Retrieval of water vapor column abundance and aerosol properties from ChemCam passive sky spectroscopy

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2018-06, Vol.307, p.294-326
• Main Authors: McConnochie, Timothy H., Smith, Michael D., Wolff, Michael J., Bender, Steve, Lemmon, Mark, Wiens, Roger C., Maurice, Sylvestre, Gasnault, Olivier, Lasue, Jeremie, Meslin, Pierre-Yves, Harri, Ari-Matti, Genzer, Maria, Kemppinen, Osku, Martínez, Germán M., DeFlores, Lauren, Blaney, Diana, Johnson, Jeffrey R., Bell, James F.
• Format: Article
• Language: English
• Subjects: Atmosphere; Earth Sciences; Mars; Planetology; Radiative transfer; Sciences of the Universe; Spectroscopy
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2017.10.043

•We measure water vapor abundances and aerosol properties at Gale Crater, Mars.•Precipitable water column is measured with a precision of ±0.6 μm.•Measured quantities include dust & ice fractions, particle sizes, and scale heights.•Results suggest substantial diurnal interactions of water vapor with the surface.•A large interannual change in water ice cloud/haze opacity is observed. We derive water vapor column abundances and aerosol properties from Mars Science Laboratory (MSL) ChemCam passive mode observations of scattered sky light. This paper covers the methodology and initial results for water vapor and also provides preliminary results for aerosols. The data set presented here includes the results of 113 observations spanning from Mars Year 31 Ls = 291° (March 30, 2013) to Mars Year 33 Ls = 127° (March 24, 2016). Each ChemCam passive sky observation acquires spectra at two different elevation angles. We fit these spectra with a discrete-ordinates multiple scattering radiative transfer model, using the correlated-k approximation for gas absorption bands. The retrieval proceeds by first fitting the continuum of the ratio of the two elevation angles to solve for aerosol properties, and then fitting the continuum-removed ratio to solve for gas abundances. The final step of the retrieval makes use of the observed CO2 absorptions and the known CO2 abundance to correct the retrieved water vapor abundance for the effects of the vertical distribution of scattering aerosols and to derive an aerosol scale height parameter. Our water vapor results give water vapor column abundance with a precision of ±0.6 precipitable microns and systematic errors no larger than ±0.3 precipitable microns, assuming uniform vertical mixing. The ChemCam-retrieved water abundances show, with only a few exceptions, the same seasonal behavior and the same timing of seasonal minima and maxima as the TES, CRISM, and REMS-H data sets that we compare them to. However ChemCam-retrieved water abundances are generally lower than zonal and regional scale from-orbit water vapor data, while at the same time being significantly larger than pre-dawn REMS-H abundances. Pending further analysis of REMS-H volume mixing ratio uncertainties, the differences between ChemCam and REMS-H pre-dawn mixing ratios appear to be much too large to be explained by large scale circulations and thus they tend to support the hypothesis of substantial diurnal interactions of water vapor with the surface. Our prelimin