Water in Middle Mars History: New Insights From MOLA Data.

Numerous water-related features and deposits are seen in the Hesperian Period of Mars: What is their nature and what can we learn about the movement of ice and water during this time? Is there a hydrologic cycle, what role did the cryosphere play, and how did they change with time? New MOLA data are providing insight into the mode of formation of these features, the volumetric significance and evolution of the deposits, and their relation to events and deposits in the earlier Noachian and later Amazonian. In the Hesperian, continued regional volcanic activity occurred in Tharsis, Syria, and Elysium, and globally distributed ridged plains of volcanic origin were emplaced from several centers characterized by paterae, and then deformed. MOLA data reveal that estimates of the volatile flux into the atmosphere from these eruptive deposits is significantly greater than previously thought. The Hesperian is characterized by dissection and erosion of the uplands by late-stage valley network formation. Extensive south polar ice-rich deposits formed then and were highly modified, with evidence for meltback of many hundreds of km, surface ponding of meltwater, and drainage into the Argyre and Hellas basins. The majority of the outflow channels formed during this period, emptying into the northern lowlands, and extensive sedimentary resurfacing and infilling of the northern lowlands took place. These observations provide evidence that during the Hesperian Period major redistribution of water took place in several modes: 1) from the cratered uplands toward the northern lowlands (late valley networks), 2) from the regional volcanic deposits into the atmosphere, 3) from the south pole across the surface to southern mid-latitudes (polar deposit meltback and lateral transport), 4) from the south pole into the subsurface aquifer (basal melting), 5) from the subsurface to the surface (outflow channels), and into the northern lowlands, 6) from the surface into the atmosphere (all of the above), and 7) from the surface and atmosphere to cold traps, such as the poles. These different deposits, their volumetric significance, and their timing provide a new and increasingly quantitative framework for the evolution of water during the Hesperian. This framework also provides insight into the origin of later Amazonian Period phases of channel formation, volatile-rich deposits, and emplacement of polar deposits.