Major episodes of the hydrologic history in the region of Hesperian Planum, Mars

The High Resolution Stereo Color camera (HRSC) data over Hesperia Planum and its surroundings reveal important details of geologic episodes and water-related processes in this region. (1) The Noachian fluvial events of Hesperia Planum depression included accumulation of water and formation of a water/ice reservoir there. Later, the reservoir was depleted in several phases reflecting diminishing amounts of water. Climate changes and/or volcanism were important in these volatile releases. (2) The massive, 0.45-1.5 × 10⁶ km³, erosion from the Hesperia depression before the main lava eruption possibly resulted in thick, 0.5-1.5 km, deposits in Hellas Planitia. (3) Measurements of the flooded craters within Hesperia Planum provide the estimates of the thickness of lavas there, about 250-500 m. The final volume of lavas within Hesperia Planum (0.4-0.7 × 10⁶ km³) is comparable with the range of some terrestrial igneous provinces such as Columbia River Basalts. (4) Extended magmatism possibly triggered formation of the outflow channel in a few locations after the lava emplacement. During this episode, about 0.04 × 10⁶ km³ of material (about 4.5-8.9% of the volume eroded in the episode of massive erosion) were removed. The thickness of the composite lava layer exposed on the walls of the outflow channels, a few hundreds of meters, corresponds well to the thickness estimates made by the measurements of the flooded craters. (5) Dispersed viscous flows (debris aprons, flow-like deposits) reflect the final fluvial events. Viscous flows from the subsurface sources in the Southwestern trough associate with Dao, Niger, and Harmakhis Valles. These flows represent the final volatile discharge from the Hesperia reservoir that mostly was depleted by the earlier events of massive erosion and formation of the outflow channels. 'Viscous surface flows are mostly associated with Reull Vallis and probably reflect redistribution of volatiles related to the late episodes of evolution of this outflow channel.