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

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

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