Cratering on Gaspra

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

Cratering on Ida

Craters on Ida have a size distribution consistent with equilibrium under conditions of crater saturation over the range 100 m to 1 km diameter. The density of craters is high and they follow a -3 differential slope. Craters show a full spectrum of degradational morphologies at both large and small sizes. Craters >5 km diameter are more prevalent on one part of Ida; a sub-equilibrium, production population should have produced appreciable regolith, at least many tens of meters deep, consistent with photogeologic interpretations. If Gaspra and Ida have similar strentghs and are subject to impact by the same projectile population, then Ida is about ten times older than Gaspra, roughly 2 b.y. old. However, Ida's presence in the Koronis family and the existence of Idas's satellite, Dactyl, suggest that an unknown fraction of Ida's craters could have formed early in its history, shortly after the break-up of the Koronis precursor body. In that case, Ida could be considerably younger and only a fraction of its crater would have been produced by general asteroid belt projectiles

Discovery and physical properties of Dactyl, a satellite of asteroid 243 Ida

Observations of stellar occultions by asteroids have suggested that some may have satellites. But given the absence of any confirmatory evidence, the prevailing view has been that although such satellites probably do exist, they are likely to be rare. Here we report the discovery by the Galileo spacecraft of a satellites associated with the asteroid 243 Ida. Although the satellite, Dactyl, is only 1.6 km across, it has been imaged with sufficient resolution for geological analysis. We describe the physical properties of Dactyl, with emphasis on its notably smooth shape, its crater population (which includes a crater chain) and its photometric properties. We find that, spectroscopically, Dactyl resembles both Ida and the other members of the Koronis asteroid family, implying a similar composition; small spectral differences may reflect a space weathering process that slightly alters the colours with time. We argue that Dactyl originated during the breakup of the Koronis parent body, and that satellites could be common around other asteroids (particularly members of asteroid families)

Color Variations on the Surface of Gaspra.

3-filter color images of Gaspra (0.41, 0.56 and 0.99mm) reveal that the asteroid's surface is spectrally heterogeneous, with violet/ green and IR/green ratios varying by up to 10% and 20-30% respectiv- ly. Two types of spectral variations are recognized and correlate spatially with resolvable morphologic features. (I) Approximately the small patches have anomalously high violet/green ratios and low IR/green ratios. Indicating a bluer spectral slope and enhanced matic absorption. Most of these patches correlate with small impact craters, suggesting that relativly fresh substrate is exposed. (II) Central portions of the asteroid's "facets" have high violet/ green and high IR/green ratios, indicating subdued spectral contrast and mafic absorptions. Bluer, mafic patches associated with small craters are noticeably absent from these latter regions. The spatial and spectral properties of these spectrally subdued areas are suggesttive of accumulation of optically altered. Laterally transported material on gravitationally low portions of the asteroid.We are investigating mechanisms that could produce this distinct type of spectral heterogeneity