CHEMICAL DIVERSITY AMONG FINE-GRAINED SOILS AT GALE (MARS): A CHEMICAL TRANSITION AS THE ROVER IS APPROACHING THE BAGNOLD DUNES?

Curiosity rover at Gale crater has approached the Bagnold Dunes on sol XXX, which is the first active dune field accessible for in situ analyses. One of the main goals is to determine or constrain the dune material chemistry as well as its provenance [1]. The ChemCam instrument [2,3] has the capability to study the chemical composition of soils at a sub-millimeter scale, thus providing an unprecedented spatial resolution for their study. More than 300 soils have been sampled so far with ChemCam [4,5]. Detailed chemical analysis as a function of grain size is of great importance in order to better constrain soils formation. This study is focusing on the dune materials as well as recent soils analyzed when approaching the dunes, for a comparison with previous soil targets, and with dunes specifically.

Soil analyses acquired closer to the Bagnold dunes are considered in this study as new soils. We are reporting on both fine-grained soils (<500 μm) and coarser grains in a separate comparison with previous studies.

Overall the recent soils, when getting closer to the dunes, have a smaller grain size than at the beginning of the traverse. Those recent soils, as well as the dune materials contain less volatile (such as H or Cl) than the soils encountered up to Pahrump Hills. Nevertheless, on a volatile-free comparison, these recent soils and dunes have a similar composition than the previous soils. Besides this similarity, ChemCam is able to distinguish two types of coarse grains: some with a felsic composition, and some with a mafic one, the latter being the most encountered. The observation of felsic coarse-grains in the dunes was surprising, but also confirmed from the MAHLI images [6].

[1] Bridges et al. (2016) LPSC XLVII; [2] Wiens et al. (2012) Space Sci. Rev. 170; [3] Maurice et al. (2012) Space Sci. Rev. 170; [4] Meslin et al. (2013) Science 341; [5] Cousin et al. (2015) Icarus 249;[6] Edgett, K.S. et al. (2016) LPSC XLVII.