CHEMCAM INVESTIGATION OF THE STIMSON DRILL SITES, GALE CRATER, MARS

This study utilizes ChemCam data of bedrock, drill hole walls, tailings, and dump piles at the four drill sites in the Stimson formation to investigate chemical variations with depth in the drill holes using the ChemCam LIBS instrument. The Stimson formation is an eolian sandstone that overlies the Murray formation and is mostly composed of decimeter to meter scale crossbedding. There is evidence that the Stimson was emplaced over existing erosional surface in the Murray, indicating that enough time had passed between the deposition of the two formations for there to be substantial reworking of the Murray formation. The rover drill system uses rotary percussion to bore to a depth of 6.5 cm. The drill fines collection system does not engage until a depth of ~2.0 cm is reached; materials from less than 2.0 cm depth are deposited on the surface as tailings. The material collected by the drill system is sieved to <150 µm and delivered to the internal instruments. Unused portions of both sieved and unsieved drill powder are later dumped back on surface and referred to as dump piles. ChemCam can also target sequences on the drill hole wall, typically sampling a depth of ~1.5 cm or less. This difference in source depth for these distinct materials allows ChemCam to sample the surface, a depth down to 2 cm, and 2 to ~6.5 cm down.

Previous work indicates that the eolian Stimson formation is chemically similar to active windblown sands in Gale Crater. ‘Halos’ created by aqueous alteration were observed in the Stimson formation; the alteration zones were characterized by loss of crystalline material and increase in amorphous material, as well as, enrichment in silica, and depletion of numerous elements including Mg, Al, Ca, and Fe. The drilling campaigns sampled a halo and then nearby ‘baseline’ Stimson; the Greenhorn and Lubango sites are in the alteration halos while the Big Sky and the Okoruso targets are in the unaltered sandstone. The surface values and drill hole walls of the alteration halos are much more enriched in silica than the baseline Stimson; however, as material from deeper in the hole was sampled, the data displayed a trend of lowering silica. The silica trend from the halo data does not become as low as what is seen in the baseline, and the MgO also remains depleted in the halo compared to the baseline. Continued work is needed to evaluate this trend.