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

Images of the Venus cloud deck from Galileo.

Images of Venus taken in spectral bands centered at 418 (violet) and 986 (NIR) nanometers show that the morphology and motions of large-scale features change with depth into the cloud deck. Equatorial zonal velocities of 101+-1 m.sec -1 are seen in the violet and 78+-2m.sec -1 in the NIR. Poleward meridional velocities are seen in both spectral regions but are much reduced in the NIR. Inthe south polar region the dominant markings in the two wavelength bands are strongly anticorrelated, while in the equatorial region the motion of a large-scale meridional NIR feature appears to be associated with the equatorial wave (Venus "horizontal" Y) feature seen in the violet. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and several equatorial strucutres are seen to evolve rapidly. The zonal flow field shows a longitudinal periodicity that may be coupled to the initiation of limited regions of small scale markings near the subsolar region. In midlatitudes the shapes of small features are seen to evolve as they move along the region that forms the arms of the "Y", indicating that advection, and notsuperimposed wave motion, is the probable cause of the striated pattern that is seen there. Limb hazes between 83 and 96 km altitude show similar behavior at both wavelengths indicating that te particulates above the main cloud deck are at least a few tenth of a micron in size. The vertical structure is similar to that previously observed by Mariner 10 but displaced to higher altitudes with less prominent layering. A search was made for optical lightning but no events were detected. The limiting optical energy/flash for lightning to show in the SSI frames is estimated at 4.10^9 J

Discovery of Grooves on Gaspra

We report the discovery of grooves in Galileo high-resolution images of Gaspra. These features, previously seen only on Mars' satellite Phobos, are most likely related to severe impacts. Grooves on Gaspra occur as linear and pitted depressions, typically 100-200 m wide, 0.8 to 2.5 km long, and 10-20 m deep. Most occur in two major groups, one of which trends approximately parallel to the asteroid's long axis, but is offset by some 15 deg.; the other is approximately perpendicular to this trends. The first of extensive flat facets identified by Thomas et al., Icarus 107. The occurence of grooves on Gaspra is inconsistent with other indications (irregular shape, cratering record) that this asteroid has evolved through a violent collisional history. The bodywide congruence of major groove directions and other structural elements suggests that the present- day Gaspra is a globally coherent body