THICK LAYERED MOUNDS ON MARS AS MARINE SULFATE PLATFORMS ANALOGOUS TO TERRESTRIAL CARBONATE PLATFORMS

Large layered mounds up to several kilometers thick have been identified in several places on Mars. Most prominent of these are the Medusae Fossae Formation between Southern Elysium Planitia and western Tharsis. Other large sedimentary mounds are found in Valles Marineris, Meridiani Planum, Henry, Bequerel, and Gale (Mt. Sharp) craters. Most of these lie within 10 degrees latitude of the equator. The superposition of these mounds on large late Noachian craters suggests an upper age limit for the deposits. Though the more prominent features are typically limited to discrete mound forms several tens to hundreds of kilometers across, Malin and Edgett (2000) suggested they are remnants of a former regional blanket that has been eroded to the present extents. Parker and Gorsline (1991) compared the regional geomorphology of the Medusae Fossae mounds to terrestrial carbonate platforms, suggesting they formed in a marine environment and might indicate the “missing sink” for the previously-modeled volume of CO2 outgassed in Mars’ early history. These mound deposits have turned out not to be carbonates, however, but rather sulfates. So, after the Opportunity rover discovered aqueous sedimentary structures within the Meridiani Planum deposits, and subsequently the Curiosity rover identified subaqueous sedimentary structures at Mt Sharp, Parker suggested that these mounds might still be best explained as marine platform deposits with a sulfate composition. Parker and coauthor Anderson have argued for years about the formation of these materials. Anderson preferred a volcanic ash origin from massive volcanic eruptions from the major volcanic centers at Tharsis, Elysium, and Syrtis that began during the late Noachian. Recently, it occurred to both that a combination of the two hypotheses might provide a convenient explanation, whereby the volcanism provides both the sulfates and the water to cause an ocean transgression that enabled sulfate platforms to develop at equatorial latitudes.