oberon is farthest left, with Titania closer in. At the right side of the frame is ruling gas giant Jupiter, flanked along a line by all four of its Galilean satellites. Farthest from Jupiter is Callisto, with Europa and Io all left of the planet's disk, while Ganymede stands alone at the right.

APOD: 2009 September 20 - Ganymede Enhanced
Explanation: What does the largest moon in the Solar System look like? Ganymede, larger than even Mercury and Pluto, has a surface speckled with bright young craters overlying a mixture of older, darker, more cratered terrain laced with grooves and ridges. Like Earth's Moon, Ganymede keeps the same face towards its central planet, in this case Jupiter. In this historic and detailed image mosaic taken by the Galileo spacecraft that orbited Jupiter from 1995 to 2003, the colors of this planet-sized moon have been enhanced to increase surface contrasts. The violet shades extending from the top and bottom are likely due to frost particles in Ganymede's polar regions. Possible future missions to Jupiter are being proposed that can search Europa and Ganymede for deep oceans that may harbor elements thought important for supporting life.

APOD: 2009 March 8 - Gibbous Europa
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA and NASA have together started preliminary development of the Europa Jupiter System Mission , a spacecraft proposed to better study Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

APOD: 2007 December 2 - Gibbous Europa
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA has started preliminary development of the Jovian Europa Orbiter, a spacecraft proposed to orbit Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

APOD: 2007 May 7 - Europa Rising
Explanation: When passing Jupiter on your way to Pluto, what should you look for? NASA pondered just this question recently, and the response from one space enthusiast was to capture the above breathtaking moonrise. The unusual vista was then actually captured by the New Horizons spacecraft in February just after it buzzed past Jupiter on its way to Pluto and the outer Solar System. Visible above is the cracked surface of Europa's expansive ice fields, visible just behind a jumble of Jupiter's swirling clouds. Europa is one of the largest moons of Jupiter and a possible host to sub-surface liquid oceans that are real candidates for containing extra-terrestrial life. During the Jupiter flyby, New Horizons also carried out scientific observations of Jupiter's cloud tops and comparative images of Io's volcanoes and its continually changing surface.

APOD: 2007 March 29 - Jupiter Moon Movie
Explanation: South is toward the top in this frame from a stunning movie featuring Jupiter and moons recorded last Thursday from the Central Coast of New South Wales, Australia. In fact, three jovian moons and two red spots are ultimately seen in the full video as they glide around the solar system's ruling gas giant. In the early frame above, Ganymede, the largest moon in the solar system, is off the lower right limb of the planet, while intriguing Europa is visible against Jupiter's cloud tops, also near the lower right. Jupiter's new red spot junior is just above the broad white band in the planet's southern (upper) hemisphere. In later frames, as planet and moons rotate (right to left), red spot junior moves behind Jupiter's left edge while the Great Red Spot itself comes into view from the right. Also finally erupting into view at the right, is Jupiter's volcanic moon, Io. To download the full 2 megabyte movie as an animated gif file, click on the picture.

APOD: 2006 March 10 - Enceladus and the Search for Water
Explanation: Based on data from Cassini spacecraft instruments, researchers are now arguing that liquid water reservoirs exist only tens of meters below the surface of Saturn's small (500 kilometer diameter) but active moon Enceladus. The exciting new results center around towering jets and plumes of material erupting from the moon's surface. The plumes originate in the long tiger stripe fractures of the south polar region pictured here. Detailed models suport conclusions that the plumes arise from near-surface pockets of liquid water at temperatures of 273 kelvins (0 degrees Celsius), even though Enceladus has a surface temperature of about 73 kelvins (-200 degrees Celsius). Clearly an important step in the search for water and the potential for the origin of life beyond planet Earth, such near-surface reservoirs of water would be far more accessible than, for example, the internal ocean detected on the Jovian moon Europa.

APOD: 2004 December 18 - Europa: Ice Line
Explanation: This bright white swath cutting across the surface of icy Jovian moon Europa is known as Agenor Linea. In all about 1000 kilometers long and 5 kilometers wide, only a section is pictured here as part of a combined color and black and white image based on data from the Galileo spacecraft. Most linear features on Europa are dark in color but Agenor Linea is uniquely bright for unknown reasons. Also unknown is the origin of the reddish material along the sides. While these and other details of Europa's surface formations remain mysterious, the general results of Galileo's exploration of Europa have supported the idea that an ocean of liquid water lies beneath the cracked and frozen crust. An extraterrestrial liquid ocean holds out the tantalizing possibility of life.

APOD: 2004 April 4 - The Lost World of Lake Vida
Explanation: A lake hidden beneath 19 meters of ice and gravel has been found near the bottom of the world that might contain an ecosystem completely separate from our own. In a modern version of Sir Arthur Conan Doyle's classic book Lost World, NASA funded scientists are now plotting a mission to drill down into the lake and remove a sample of water from the lake for analysis. Lake Vida, buried under Antarctic ice for over 2,500 years, is liquid only because of a high salt content that results from salt being expelled from water above as it turns to ice. Previously, scientists drilled to within a few meters of the lake and indeed found frozen microbes. Their existence bolsters speculation that similar microorganisms could be found in frozen brine beneath the surface of Mars. If living organisms are found in Lake Vida, they may give an indication that life might even still exist under similar frozen ice-sheets, such as under the larger Lake Vostok, parts of Mars, and even moons of Jupiter such as Europa. Pictured above, a robot meteorological station continues to monitor surface conditions over the ice-sealed lake.

APOD: 2003 December 4 - New Horizons at Jupiter
Explanation: Headed for the first close-up exploration of the Pluto-Charon system and the icy denizens of the Kuiper belt, NASA's New Horizons spacecraft is pictured here in an artist's vision of the robot probe outward bound. The dramatic scene depicts the 465 kilogram spacecraft about one year after a planned 2006 launch, following a flyby of gas giant Jupiter. While the Jupiter flyby will be used as a gravity assist maneuver to save fuel and cut travel time to the outer reaches of the Solar System, it will also provide an opportunity to test instruments and study the giant planet, its moons, and magnetic fields. The Sun is seen from eight hundred million kilometers away, with inner planets Earth, Venus, and Mercury aligned on the left. A dim crescent of outermost Galilean moon Callisto, orbiting Jupiter just inside of the spacecraft's trajectory, appears to the upper right of the fading Sun. Left of Jupiter itself is Europa and in the distant background are the faint, unresolved stars and dust clouds of the Milky Way. New Horizons' planned arrival at Pluto-Charon is in the summer of 2015.

APOD: 2003 September 19 - Galileo's Europa
Explanation: Launched in 1989 and looping through the jovian system since late 1995, the voyage of NASA's Galileo spacecraft will soon come to an end. The spacecraft has been targeted to plunge directly into Jupiter this Sunday, September 21st, at about 30 miles per second. Its components will be vaporized in the gas giant's outer atmosphere. While Galileo's long voyage of exploration has resulted in a spectacular scientific legacy, the spacecraft's ultimate fate is related to perhaps its most tantalizing discovery -- strong evidence for a liquid ocean beneath the frozen surface of Jupiter's moon Europa. Galileo is now almost completely out of fuel for maneuvers, so this intentional collision with Jupiter will prevent any unintentional future collision with Europa and the possibility of contaminating the jovian moon with microbes from Earth hardy enough to survive in interplanetary space. Color image data from the Galileo mission recorded between 1995 and 1998 was used to create this depiction of Europa's cracked and icy surface. The inset shows dark reddish, disrupted regions dubbed Thera and Thrace.

APOD: 2003 September 6 - Jupiter Unpeeled
Explanation: Slice Jupiter from pole to pole, peel back its outer layers of clouds, stretch them onto a flat surface ... and for all your trouble you'd end up with something that looks a lot like this. Scrolling right will reveal the full picture, a color mosaic of Jupiter from the Cassini spacecraft. The mosaic is actually a single frame from a fourteen frame movie constructed from image data recorded by Cassini during its leisurely flyby of the solar system's largest planet in late 2000. The engaging movie approximates Jupiter's cloud motions over 24 jovian rotations. To make it, a series of observations covering Jupiter's complete circumference 60 degrees north and south of the equator were combined in an animated cylindrical projection map of the planet. As in the familiar rectangular-shaped wall maps of the Earth's surface, the relative sizes and shapes of features are correct near the equator but become progressively more distorted approaching the polar regions. In the Cassini movie, which also features guest appearances by moons Io and Europa, the smallest cloud structures visible at the equator are about 600 kilometers across.

APOD: 2003 January 28 - The Lost World of Lake Vida
Explanation: A lake hidden beneath 19 meters of ice and gravel has been found near the bottom of the world that might contain an ecosystem completely separate from our own. In a modern version of Sir Arthur Conan Doyle's classic book Lost World, scientists are now plotting a mission to drill down into the lake and take out a small part to see what's there. Lake Vida, buried under Antarctic ice for over 2,500 years, is liquid only because of its high salt content. Previously, scientists drilled to within a few meters of the lake and indeed found frozen microbes. Their existence bolsters speculation that similar microorganisms could be found in frozen brine beneath the surface of Mars. If living organisms are found in Lake Vida, they may give an indication that life might even still exist under similar frozen ice-sheets, such as under the larger Lake Vostok, parts of Mars, and even moons of Jupiter such as Europa. Pictured above, a robot meteorological station continues to monitor surface conditions over the ice-sealed lake.

APOD: 2002 November 1 - Europa's Freckles
Explanation: Europa, one of Jupiter's large Galilean moons, may well possess an ocean of liquid water hidden beneath its icy surface -- and so holds the tantalizing possibility of life. In this image, constructed with data recorded in 1996 and 1997 by the Galileo spacecraft, Europa's characteristic surface ridges and cracks are seen along with domes and dark reddish spots called lenticulae from the Latin word for freckles. The freckles are about 10 kilometers across and are believed to be blobs of warmer ice from below that have gradually risen through the colder surface layers, analogous to the motions in a lava lamp. If the freckles do represent material from deeper ice layers closer to the hidden ocean, future space missions to investigate Europa's interior could sample the relatively accessible freckles rather than drill through Europa's potentially thick ice shell.

APOD: 2002 June 2 - Cracks and Ridges on Europa
Explanation: Which way to the interstate? What appears to be a caricature of a complex highway system on Earth is actually a system of ridges and cracks on the icy surface of Jupiter's moon Europa. The distance between parallel ridges in the above photograph is typically about 1 kilometer. The complexity of the cracks and ridges tell a story of Europa's past that is mostly undecipherable -- planetary geologists try to understand just the general origin of the overall features. One noteworthy feature is the overall white sheen, possibly indicating the presence of frost. Another is the dark centers between parallel ridges, which might indicate that dirty water from an underground ocean recently welled up in the cracks and froze. Recent research indicates that enough carbon exists to support an underwater biosphere, but that Europa's ice crust may be over three kilometers thick in some places.

APOD: 2002 April 13 - Pwyll: Icy Crater of Europa
Explanation: The impact crater Pwyll (a name from Celtic Mythology) is thought to represent one of the youngest features on the surface of Jupiter's moon Europa. A combination of color and high resolution black and white data from NASA's Galileo spacecraft was used to produce this view looking down on the crater with the sun illuminating the scene from the right. Pwyll's visible dark central region is about 26 kilometers in diameter, while brilliant white rays of debris blasted from the impact site extend outward for hundreds of kilometers. The white debris or ejecta clearly overlays everything else on the surface - indicating that this impact crater is younger than all surrounding features. The bright white color suggests a composition of water ice particles. Galileo's instruments have uncovered substantial evidence that water in liquid form exists below Europa's icy surface. If Europa has a subsurface ocean, could it harbor life?

APOD: 2002 February 25 - Crescent Europa
Explanation: Although the phase of this moon might appear familiar, the moon itself might not. In fact, this crescent shows part of Jupiter's moon Europa. The passing robot spacecraft Voyager 2 captured this image in 1979. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much more smooth, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, currently orbiting Jupiter, indicate that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, NASA has started preliminary development of the Europa Orbiter, a spacecraft that would use radar to help determine the thickness of the surface ice. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.

APOD: 2001 July 31 - Oceans Under Jupiters Callisto
Explanation: Why does Jupiter's moon Callisto alter the magnetic field of Jupiter in its vicinity? Callisto itself does not have a strong magnetic field. One possible answer is that Callisto harbors sub-surface oceans of electrically conducting salt-water. This hypothesis was bolstered recently by a new analysis of how Callisto creates and dissipates heat. Callisto is thought to create heat by the radioactive decay of internal rock -- a process that keeps the Earth's mantle molten. Callisto may not be able to dissipate this heat very efficiently, however, as it has thick layers of ice and rock on its surface. Perhaps this heat is enough to keep sub-surface water from freezing into ice. With this hypothesis, Callisto joins two other of Jupiter's moons, Europa and Ganymede, in candidates for sub-surface oceans. Callisto's oceans, however, might prove too hostile to support Earth-like life.

APOD: 2001 February 15 - Jupiter Unpeeled
Explanation: Slice Jupiter from pole to pole, peel back its outer layers of clouds, stretch them onto a flat surface ... and for all your trouble you'd end up with something that looks a lot like this. Scrolling right will reveal the full picture, a color mosaic of Jupiter from the Cassini spacecraft. The mosaic is actually a single frame from a fourteen frame movie constructed from image data recorded by Cassini during its leisurely flyby of the solar system's largest planet late last year. The engaging movie approximates Jupiter's cloud motions over 24 jovian rotations. To make it, a series of observations covering Jupiter's complete circumference 60 degrees north and south of the equator were combined in an animated cylindrical projection map of the planet. As in the familiar rectangular-shaped wall maps of the Earth's surface, the relative sizes and shapes of features are correct near the equator but become progressively more distorted approaching the polar regions. In the Cassini movie, which also features guest appearances by moons Io and Europa, the smallest cloud structures visible at the equator are about 600 kilometers across. (Note: Downloading a large gif or quicktime version of the movie may take 15 minutes or longer.)

APOD: 2001 January 16 - Europa Rotating
Explanation: Evidence has been mounting that beneath the vast planes of ice that cover Europa lies water -- liquid oceans that might be home to alien life. The smallest of Jupiter's Galilean Moons (which include Io, Ganymede, and Callisto), Europa's deep interior is composed of mostly of silicate rock. Upon close inspection, many surface cracks stop abruptly only to continue on somewhere else -- indicating surface plates that might be sliding. The above time-lapse sequence is a composite of images taken during the Voyager spacecraft flyby of the moon twenty years ago. Not all regions are resolved in high detail. The movie shows Europa during a complete rotation, which corresponds to a complete revolution around Jupiter since Europa always keeps the same face toward the giant planet. The cause of many of the surface colors on Europa also remains a topic of research.

APOD: 2001 January 2 - Jupiter, Europa, and Callisto
Explanation: As the robot Cassini spacecraft rounds Jupiter on its way toward Saturn, it has taken a sequence of images of the gas giant with its four largest moons. Previously released images have highlighted Ganymede and Io. Pictured above are the two remaining Galilean satellites: Europa and Callisto. Europa is the bright moon superposed near Jupiter's Great Red Spot, while Callisto is the dark moon near the frame edge. Callisto is so dark that it would be hard to see here if its brightness was not digitally enhanced. Recent evidence indicates that both moons hold salt-water seas under surface ice that might be home to extra-terrestrial life. By noting the times that moons disappeared and reappeared behind Jupiter in 1676, Ole Roemer was able to make the first accurate estimation of the speed of light.

APOD: 2000 December 19 - A Close Up of Aurora on Jupiter
Explanation: Jupiter has aurorae. Like Earth, the magnetic field of the gas giant funnels charged particles released from the Sun onto the poles. As these particles strike the atmosphere, electrons are temporarily knocked away from existing gas molecules. Electric force attracts these electrons back. As the electrons recombine to remake neutral molecules, auroral light is emitted. In the above recently released photograph by the Hubble Space Telescope taken in ultraviolet light, the aurorae appear as annular sheets around the pole. Unlike Earth's aurorae, Jupiter's aurorae include several bright streaks and dots. These marks are caused by magnetic flux tubes connecting Jupiter to its largest moons. Specifically, Io caused the bright streak on the far left, Ganymede caused the bright dot below center, and Europa caused the dot to its right.

APOD: 2000 December 18 - Oceans Under Jupiter's Ganymede
Explanation: The search for extraterrestrial life came back into our own Solar System last week with the announcement that there may be liquid oceans under the surface of Jupiter's moon Ganymede. Ganymede now joins Callisto and Europa as moons of Jupiter that may harbor seas of liquid water under layers of surface ice. The ocean hypothesis surfaced as an explanation for Ganymede's unusually strong magnetic field. Ganymede, the largest moon in the Solar System, also has the largest measured magnetic field of any moon. Some exobiologists hypothesize that life may be able to emerge in such an ocean, much as it did in the oceans of ancient Earth. Above, a frame from a computer simulation shows what it would look like to fly over the surface of Ganymede, as extrapolated from photographs of the grooved moon taken by the robot spacecraft Galileo currently orbiting Jupiter.

APOD: 2000 November 18 - Jupiter And Family
Explanation: This composite image features classic portraits of members of one of the Solar System's most prominent families - Jupiter and its four large "Galilean" moons. Starting from the top the moons are Io, Europa, Ganymede, and Callisto. The top-to-bottom order is also the order of increasing distance from Jupiter. These are big moons indeed which attend the largest planet. The smallest of the lot, Europa, is the size of Earth's moon while Ganymede is the largest moon in the Solar System. In fact, Ganymede with a diameter of 3,100 miles, is larger than the planets Mercury and Pluto. The swirling Great Red Spot appears at the edge of Jupiter. A hurricane-like storm system that has persisted for over 300 years, two to three earths could fit inside it. Battered Callisto's image was recorded during the 1979 flyby of Voyager. The other portraits were taken by the Galileo spacecraft which began exploring the Jovian system in 1995.

APOD: 2000 October 24 - Io Rotating
Explanation: The surface of Io is continually changing. Jupiter's moon is the home to many powerful volcanoes so active they are effectively turning the moon inside out. The above time-lapse sequence is a composite of images taken during two space missions that approached the violent moon: Voyager and Galileo. The sequence shows Io during a complete rotation, which corresponds to a complete revolution around Jupiter since Io always keeps the same face toward the giant planet. The rampant volcanism is thought to be caused by Jupiter's more distant Galilean Moons (Europa, Ganymede, and Callisto) pulling on Io and continually distorting its shape, causing internal frictional heating. Io is composed mostly of rock, with the yellow color originating from sulfur. The causes of many of the other colors remain a topic of research.

APOD: 2000 October 20 - North Pole Below
Explanation: Orbiting over the north pole of planet Earth on May 5, the MODIS instrument on-board the Terra spacecraft, recorded this view of the ice cap 700 kilometers below. A radial grid centered on the pole is shown on top of the approximately true color image where each pixel covers about one square kilometer. Frozen sea ice appears whitish while open water or newly refrozen ice looks black. An impressive criss-crossing network of cracks in ice shifting above a liquid water ocean is visible, traced by the meandering dark lines. In fact, the dark network of cracks in the sea ice is reminiscent of another world in our solar system which may also harbor a liquid water ocean -- Jupiter's ice moon Europa.

APOD: 2000 October 11 - Cassini Spacecraft Approaches Jupiter
Explanation: A new spacecraft has entered the outer Solar System: Cassini. Launched in 1997 and bound for Saturn in 2004, Cassini sent back the above image last week while approaching the giant planet Jupiter. Cassini joins the Galileo spacecraft currently in orbit around Jupiter in studying the gas giant and its moons. In fact, observations involving both spacecraft simultaneously are planned in the coming months. This color picture was taken when Cassini was 81.3 million kilometers from Jupiter. The alternating dark and bright bands characteristic of Jupiter's cloud tops can be easily seen. Jupiter's moon Europa is also seen at the far right of the image casting a round shadow on the planet.

APOD: 2000 August 25 - Folding Europa
Explanation: Astypalaea Linea on Jovian ice moon Europa is the broad smooth region running through these images recorded by the Galileo spacecraft in 1998. The pictures are different computer processed versions of the same mosaic -- on the left, small scale details have been enhanced while on the right, large scale features are emphasized. In both versions, the bold criss-crossing ridges believed to result from the upwelling of new material through cracks in the surface ice are apparent. But more easily seen on the right are recently recognized gentle rises and dips, about 15 kilometers across, which likely formed as the icy surface was compressed by the addition of the new material. Further evidence that stress is folding Europa's surface is offered by the presence of smaller cracks and wrinkles more easily seen on the left. These span the width of the broad swells suggestive of anticlines and synclines familiar to geologists on planet Earth. Though ice covered, the surface of Europa is thought to be geologically active, riding over a substantial ocean of liquid water.

APOD: 2000 August 10 - Other Worlds and HD 38529
Explanation: After the latest round of discovery announcements, the list of known worlds of distant suns has grown to 50. While extrasolar planet discoveries are sure to continue, none - so far - points clearly to another planetary system like our own. Take, for example, the newly discovered parent star HD38529. Shining in Earth's night sky at 6th magnitude, this sun-like star lies 137 light-years away in the constellation Orion. Like most of the known extrasolar planets, HD38529's planet was discovered by detecting the telltale Doppler wobble in the parent star's spectrum. The data reveal that this planet orbits once every 14.3 days at an average of only 0.13 times the Earth-Sun distance and has a minimum of 0.77 Jupiter masses (about 240 Earth masses). There is even evidence in the wobble data that HD38529, and other stars with one known planet have additional massive planets orbiting them. In this dramatic artist's vision, HD38529 and its newfound world are viewed from the moon of another massive ringed planet orbiting farther out. The ringed planet's moon is imagined to have a thin atmosphere and a surface covered with icy sheets and ridges similar to those found on Jupiter's moon Europa.

APOD: 2000 July 14 - Crater On Ice
Explanation: Impact craters are common on Earth's moon but on Jupiter's large ice moon Europa, they are very rare. Over time, both bodies have been subjected to an intense pounding by the solar system's formative debris, but geological activity on Europa's surface seems to have erased most of these impact scars. This false-color infrared image from the Galileo spacecraft's NIMS instrument shows a newly discovered crater on Europa as a light red ring feature near center surrounding a dark core. For scale, the dark core is about 29 kilometers in diameter. Only seven comparably large craters have now been identified on Europa's surface. Red colors in the image represent a relatively pure water ice composition while blue colors indicate that other minerals are present. The crater's central dark area may contain the remnants of the impacting body. The icy crust of Europa is of great interest, as evidence mounts that it covers an ocean of liquid water, possibly providing suitable conditions for life.

APOD: 2000 May 8 - Jupiters Moons Thebe, Amalthea, and Metis
Explanation: The robot spacecraft Galileo in orbit around Jupiter has recently photographed the inner moons of Jupiter in greater detail than ever before. These pictures of Thebe, Amalthea, and Metis are shown to scale, and reveal details as small as three kilometers across. Amalthea, by contrast, has a total length of about 200 kilometers. The moons are composed mostly of ice, are much smaller than Jupiter's more famous Galilean satellites (Io, Europa, Ganymede, and Callisto), and orbit between Io and Jupiter's rings. Thebe appears dominated by a huge impact crater 40 kilometers across. Astronomers are uncertain of the origin of the unusual white gash at the bottom of Amalthea.

APOD: 2000 April 18 - Europa: Ice Line
Explanation: This bright white swath cutting across the surface of icy Jovian moon Europa is known as Agenor Linea. In all about 1000 kilometers long and 5 kilometers wide, only a section is pictured here as part of a combined color and black and white image based on data from the Galileo spacecraft. Most linear features on Europa are dark in color but Agenor Linea is uniquely bright for unknown reasons. Also unknown is the origin of the reddish material along the sides. While these and other details of Europa's surface formations remain mysterious, the general results of Galileo's exploration of Europa have supported the idea that an ocean of liquid water lies beneath the cracked and frozen crust. An extraterrestrial liquid ocean holds out the tantalizing possibility of life.

APOD: December 15, 1998 - Plains and Ridges on Europa
Explanation: The ridges on Europa may be caused by cold water volcanoes. Europa, one of the largest moons of Jupiter, has been the source of intense scrutiny since speculation increased of there being oceans beneath its icy surface. Currently the Galileo spacecraft orbiting Jupiter is on an extended mission designed, in part, to study Europa's surface in greater detail. The above image highlights features common to Europa's surface: pure blue water ice beneath lighter ridges that run for many kilometers. These ridges may result from volcanic cracks in the ice where emerging liquid water froze upon exposure to the cold of deep space. The reasons for the colors of the ridges remain uncertain.

APOD: November 6, 1998 - Cutaway Callisto: Ice, Rock, and Ocean
Explanation: Cruising past the moons of reigning gas giant Jupiter, Voyager and Galileo have returned tantalizing evidence for a liquid water ocean beneath the surface of Europa. Now researchers are reporting telltale indications that the battered Jovian moon Callisto may also harbor a subsurface ocean. This cutaway view of Callisto shows a whitish 200 kilometer thick band of ice just beneath the moon's surface. The hypothetical ocean - indicated by the underlying light blue stripe - is potentially a salty layer of liquid water up to 10 kilometers thick, while the rest of the interior is seen as a jumble of rock and ice. Why a salty subsurface ocean? Magnetic measurements made during Galileo flybys so far indicate Callisto's magnetic field is variable, analogous to results during Europa passes, and a plausible explanation is that Callisto too has a subsurface liquid layer. If the liquid were salt water it could easily carry electrical currents and produce the changing magnetic field.

APOD: September 10, 1998 - Europa: Ridges and Rafts on a Frozen Moon
Explanation: The large Jovian moon Europa may harbor liquid water beneath its frozen crust. Controversy surrounding this idea has been recently fueled by dramatic images of Europa's surface from the Galileo spacecraft. This alluring color image was produced by combining low resolution color data with higher resolution mosaics recorded during three separate flybys and covers about 120 by 150 miles. The eerie terrain of grooved linear ridges and crustal plates which seem to have broken apart and rafted into new positions could indicate subsurface water or slush. Blue tints represent relatively old ice surfaces while reddish regions may contain material from more recent internal geological activity. White splotches are bright material blasted from the young impact crater Pwyll located about 600 miles south (to the right) of this area. Many believe that large reservoirs of water hold out the tantalizing possibility of organisms living on this dim, distant world.

APOD: June 9, 1998 - Ice Cusps on Europa
Explanation: Europa's icy crust has many unusual features. Pictured above is part of Europa's southern hemisphere photographed by the Galileo spacecraft currently orbiting Jupiter. Europa is one of the largest moons of Jupiter, and is thought to have oceans of water underneath its ice-covered surface. Among many cracks and ridges appear dark cusp-shaped features running from the lower left toward the upper right. The origin of these features is not known for sure, but their shape is thought to indicate that large portions of Europa's crust move together, similar to tectonic motion of our Earth's crust.

APOD: March 10, 1998 - Cracks and Ridges on Europa
Explanation: Which way to the interstate? What appears to be a caricature of a complex highway system on Earth is actually a system of ridges and cracks on the icy surface of Jupiter's moon Europa. The distance between parallel ridges in the above photograph is typically about 1 kilometer. The complexity of the cracks and ridges tell a story of Europa's past that is mostly undecipherable -- planetary geologists try to understand just the general origin of the overall features. One noteworthy feature is the overall white sheen, possibly indicating the presence of frost. Another is the dark centers between parallel ridges, which might indicate that dirty water from an underground ocean recently welled up in the cracks and froze.

APOD: March 3, 1998 - 560 Kilometers Above Europa
Explanation: This is the closest photograph ever taken of Europa. Last December, NASA's spacecraft Galileo swept past the Jovian moon and took photographs which were released yesterday. Speculation that life-bearing oceans exist beneath Europa's surface caused NASA to put Galileo on orbits that approach Europa. The above photograph shows new details on Europa's surface, indicating that much of Europa as strewn with bumps and hills of ice laced with long fractures. Dark circular features may be impact craters.

APOD: January 2, 1998 - Europa's Disconnected Surface
Explanation: Jupiter's moon Europa is so exciting that the Galileo spacecraft orbiting Jupiter has now embarked on an extended mission to study it. Oceans that might exist beneath Europa's surface are thought to be one of the best places to look for life in our Solar System. The Galileo Europa Mission has planned eight close fly-bys frozen moon. The first close encounter of this extended mission occurred last December and the next will occur in February. The above enhanced color picture shows a small region of Europa's Conamara region. The white and blue colors highlight regions covered by ice dust from the collision that created the Pwyll Crater. Disconnected islands of ice are visible that have apparently rafted to new positions.

APOD: September 29, 1997 - Jupiter And Family
Explanation: This composite image features classic portraits of members of one of the Solar System's most prominent families - Jupiter and its four large "Galilean" moons. Starting from the top the moons are Io, Europa, Ganymede, and Callisto. The top-to-bottom order is also the order of increasing distance from Jupiter. These are big moons indeed which attend the largest planet. The smallest of the lot, Europa, is the size of Earth's moon while Ganymede is the largest moon in the Solar System. In fact, Ganymede with a diameter of 3,100 miles, is larger than the planets Mercury and Pluto. The swirling Great Red Spot appears at the edge of Jupiter. A hurricane-like storm system that has persisted for over 300 years, two to three earths could fit inside it. Battered Callisto's image was recorded during the 1979 flyby of Voyager. The other portraits were taken by the Galileo spacecraft which began exploring the Jovian system in 1995.

APOD: August 15, 1997 - Impact on Europa
Explanation: This bull's-eye pattern marks the impact of a mountain-sized comet or asteroid on the icy surface of Jupiter's moon Europa. Recorded by the Galileo spacecraft in April of this year, the composite false color image clearly reveals the telltale concentric fractures which cover about 86 miles - roughly equivalent to the Island of Hawaii. The fat reddish and finer blue-green lines overlay the impact fractures and must therefore be younger surface features formed after the impact. The dark red color may be the result of a a relatively dirty water-ice mixture. The possibility of liquid water below the ice has fueled speculation that life may exist on this large and distant moon.

APOD: June 12, 1997 - Jupiter's Dry Spots
Explanation: Known for its spectacular images of Jupiter's moons, Io, Ganymede, Callisto, and Europa, the robot spacecraft Galileo has also aggressively explored the Jovian atmosphere. In December of 1995, Galileo's atmospheric probe descended into Jupiter's clouds and reported a surprising absence of water. It is now believed that the probe entered through one of Jupiter's dry spots, similar to the dark region in this image of the swirling Jovian cloud deck. The smallest features visible here are tens of miles in size. These dry regions appear to correspond to locations where winds converge creating downdrafts. The downdrafts generate local cloudless clearings through which Jupiter's deeper warmer layers can be glimpsed. Just as the dark areas are extremely dry, the surroundings are full of moisture. The contrast is analogous to the desert and tropics of Earth.

APOD: May 26, 1997 - Old Faithful Meets Hale-Bopp
Explanation: As Comet Hale-Bopp leaves our Northern Skies, it provides us with yet another burst of joy. On May 11th the fading comet was photographed behind the famous "Old Faithful" water geyser of Yellowstone National Park, Wyoming, USA, Planet Earth. Perhaps more familiar to Earth Dwellers than the dark geysers on Neptune's moon Triton, the gas geysers on Jupiter's moon Io, and the dirty water geysers hypothesized on Jupiter's moon Europa, Earth's Old Faithful is also reliable - every 60-80 minutes it gushes a plume of water and steam high into the air. Comet Hale-Bopp will continue to be visible to observers in the Southern Hemisphere as it moves away from the Sun towards the outer Solar System.

APOD: April 17, 1997 - Pwyll: Icy Crater of Europa
Explanation: The impact crater Pwyll (a name from Celtic Mythology) is thought to represent one of the youngest features on the surface of Jupiter's moon Europa. A combination of color and high resolution black and white data from NASA's Galileo spacecraft was used to produce this view looking down on the crater with the sun illuminating the scene from the right. Pwyll's visible dark central region is about 24 miles in diameter, while brilliant white rays of debris blasted from the impact site extend outward for hundreds of miles. The white debris or ejecta clearly overlays everything else on the surface - indicating that this formation is younger than all surrounding features. Interestingly, the bright white color suggests a composition of fresh water ice particles. Does water in liquid form exist below Europa's icy surface?

APOD: April 10, 1997 - Europa's Ice Rafts
Explanation: The surface of Jupiter's moon Europa has shifted. Photographs released yesterday of Europa's surface taken by the spacecraft Galileo show that the smooth moon's icy surface is sometimes like a giant scrambled jigsaw puzzle. Pieces of Europa's surface, identifiable above, have rafted to new locations. Also visible are larger areas that are clearly shifted from their original alignment. What could cause this surface shuffle? One leading explanation is water - oceans of water just beneath Europa's ice plains. This discovery again fuels speculation that life might exist far from the comfort of Earth.

APOD: April 9, 1997 - Oceans Under Jupiter's Europa?
Explanation: Is there life beneath Europa? Today, new results are being announced (at about 2 pm EST) about the possibility of oceans under Jupiter's moon Europa. The existence of such oceans increases the likelihood that some sort of life exists beneath the fractured ice planes of Jupiter's smoothest satellite. Results from the February 20th flyby of the robot spacecraft Galileo past Europa now clearly indicate that either large bodies of water or slush exist under a relatively thin ice sheet covering the moon's surface, at least in some regions. Areas on the surface are marked by very few craters, indicating that water flowed there in the time since most craters formed.

APOD: January 17, 1997 - Europa: The Latest From Galileo
Explanation: Today, NASA revealed recent results from the Galileo Probe's December 19th flyby of Europa, Jupiter's ice-covered moon - including this close-up image of fractured and frozen terrrain. The highest resolution picture ever made of Europa, the snapshot shows a complex array of surface features in a 5.9 x 9.9 mile area near the moon's equatorial region. North is toward the top and the Sun illuminates the region from the right. The image was made from a distance of 2,060 miles. At the upper left are linear criss-crossing ridges and grooves probably caused by movements of the surface ice. Serpentine valleys and lumpy features of unknown origins are also visible. Only a few impact craters are apparent though, implying a geologically young surface. So far, Galileo's findings lend support to the exciting possibility that liquid water once existed and may still exist beneath Europa's surface.

APOD: December 23, 1996 - The Hills of Ganymede
Explanation: This computer generated 3D close-up view of Jupiter's large moon Ganymede was created using image data from NASA's Galileo spacecraft. Simulating stereo vision by combining two recent images recorded from different angles, 3 dimensional information was reconstructed for a section of Ganymede's surface. The result shows the furrows, craters, and hills in the region appropriately known as "Galileo Regio" with a resolution of about 250 feet. Currently exploring the Jovian System, the Galileo spacecraft just completed a flyby of Europa. It is scheduled to return to Ganymede in April next year.

APOD: November 20, 1996 - Europa Full Face
Explanation: What mysteries might be solved by peering into this crystal ball? This crystal ball is quite unusual because it is actually a moon of Jupiter, the crystals are ice-crystals, and the ball is not only dirty and opaque but cracked beyond repair. Nevertheless, speculation is rampant that oceans exist under these tortured ice-plains that could support life. Europa, the smallest of Jupiter's Galilean moons, was photographed last month in natural color by the robot spacecraft Galileo, now in orbit around Jupiter. The brown patches are what one might think: dirt -- tainting an otherwise white ice-crust. Europa, nearly the same size as Earth's Moon, similarly keeps one face toward its home planet. The hemisphere of Europa shown above is the one that always trails. Why is Europa's surface the smoothest in the Solar System? Where are Europa's craters?

APOD: October 25, 1996 - A Flyby View of Ganymede
Explanation: This is what it would look like to fly over the surface of Jupiter's moon Ganymede. NASA's robot spacecraft Galileo recently approached only 6000 miles from this frozen ice-ball of a moon. The above image is a digital reconstruction from two images taken during this flyby. The blue color of the sky is artificial. Deep furrows cover Ganymede's surface, and several impact craters are also present. Galileo continues to orbit Jupiter and send back breathtaking photos of Ganymede, Io, Europa, Callisto, and, of course, Jupiter itself.

APOD: October 22, 1996 - The Cracked Ice Plains of Europa
Explanation: What caused the cracks in this giant ice-ball? Jupiter's moon Europa has smoothest surface in the solar system and is composed mostly of cracked water-ice. In the above false-colored picture released last week by the NASA team in charge of the Galileo mission, blue hues represent ice plains divided by dirty red and brown bands of mottled terrain. As the robot Galileo spacecraft orbits Jupiter, it sends back revealing pictures of Jupiter and its large moons including Europa, Io, Ganymede, and Callisto. The region of Europa highlighted above is known as Minos Linea. The cause for many of the cracks remains unknown but may involve shifting stresses from gravity and temperature variations. The new Galileo pictures have increased evidence that liquid oceans may indeed exist under these giant ice-sheets, a place possibly ripe for the development of life.

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: August 13, 1996 - Europa's Surface
Explanation: Voyager spacecraft images of Europa's surface, like the one above, are suggestive of sea ice on Earth. The criss-crossing dark streaks may indeed be cracks in its ice-covered surface caused by Jupiter's tidal stresses accompanied by the freezing and expansion of an underlying layer of water. This tantalizing prospect of oceans of liquid water beneath its frozen surface has helped make the smallest of the Galilean moon's of Jupiter a planned focus of the Galileo spacecraft's ongoing mission to explore the Jovian system. New Europa images and results from the Galileo mission were released today revealing details which further suggest that Europa's icy surface was once - and may still be - supported on slush or liquid water.

APOD: August 6, 1996 - Europa: Oceans of Life?
Explanation: Is there life beneath Europa's frozen surface? Some believe the oceans found there of carbon-enriched water are the best chance for life, outside the Earth, in our Solar System. Europa, the fourth largest moon of Jupiter, was recently discovered to have a thin oxygen atmosphere by scientists using the Hubble Space Telescope. Although Earth's atmospheric abundance of oxygen is indicative of life, astronomers speculate that Europa's oxygen arises purely from physical processes. But what an interesting coincidence! The above picture was taken by a Voyager spacecraft in 1979, but the spacecraft Galileo is currently circling Jupiter and has been photographing Europa. The first of these pictures will be released two days from today. Will they show the unexpected?

APOD: August 5, 1996 - Erupting Volcanoes on Io
Explanation: Jupiter's moon Io has active volcanoes. The Voyager spacecraft caught several erupting when they passed the energetic moon in 1979. In the above picture, several of Io's volcanoes are visible and one is seen actually erupting. Debris from this explosive event can be seen on the upper left of the photo, just beyond Io's edge. Io's volcanism is thought to be caused by the large tidal distortions raised by Jupiter, Europa, and Ganymede. These tides stretch Io, cause internal friction, and thus heat the interior. The hot interior then expands and forces its way out through volcanoes. Currently, the spacecraft Galileo is orbiting Jupiter and photographing Jupiter's Galilean moons.

APOD: July 12, 1996 - Ancient Cratered Plains on Ganymede
Explanation: The largest moon in the Solar System shows regions that are ancient and battered. The high density of craters demonstrate that patches of Ganymede are indeed billions of years old. This photo is one of a series released by NASA two days ago from the Galileo spacecraft orbiting Jupiter. The large impact crater on the left is 19 kilometers across, while dark indentations in Ganymede's crusty surface-ice run diagonally. Ganymede is composed of half rock and half water-ice. The Galileo spacecraft will continue to orbit Jupiter over the next 16 months and send back data about Jupiter and its four largest moons: Io, Europa, Ganymede, and Callisto.

APOD: July 10, 1996 - Galileo Photographs Ganymede
Explanation: Ganymede's surface is slowly being pulled apart. This photo of Ganymede was released earlier today by the Galileo team at NASA. The Galileo Spacecraft arrived at Jupiter in December 1995. In late June, the spacecraft passed within 10,000 kilometers of Ganymede's icy surface, and took pictures showing complex surface details for the first time. The line-like features in this photo are sunlit ridges rising above Ganymede's ice-plains. The circular features are impact craters. Ganymede is the largest moon of Jupiter and hence the largest of the four Galilean satellites: Io, Europa, Ganymede, and Callisto.

APOD: November 3, 1995 - Jupiter's Moon Amalthea
Explanation: Amalthea is Jupiter's fifth largest moon, much smaller than the four Galilean satellites Io, Europa, Ganymede, and Callisto. The orbit of Amalthea is inside of these moons, and with its long axis always pointing toward Jupiter. It's dark surface color is probably due to sulfur being expelled from Io. Amalthea is not massive enough for its internal gravity to make it spherical. Amalthea was discovered in 1892 by Edward Barnard.

APOD: September 6, 1995 - Callisto: Dark Smashed Iceball
Explanation: Callisto is a dirty battered world, showing the most beaten surface of Jupiter's major moons. Made of a rocky core covered by fractured ice, Callisto's past collisions with large meteors are evident as large craters surrounded by concentric rings. The four largest moons of Jupiter: Io, Europa, Ganymede, and Callisto were all discovered by Galileo and Marius in 1610 with early telescopes and are now known as the Galilean satellites. The NASA spacecraft Galileo is scheduled to arrive at Jupiter is December of 1995.

APOD: September 5, 1995 - Europa: Ancient Water World
Explanation: Beneath the cold icy surface of Jupiter's moon Europa are probably the only oceans of water in our Solar System outside of Earth. These oceans, possibly 50 kilometers deep, might also be the most likely local place to find extra-terrestrial life. Europa's smooth surface is unlike any other known planet or moon, giving evidence for relatively few craters or mountains. Europa was discovered by Galileo and Marius in 1610. The NASA spacecraft Galileo is scheduled to arrive at Jupiter in December of 1995.