ABSTRACT
Images obtained by the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) cameras onboard the Rosetta spacecraft reveal that asteroid 21 Lutetia has a complex geology and one of the highest asteroid densities measured so far, 3.4 ± 0.3 grams per cubic centimeter. The north pole region is covered by a thick layer of regolith, which is seen to flow in major landslides associated with albedo variation. Its geologically complex surface, ancient surface age, and high density suggest that Lutetia is most likely a primordial planetesimal. This contrasts with smaller asteroids visited by previous spacecraft, which are probably shattered bodies, fragments of larger parents, or reaccumulated rubble piles.
ABSTRACT
Venus is completely covered by a thick cloud layer, of which the upper part is composed of sulphuric acid and some unknown aerosols. The cloud tops are in fast retrograde rotation (super-rotation), but the factors responsible for this super-rotation are unknown. Here we report observations of Venus with the Venus Monitoring Camera on board the Venus Express spacecraft. We investigate both global and small-scale properties of the clouds, their temporal and latitudinal variations, and derive wind velocities. The southern polar region is highly variable and can change dramatically on timescales as short as one day, perhaps arising from the injection of SO2 into the mesosphere. The convective cells in the vicinity of the subsolar point are much smaller than previously inferred, which we interpret as indicating that they are confined to the upper cloud layer, contrary to previous conclusions, but consistent with more recent study.
