DESCRIPTION AND
DATA SHEET

Isis
Ganymede, Jupiter

This Voyager 2 stereo image of Jupiter's large icy moon Ganymede features the central pit crater Isis, as well as the two main geologic terrain types seen on Ganymede. Isis is 75 kilometers wide and roughly 2 kilometers deep. Central pit craters are seen in abundance only on the large icy satellites Ganymede and Callisto and are sometimes observed on Mars (at Ophir Planum).

Central pit craters differ from complex (or central peak) craters on the Moon, Mars, and Venus, because the conical central peak has been replaced by a small rimmed depression. The origin of central pits is not understood. Central peaks form when the floor of the crater rebounds upward during crater formation. The subsurface geology on Ganymede and Callisto (and parts of Mars) may differ from that of the Moon, preventing or destroying a central peak as it forms.

Ganymede is roughly evenly divided between dark terrain (the triangular-shaped regions) and swaths of bright terrain. Dark terrain is older and more heavily cratered than bright terrain. Bright terrain probably formed in extensional graben, which have been partially filled with bright material. This bright material may be lava flows composed of water. In some areas, bright terrain is depressed roughly 500 meters below the elevation of dark terrain.

Topographic relief on Ganymede, including the largest impact basins, rarely exceeds 2 kilometers. In contrast, the largest impact basins on the Moon are 12 kilometers deep. The low relief on Ganymede is probably related to the ice-rich composition of Ganymede's outer layers. Ice is much weaker than ordinary rock and cannot support high topography.

DATA SHEET (Top)

Location:
     68.0 S, 192.0 W
Quadrangle:
     Jg-15 (Hathor)
Mission:
     Voyager 2
Image Numbers:
     20636.38, 20640.33
Image Resolution
(Full-Sized View):
     467 meters/pixel
Image Width:
     400 kilometers
Vertical Exaggeration:
     6.3 × Normal
Vertical Resolution:
     109 meters
Spacecraft Altitude:
     59,700 kilometers
Convergence Angle:
     64°