The blue-sky horizon is artificial. Even though the Sun is shining from the south, the north-facing walls of the ridges and craters are brighter than the walls facing the Sun. Palimpsest secondary craters on Ganymede T his region of grooved terrain on Jupiter's moon Ganymede has been partially covered by ejecta from an ancient impact structure known as a palimpsest. Palimpsests are circular, bright, low relief features formed long ago by impacts into Ganymede's surface when this surface behaved differently than it does today. A lower resolution Voyager image of the same area is in the background.
North is to the top.
The edge of the palimpsest crosses this picture from lower left to upper right. Secondary craters 5 to 7 km across produced during the palimpsest-forming impact are visible outside the palimpsest, in the lower right half of the picture. Similarly, the southwest-northeast trending grooves seen clearly outside the palimpsest can be seen with difficulty for a short distance within the palimpsest, where they have been covered by ejecta. This shows that the edge of the palimpsest is the edge of an ejecta blanket. Lower resolution images from the NASA's Voyager mission in 1979 revealed the palimpsests on Ganymede, but did not provide enough detail to allow them to be understood completely.