Alpha Particle X-ray Spectrometer (APXS) Investigation of Chlorine and Bromine Enrichment at Gale Crater, Mars.

Abstract:

The Alpha Particle X-ray Spectrometer (APXS) on board the Mars science Lab (MSL) Curiosity rover has been investigating targets on Mars for almost 8 years at a rate of a little more than 100 a year on average. Among those, to date, are 29 drilled holes (including the mini-drill holes) which nominally include a pair of unbrushed/brushed rock targets of the to be drilled spot proceeding the drilling and 105 paired unbrushed/brushed pairs of targets, brushed with the Dust Removal Tool (DRT). Earlier the APXS team presented (LPSC LI, abstract #2025) work regarding the nature of the chlorine enrichments in Gale crater, and concluded that the enrichments for the majority of the targets existed as surface rinds, rather than in the bulk rock and/or dust, which is consistent with observations from Meridiani and Gusev (Clark et. al., 2005, EPSL 240). Herein we extend the earlier work on 26 drill hole targets and 81 paired unbrushed/brushed targets through the traverse onto and off of the Greenheugh pediment, adding three drill target unbrushed/brushed/drill tailing groupings and 24 unbrushed/brushed rock pairs, continuing to investigate the nature of the chlorine and additionally, the nature of the bromine. The APXS dataset for Gale, with approximately ~ 900 analyses, has max, mean, and minimum values of approximately 3.4, 1.1 and 0.3 wt%, respectively for chlorine and ppm values of approximately 1980, 158 and 5, respectively for bromine and the highest measured values for both tend to correlate. The additional targets and the additional analysis of bromine content, mean values for Cl (wt%) and Br (ppm) of approximately 0.7 and 100, 1.3 and 209, and 1.1 and 194, in the drill tailings, pre-, and post-DRT surfaces respectively, reinforces the earlier conclusion regarding chlorine enriched surface rinds, and we can conclude those surface rinds are also enriched in bromine via a similar pathway. The earlier reported relationship, approximately 60/40% post-DRT dust removal, increase in observed chlorine content, holds for the paired unbrushed/brushed and unbrushed/brushed drill targets, respectively, and bromine appears to be mobilized and deposited penecontemporaneously with the chlorine in said surface rinds.