Monitoring Condensation Flow on Mars with Landed X-ray Spectrometers: A Summary of 11,000 Sols Across Three Landing Sites

Alpha Particle X-ray Spectrometers (APXS) were an integral component of the science payload that flew on the twin Mars Exploration Rovers (MER) Spirit and Opportunity. An updated version of the MER APXS instrument, further optimized for in situ geochemical analyses on Mars, is currently operational within Gale crater onboard the Mars Science Laboratory (MSL) rover Curiosity. APXS on MER and MSL were designed and calibrated for high-precision in situ analyses of geologic materials on Mars. The use of curium-244 sources provides high sensitivity to lower-Z elements. This low-Z sensitivity is important for characterizing the abundance of rock forming elements such as Na, but also enables analyses of Ar, which makes up ~2% of the Martian atmosphere, and thus ~40% all non-condensable gas species. Atmospheric dynamics on Mars are driven in large part by condensation flow. The temperature and pressure at the winter pole leads to the deposition of carbon dioxide (which makes up ~95% of the atmosphere) onto the polar cap. The following spring, carbon dioxide sublimates from the cap, a cycle which creates a pressure gradient across the planet. Non-condensable gases, such as Ar, are not deposited on the polar cap and become enriched relative to carbon dioxide. Most environmental monitoring hardware flown to Mars can measure the absolute pressure of the atmosphere, but not specifically the abundance of non-condensable species. In the case of the Sample Analysis at Mars (SAM) instrument on MSL, atmospheric constituents can be deduced with great accuracy, but not with a high frequency. We summarize efforts on MER and MSL to characterize variability in non-condensable gas density on Mars using instruments designed to measure the composition of rocks and regolith. Analyses by Spirit enabled calibration of the MER APXS for atmospheric analyses. The Opportunity mission, spanning ~5000 sols, acquired around 2250 hours of atmospheric data with its APXS. This data set revealed an annual short-lived Ar enrichment occurring around Ls 150, previously unreported in the literature and not present in climate models at that time. This phenomenon has since been regularly targeted on MSL with APXS (and SAM), with ~800 hours of atmospheric analyses conducted by APXS thus far. We report recent findings from Mars Year 37, where dedicated high-frequency APXS atmospheric campaigns were conducted, coinciding with solar conjunction and extended holiday plans, significantly improved constraints on the timing of this short-lived enrichment at Gale crater, and compare the observed results to those from Opportunity.