Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

July 22, 1998
See Explanation.  Clicking on the picture will download 
 the highest resolution version available.

Dark Craters on Ganymede
Credit: Galileo Project, JPL, NASA

Explanation: Ganymede has craters within craters within craters. The old surface of the largest moon in the Solar System shows its age by the large amount of these impact features. The above picture released last week shows two old craters with dark floors located in a relatively bright region known as Memphis Facula, a region itself thought created by an ancient collision. The strange dark floors of these craters were themselves created long ago and now house craters of their own. Crater Chrysor, on the left, spans about 6000 meters, about half that of crater Aleyn on the right. The robot spacecraft Galileo took the above photograph during a flyby of this moon of Jupiter in June 1996.

Tomorrow's picture: X-Ray Pulsar

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
&: Michigan Tech. U.