BOXWORK STRUCTURES AND GROUNDWATER VOLUME ESTIMATES ON MOUNT SHARP, MARS

Boxwork structures are mapped in a distinctive sedimentary layer exposed on Mount Sharp in shallow depressions about 700 m above Curiosity’s landing ellipse in Gale Crater. This layer, exposed over about 1 km², is characterized by penetrative fracture networks expressed as ridges and separated by shallow depressions (2-10 m diameter) that are filled with dark windblown sand. Ridges are light-toned, averaging 4-5 m in width and sometimes marked by a thin dark strip (0-1.5 m across) in the center of the ridge. These are interpreted as large-scale boxwork structures, formed when saturated groundwater flowed through the fractured host rock and crystals precipitated within fractures and pores in the host rock, making the fractures more resistant to weathering than unaltered host rock. After weathering, the fracture fills stand as topographically higher rims around eroded host rock.

Formation hypotheses are proposed in order to calculate a conservative estimate for the amount of water required to form the boxwork layer. The volume of the boxwork ridges is estimated based on aerial exposure (20-50% of the observed 1 km² of boxwork structures), an assumed minimum 5 m depth, and a hypothesis that the ridges are either porosity-occluding cements, making up ~ 30% of the ridge volume, or space-filling cements, making up the full ridge volume. A cement composition is assumed based on a brine composition derived from the mineral assemblage in the evaporate-rich Burns Formation, Meridiani Planum, as measured by the Opportunity rover. The evaporation of this derived brine was modeled by Tosca et al. using Geochemists Workbench to obtain the mass of cement relative to the percent of water that evaporated (Tosca et al., JGR, 2011). Assuming 90-99% of the water evaporates, 800-3500 “pore volumes,” or cement-volume-equivalent volumes of water, are required before the volume is filled with cement. For a porosity-occluding cement, that means 0.23 – 2.62 km³ of water are required, or for a volume-filling cement, 0.77 – 8.75 km³ of water are required to form the exposed boxwork deposit. This is a significant amount of water 700 m above ground-level, and possible future investigation of these and similar deposits with the Mars Science Laboratory rover could provide detailed information concerning the history of groundwater chemistry in this region.