Cratering on Gaspra

Galileo flyby images of 951 Gaspra show a crater population dominated by fresh craters several hundred meters in diameter and smaller. They must represent a production population because their spatial density is low (few overlaps) and because degraded craters are underabundant; equilibrium may be attained at diameters near to or below the resolution limit of the best image. We have counted, measured, and classified craters from the highest resolution, 'high phase' image, which shows >600 craters in 90 [km.sup.2]. The differential population index (0.2-0.6 km) for the fresh, obvious craters is very 'steep' (-4.3 [+ or -] 0.3). It probably reflects the index of asteroidal projectiles; it is much steeper than the theoretical value of -3.5 for collisional equilibrium. Gaspra's crater population differs from that observed on Phobos but resembles those observed on the Moon and Mars at these sizes (consistent also with the near-Earth asteroid population). Gaspra's fresh craters are superposed on a landscape that appears 'smoothed' at a vertical scale of hundreds of meters. Some 'soft,' subdued crater-like features, commonly [approximately]>500 m across, are visible. Some of these are associated with the linear grooves on Gaspra and may be endogenic features. Many others are probably pre-existing impact craters deeply blanketed or otherwise much degraded. Gaspra's largest-scale shape is highly irregular, perhaps faceted. The biggest facet exceeds the largest crater (relative to body radius) ever observed on a satellite or expected from collisional fragmentation models. Facets cannot be successive crater-forming impacts; later scars would have destroyed earlier ones. Far-encounter images show a more lumpy than faceted visage of Gaspra; two craters are [approximately]3 km in diameter, but not even half the radius of Gaspra. We expect that Gaspra was created by collisional fragmentation of a larger parent body. Its gross configuration may reflect collisional spallation of the parent. Certainly, megaregolithic processes of reaccumulation and blanketing and/or shaking are evident, due to subsequent sub-catastrophic collisions. Gaspra's subdued craters peek through the effects of the last such collision. That smoothed surface has been cratered ever since by the steep production function, which, however must become shallower again below 10 m. Since the overall density of fresh craters is low, Gaspra must be relatively youthful. Scaled to a calculated 0.5 Gyr age for bodies of its size, based on asteroid collision models and assuming that Gaspra does not have metallic strength, its cratering lifetime is [approximately]0.2 Gyr, with large modeling uncertainties. The cumulative volume of all visible craters could create a regolith only