A UNIQUE MEASUREMENT OF MARTIAN DUST BY THE ALPHA PARTICLE X-RAY SPECTROMETER ON CURIOSITY IN GALE CRATER, MARS
Deriving elemental concentrations from APXS on the o-tray requires approaches not normally applied to in situ rocks and soils. O-tray samples are usually thin (< 50 µm) compared to the APXS information depth for heavy elements (> 90 µm). Thus the standard calibration approach cannot be applied. Assuming the thin sample is homogeneous and uniform, a physical model (calculated with software APXyield) can predict the elemental signal from a layer with a given composition. This was applied to the Sol 177 o-tray dust, which appears uniform in MAHLI images. The Sol 89 Rocknest fines (Portage in situ) were set as the model composition, which is nearly identical to typical martian soils at all other rover sites studied with the APXS. Comparing the o-tray dust to Portage enables conclusions about the composition of the airborne dust.
In the Sol 177 dust, Na, Mg, Al, Si, Ca, and Fe have signals consistent with a modeled 2 – 3 µm layer of Portage composition on a Ti substrate. In contrast, S and Cl are enriched relative to Portage by 12 ± 8% and 14 ± 27%, respectively. A drop in signal for all elements indicates the dust covers 33 ± 5% of the FOV.
This is evidence that dust which settled on the o-tray in Gale on Sols ~100 – 177 has a composition similar to Rocknest fines and other martian soils. We found slight, parallel enrichments in S and Cl that are within the typical range of reported variability of S and Cl phases in martian soils. Other MSL-APXS campaigns have found that S is enriched in surface dust relative to Gale rocks. In particular, brushing away dust from rock surfaces with the Dust Removal Tool caused significant drops in SO3 (e.g., 5.6 to 1.0 apparent wt%; Wernecke Sols 168 – 173). Airborne dust is therefore an important component of the modern S flux in Gale. The dust environment in Gale has differences from other rover sites, but MSL-APXS results are similar to findings by the MER rovers.