ESTIMATING ROCK STRENGTH PARAMETERS FROM ROCK ABRASION TOOL (RAT) GRINDS

Each Mars Exploration Rover carries a Rock Abrasion Tool (RAT) whose intended use was to abrade the outer surfaces of rocks to expose more pristine material. Motor currents drawn by the RAT motors are related to strength and hardness of rock surfaces undergoing abrasion, and this data can be using to infer more about a target rock’s physical properties. However, no calibration of the RAT exists. Here, an empirical calibration is derived using an assemblage of terrestrial rocks and apply this calibration to data returned by the rover Spirit. The calibration results demonstrate a positive correlation between rock strength and RAT grind energy for rocks with compressive strengths less than about 150 MPa, a category that includes all but the strongest intact rocks. Applying this calibration to rocks abraded by Spirit’s RAT, the results indicate a large divide in strength between more competent basaltic rocks encountered in the plains of Gusev crater (Adirondack Class rocks) and the weaker variety of rock types measured in the Columbia Hills. Adirondack Class rocks have estimated compressive strengths in the range of 110-160 MPa and are significantly less strong than fresh terrestrial basalts; this may be indicative of a degree of weathering-induced weakening. Rock types in the Columbia Hills (Wishstone, Watchtower, Clovis, and Peace class) all have strengths < 50 MPa and are consistent with impactites or volcanoclastic materials. In general, when considered alongside chemical, spectral, and rock textural data, these inferred strength results help inform our understanding of martian rock origin and modification history.