Seasonal Atmospheric Argon Variability Measured in the Equatorial Region of Mars by the Mars Exploration Rover Alpha Particle X‐Ray Spectrometers: Evidence for an Annual Argon‐Enriched Front

Abstract

AbstractThe Mars Exploration Rover Opportunity (MER‐B) has been exploring the surface of Mars since landing in 2004. Its Alpha Particle X‐ray Spectrometer (APXS) is primarily used to interrogate the chemical composition of rocks and soil samples in situ. Additionally, the APXS has measured the atmosphere of Mars with a regular cadence, monitoring the change in relative atmospheric argon density. Atmospheric measurements with the MER‐B APXS span over six Mars years providing an unprecedented level of statistics for careful study of the ubiquitous APXS spectral background. Several models were applied to high‐frequency long‐duration Spirit rover atmospheric APXS measurements. The most stable model with the least uncertainty was applied to the MER‐B data set. Seasonal variation of 10–15% in equatorial atmospheric argon density was observed ‐ in agreement with existing literature and global climate models. Unseen in previous work and global climate models, an abrupt deviation from the model‐predicted annual mixing ratio was measured by the MER‐B APXS around Ls 150. The sharp change, ~10% over 10° Ls, provides strong evidence for a northward migrating front, enriched in argon, sourced from the south pole at the end of southern winter. A similar weaker front is possibly observed around Ls 325, sourced from the northern polar region.