Arecibo radar imagery of Mars: II. Chryse–Xanthe, polar caps, and other regions

Abstract

We conclude our radar imaging survey of Mars, which maps spatial variations in depolarized radar reflectivity using Arecibo S-band (λ12.6-cm) observations from 2005–2012. Whereas our earlier paper (Harmon et al., 2012, Arecibo radar imagery of Mars: The major volcanic provinces. Icarus 220, 990–1030) covered the volcanic regions of Tharsis, Elysium, and Amazonis, this paper includes non-volcanic regions where hydrologic and impact processes can be the dominant resurfacing agents affecting radar backscatter. Many of the more prominent and interesting radar-bright features outside the major volcanic provinces are located in and around Chryse Planitia and Xanthe Terra. These features are identified with: a basin in northeast Lunae Planum containing the combined deposits from Maja Vallis and Ganges Catena outflows; channel outwash plains in western and southern Chryse basin; plateaus bordering chasma/chaos zones (Simud and Shalbatana Valles, Ganges Chasma, Aromatum Chaos, Orson Welles Crater), where surface roughening may have resulted from hydrologic action associated with incipient chaos formation; and some bright-ejecta craters in Chryse basin, of a type otherwise rare on Mars. Dark-halo craters have also been identified in Chryse and elsewhere that are similar to those seen in the volcanic provinces. Although the cratered highlands are relatively radar-bland, they do exhibit some bright depolarized features; these include eroded crater rim walls, a few unusual fluidized ejects lobes (including at least one clear case of impact melt), and terrain-softened plains. The rims of large impact basins (Hellas, Argyre, Isidis) show a variety of radar-bright features provisionally identified with massif slopes, erosion sediments, eroded pyroclastics, impact melts, and glacial deposits. The interiors of these basins are largely radar-dark, which is consistent with mantling by fine sediments. Tempe Terra and Acheron Fossae show bright features possibly associated with rift volcanism or eroded tectonic structures, and the Mareotis region of northwest Tempe Terra shows one very bright feature possibly associated with glacial or other ice processes. The first delay-Doppler images of the radar-bright features from the north and south polar icecaps are presented. There is a close correspondence between radar-bright regions and high-albedo residual ice, although some enhanced backscatter extends into the low-albedo polar layered deposits. Both poles show the circular polarization inversion and high reflectivity characteristic of coherent volume backscatter from relatively clean ice, although their S-band reflectivities are significantly less than the X-band (λ3.5-cm) reflectivity of the south polar icecap. The north polar region shows radar-bright ice features from Korolev Crater and a few other outlier ice deposits.