The Martian O₂ and H₂O cycles observed with ChemCam Passive Sky Spectroscopy

The Mars Science Laboratory's (MSL) ChemCam spectrometer, designed primarily for laser-induced breakdown spectroscopy, has proven to also be every effective as a passive spectrometer. We will describe measurements O2, H2O, and aerosols from ChemCam passive sky observations. We will then discuss the evidence they provide for a seasonally active Martian O2 cycle as well as their implications for the Martian water cycle. ChemCam passive sky observations proceed by acquiring spectra of scattered light from the atmosphere at two different elevation angles, constructing a ratio spectrum to eliminate the solar spectrum and instrument response uncertainties, and then fitting a discrete ordinates multiple scattering radiative transfer model. O2 was thought to have a long, greater than 10 Martian year, photochemical lifetime (e. g. Krasnopolsky 2010, Icarus 207) and thus was expected to show a seasonal behavior identical to other non- condensable inert gases. ChemCam measurements demonstrate that this is not the case, showing that O2 decreases rapidly relative to SAM-QMS- measured Ar and CRISM-measured CO during the Ls = 350° to Ls = 30° period and then doubles its mixing ratio relative to those gases during the Ls = 50° to Ls = 130° period. SAM-QMS O2 measurements confirm the ChemCam results, showing essentially identical O2 mixing ratios and seasonal variations. The ChemCam water vapor measurements, when combined with the in-situ sampling of the REMS humidity sensor and with global scale orbiter data sets, provide information on the vertical distribution of water vapor and yield insights into the local and regional scale dynamics in the vicinity of Gale Crater. We observe a close match between mixing ratios inferred from REMS humidity sensor measurements and column average mixing ratios inferred from ChemCam in some seasons, but significant differences in other seasons, suggesting seasonal variations in vertical mixing processes.