DUSTY ROCKS IN GALE CRATER: ESTIMATING COVERAGES ON APXS TARGETS FROM MAHLI IMAGES AND THE INFLUENCE OF DUST ON ELEMENTAL COMPOSITIONS

A global unit of fine-grained S- and Cl-rich airfall dust is abundant on the surface of Mars. A thin, but discontinuous layer of this dust covers rock surfaces examined by the Mars Science Lab (MSL) rover Curiosity, and complicates our ability to interpret rock textures in Mars Hand Lens Imager (MAHLI) images. In addition, Alpha Particle X-ray Spectrometer (APXS) elemental compositions are affected by surface dust because the lightest elements (Na, Mg, Al, Si) are measured only from the outermost 2-3 µm of rock surfaces (heavier elements are less affected). Using three different image processing methodologies, we have estimated dust coverages in MAHLI images for all APXS targets to sol 1512 (191 total images). Overall, dust coverages of ‘as is’ rock targets range from 6 to 77% coverage (±5% estimated error) with vertical faces at the lower end of that range (8 to 53%). Targets that have been brushed by the Dust Removal Tool (DRT) range to lower dust coverages than ‘as is’ targets, but coverage varies as a function of rock surface type; brushed, fine-grained mudstones have the narrowest range and lowest coverages (11-25%), and dust removal from coarser grained sandstones is more variable, ranging to higher values (12-58% coverage). We find that when assessing coverages for targets within an APXS class, there is a strong correlation between volatile element (S and Cl) concentrations and dust coverage (up to r=0.985 for SO₃/Cl ratios). By comparing paired ‘as is’ and DRT APXS analyses, using dust coverages determined by image analysis, and finding a best fit dust thickness (generally found to be ~10 µm), we can separate the relative contributions of the dust and bedrock and extrapolate estimates of dust-free rock compositions. Dust compositions determined by this method are basaltic with high S and Cl, and are in agreement with those estimated by Berger et al. (2016) by looking at dust accumulated on a piece of rover hardware. In general, the dust-free rock compositions tend to have higher SiO₂ and Na₂O (as much as 6.5 wt% and 0.5 wt% higher, respectively) and lower SO₃ and CaO (up to 5.5 wt% and 1.3 wt% lower, respectively).