Recent results from ChemCam Passive Sky Spectroscopy at Gale Crater, Mars

The Mars Science Laboratory's (MSL) ChemCam spectrometer measures atmospheric aerosol properties as well as O₂ and H₂O gas abundances by operating in passive mode and observing scattered sky light at two different elevation angles [1]. To date, the most important ChemCam passive sky results include: In-situ water vapor (measured by the REMS humidity sensor) is depleted in the pre-dawn hours relative to column water vapor (measured by ChemCam); this is likely caused by nighttime adsorption onto soil grains [2] and appears to vary with surface properties [3]. Dust aerosol particles sizes are found to be very large at the peak of the 2018 global dust storm, in good agreement with other MSL sky observations during this time [4,5]. Outside of major dust events, the ChemCam dust aerosol particle sizes show a smooth seasonal variation as well as inter-annual variability that seems to be correlated with ChemCam-measured inter-annual variability in aphelion season ice cloud opacity. This is well-corroborated by REMS UV sensor dust particle size results [4,5,6]. O₂ column-averaged volume mixing ratios are unexpectedly high and show variability well beyond expectations for a non-condensable trace gas. To-date, interpretation of these O₂ results remains complicated by not-yet-fully-understood uncertainties.

While ChemCam passive sky observations are ongoing, all of these results are based largely on the first 1300 sols of MSL operations, plus a small amount of additional analysis around the 2018 global dust storm. We will report on the results of a new research effort to improve our methodology, most importantly for O₂, and to extend the available data set to cover the last ~1.5 Martian years of ChemCam sky observations.

REFERENCES: [1] McConnochie, et al. (2018), Icarus, 307. [2] Savijarvi et al. (2019) Icarus 319. [3] Savijarvi et al. (2019) Icarus 326. [4] Lemmon et al., submitted to GRL. [5] Lemmon et al. (2019) 9th Int. Conf. on Mars #6298. [6] Vicente-Retortillo et al. (2017), GRL, 44.