Volumetric estimates of ancient water on Mount Sharp based on boxwork deposits, Gale Crater, Mars

While the presence of water on the surface of early Mars is now well known, the volume, distribution, duration, and timing of the liquid water have proven difficult to determine. This study makes use of a distinctive boxwork-rich sedimentary layer on Mount Sharp to map fluid-based cementation from orbital imagery and estimate the minimum volume of water present when this sedimentary interval was formed. The boxwork structures on Mount Sharp are decameter-scale light-toned polygonal ridges that are unique compared to previous observations of Martian fractured terrain because they are parallel-sided ridges with dark central linear depressions. This texture and the sedimentary setting strongly imply that the ridges are early diagenetic features formed in the subsurface phreatic groundwater zone. High-resolution orbital imagery was used to map the volume of light-toned cemented ridges. Based on the cemented volume, a minimum of 5.25 x 105m(3) of cement was deposited within the fractures. Using a brine composition based on observations of other Martian cements and modeling the degree of evaporation, each volume of cement requires 800-6700 pore volumes of water, so the mapped boxwork ridge cements require a minimum of 0.43km(3) of water. This is a significant amount of groundwater that must have been present at the -3620m level, 1050m above the current floor of Gale Crater, providing both a new constraint on the possible origins of Mount Sharp and a possible future science target for the Curiosity rover where large volumes of water were present, and early mineralization could have preserved a once-habitable environment.