Evidence for multiple ice deposits on the northeastern rim of Hellas basin, Mars

Four prominent fluvial systems, Dao, Niger, and Harmakhis Valles, and a system of smaller channels between Niger and Harmakhis Valles characterize a wide topographic trough that connects Hesperia Planum and Hellas Planitia (Hesperia–Hellas trough, HHT). The source depressions of the canyons of Dao and Harmakhis Valles are at the lowermost level (about − 5.4 km and lower), the system of Niger Vallis is at medium elevation (about − 3 to − 4 km), and the small channels occur at the highest topographic levels (about − 2.5 to − 3.2 km). All systems are among the youngest features in the region. They formed due to the release of subsurface water and apparently have their sources at different topographic levels below or within the layered suite of volcanic deposits in HHT. Two scenarios can explain the subsurface nature of their sources: (1) flow of liquid ground water from extensive watershed toward the source areas and accumulation of water there and (2) deposition of ice-bearing materials in and around the sources. The first scenario requires the existence of extensive confining layers at different topographic levels corresponding to the levels of the floors of the canyons/channels. Because the presence of such confining layers on Mars is problematic, we consider the first scenario to be less likely. We believe that the deposition of ice-bearing materials at different levels during the growth of the lava plateau in HHT can better explain the key features of the fluvial systems: the subsurface nature of the sources and the relatively young age. In this scenario, the higher topographic position of the ice-bearing deposits corresponds to the younger age of deposition. The possible accumulations of ice were buried by successive volcanic deposits, including lava flows in HHT and later were heated from below by magmatic intrusions that caused melting of the icy deposits, release of water, and formation of the fluvial structures.