Aqueous Processes on Mars: Results From the Mars Exploration Rover Mission

The Mars Exploration Rovers Spirit and Opportunity have both yielded evidence for aqueous processes at their landing sites. In Gusev crater, Spirit found only trace evidence for the action of water on the basalt-covered plains of the crater floor. This water action left thin salt-rich deposits on the surfaces of rocks and in fractures within rocks, and what appear to be magnesium sulfate salt concentrations in soils. The older rocks in the Columbia Hills, however, show evidence for much more substantial aqueous alteration. At Meridiani Planum, Opportunity has found layered sedimentary rocks that we interpret to be "dirty" evaporites. Environmental conditions that they record include episodic inundation by shallow surface water, evaporation and desiccation. After deposition, these rocks underwent a complex diagenetic history that resulted in recrystallization, vug formation, and growth of hematite-rich concretions. A stratigraphic section obtained within Endurance crater shows significant variation of rock chemistry and texture as a function of depth, indicating changes in depositional and/or diagenetic processes with time.