Depth Distribution of Chlorine at Gale Crater, Mars, as Derived From the DAN and APXS Experiments Onboard the Curiosity Rover

The subsurface chlorine depth distribution at Gale crater has been studied using measurements from the Alpha Particle X-ray Spectrometer (APXS) and Dynamic Albedo of Neutrons (DAN) instruments onboard NASA's Curiosity Mars rover. These measurements have been acquired at or near similar to 30 drill holes made by Curiosity along its 30 km drive. APXS measured a similar to 15 micron surface layer of bedrock and shallow depth of similar to 5 cm of drill tailings. DAN is sensitive to the bulk (similar to 50 cm depth) chlorine abundance and that of other neutron absorbing elements. Joint analysis of APXS and DAN data shows that the chlorine distribution at Gale crater consists of two components: surficial chlorine with concentrations >1 wt.%, possibly controlled by aeolian deposition of Cl-rich dust and surface alteration, and subsurface chlorine with concentrations <1 wt.%, likely a result of groundwater activity in the past.