PHOBOS: NATURE OF CRATER POPULATIONS AND POSSIBLE EVIDENCE FOR INTERIOR VOLATILES

Crater populations on Phobos appear to be at or near the saturation equilibrium level. This level is defined from other cratered surfaces in the solar system, and also by numerical modeling of crater populations (Hartmann and Gaskell 1997). In the region of Mars, craters smaller than ~ 90 meters go into saturation equilibrium within about 1 Gy, so the small-crater populations do not provide tight constraints on Phobos age. Near-saturation levels for the few larger craters suggest a very old surface.