Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometers show that the morphology and motions of large-scale features change with depth in the cloud deck. Poleward meridional velocities, seen in both spectral regions, are much reduced in the NIR In the south polar region the markings in the two wavelength bands are strongly anticorrelated. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large-scale planetary waves. No optical lightning was detected.
Simultaneous observations of auroral emissions and particle precipitation in the 19–24 MLT sector by instruments on the Isis 2 satellite are reported. The optical intensity of the diffuse aurora is found to be produced mainly by a relatively uniform precipitation of low‐energy electrons between about 100 eV and 10 keV with monotonic energy spectra. Discrete auroras are embedded within but usually in the poleward portion of the diffuse aurora and are caused by highly structured and intense electron precipitation. The corresponding electron energy spectrum consists of a monotonic component similar to that in the diffuse aurora and a peaked component often narrower than a Maxwellian spectrum. Several characteristics of the precipitation associated with the diffuse aurora support the identification of the diffuse aurora as the optical image of the earthward termination of the plasma sheet at the auroral altitudes. This suggests that the source particles of discrete auroras lie along the geomagnetic field lines within but usually near the poleward boundary of the plasma sheet. The background boundary for energetic electrons (E > 40 keV) is frequently observed to extend up to the location of discrete auroras, some cases of clearly discernible pancake type pitch angle anisotropies suggesting that some of the discrete auroras are on closed field lines. Protons precipitate in both the discrete and the diffuse auroral region but do not contribute significantly to the total energy flux, except near the equatorward boundary of the diffuse aurora, where the proton flux can be dominant in some cases.
Galileo images of Gaspra reveal it to be an irregularly shaped object (19 by 12 by 11 kilometers) that appears to have been created by a catastrophic collisional disruption of a precursor parent body. The cratering age of the surface is about 200 million years. Subtle albedo and color variations appear to correlate with morphological features: Brighter materials are associated with craters especially along the crests of ridges, have a stronger 1-micrometer absorption, and may represent freshly excavated mafic materials; darker materials exhibiting a significantly weaker 1-micrometer absorption appear concentrated in interridge areas. One explanation of these patterns is that Gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas.
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