Evidence for remnants of ancient ice-rich deposits: Mangala Valles outflow channel, Mars

Outflow channels represent one of the most dramatic manifestations of the presence, during the Late HesperianAmazonian periods, of huge quantities of liquid water on the surface of Mars. Features associated with outflow channels show evidence for catastrophic water release from the subsurface, extensive overland flow and erosion, ponding in low-lying regions and ultimate return to other water reservoirs (e.g. Baker et al., 1991; Carr, 1996; Kreslavsky and Head, 2002). The fate of the water released and emplaced on the surface in these events has always been a matter of controversy (e.g. Carr, 1996) and the nature of such groundwater and its relation to potential exobiological environments has been of extreme interest (e.g. McKay and Stoker, 1989; McKay and Davis, 1991). High-resolution images from MGS and Odyssey reveal an unusual unit on the floor of the Mangala Valles outflow channel (Figs 1–7). In contrast to abundant terrain showing scour and hydrodynamic shaping typical of the floors and margins of Mangala Valles and other outflow channels (e.g. Tanaka and Chapman, 1990; Zimbelman et al., 1992, 1994; Craddock and Greeley, 1994; Ghatan et al., 2005), this unit (Fig. 3) is smooth-surfaced, has arcuate and cuspate margins, and has a host of unusual surface features including round pits and ring structures often containing huge angular blocks (Fig. 7). We assess several possible origins for this unit and the associated features, and conclude that the most plausible explanation is an icerich remnant created by a combination of ponding and proximal ice-cover deflation during the waning stages of the outflow channel flood event.