Cratering on Gaspra

Abstract

Galileo flyby images of 951 Gaspra show a crater population dominated by fresh craters several hundreds meters in diameter and smaller. They must represent 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 highest resolution, "high phase" image, which shows >600 craters in 90 km_2. The differential population index (0.2 - 0.6 km) for the fresh, obvious crater is very "steep" (-4.3 +- 0.3). It probably reflects the index of asteroidal projectiles; it is much steeper than the theoretical valueof -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 >500m across, are visible. Some of these are associated with the linea 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 that faceted visage of Gaspra; the two craters are 3 km in diameter, not even half the radius of Gaspra. We expect that Gaspra was created by collisional fragmentation of a larger parent body.