Mysterious Features on Ganymede . ... 9.12.1997 . ... Credit Copyright: The Galileo Project , NASA Explanation: Where is the rest of the circle? Jupiter 's largest moon Ganymede has some truly unusual terrain, including the pictured half circle above cut by nearly parallel curves. Full circles can be easily explained by impact craters , but partial circles imply that some resurfacing has occurred since the original impact. The diameter of the half circle is about 32 kilometers. ...