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

Abstract

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 L s = 291° (March 30, 2013) to Mars Year 33 L s =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 CO 2 absorptions and the known CO 2 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 preliminary aerosol results, meanwhile, show the expected seasonal pattern in dust particle size but also indicate a surprising inter-annual increase in water-ice cloud opacities. Highlights:• 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.

Large Dust Aerosol Sizes Seen During the 2018 Martian Global Dust Event by the Curiosity Rover

“…Highs near L s 220 and 340 were seen in both MY 32 and 33, with identical behavior from L s 150 until 60. For MY 32, the retrieved sizes were similar to those reported by Chen‐Chen et al () from 650‐nm diffraction observations, by Vicente‐Retortillo et al () from ultraviolet fluxes, and spectral determination of opacity by McConnochie et al (). In each case, the seasonal pattern of variation is the same as seen in this work.…”

“…Future work will examine more aspects of constraints on the size distribution. McConnochie et al (2018). In each case, the seasonal pattern of variation is the same as seen in this work.…”

“…Dust properties with time. (a) Retrieved dust r eff for MY 31 (blue), 32 (black), 33 (red), and 34 (green) is shown: circles, this work; triangles, Vicente‐Retortillo et al (); diamonds, McConnochie et al (); and squares, Chen‐Chen et al (). (b) The 440‐nm optical depth (blue diamonds) and retrieved dust r eff (gray circles, every third error bar) is shown.…”

“…The composition, shape, and size of the dust particles govern their interaction with radiation in the atmosphere (e.g., Lemmon et al, 2015;Smith, 2008;Wolff et al, 2006). These dust particle properties can be retrieved directly from observations of the Martian atmosphere (specifically the wavelength-dependent way it scatters, absorbs, and/or emits radiation) and have been done so by a broad set of spacecraft missions and instruments from both the surface and orbit (see Dlugach et al, 2003, for a review): Mariner 9 (Pang et al, 1976;Toon et al, 1977), Viking orbiters (Clancy & Lee, 1991;Ockert-Bell et al, 1997), Viking landers (Pollack et al, 1995), Pathfinder (Tomasko et al, 1999), the Mars Exploration Rovers (Lemmon et al, 2004), Mars Global Surveyor Thermal Emission Spectrometer Clancy et al, 2010;Wolff et al, 2006;, Mars Express (Määttänen et al, 2013;Rannou et al, 2006), the Mars Reconnaissance Orbiter (Guzewich et al, 2014;Wolff et al, 2009), and the Mars Science Laboratory Curiosity rover (Chen-Chen et al, 2019;McConnochie et al, 2018;Vicente-Retortillo et al, 2017). With a few exceptions, most of these results have found that dust particle size varies by approximately 50% around a canonical value of~1.5 μm in effective radius (r eff , a term of convenience in describing the particle size distribution; e.g., Deirmendjian, 1964).…”

Dust particle size and optical depth on Mars retrieved by the MSL navigation cameras

“…The uncertainty of the calibration procedure was estimated by comparing the radiance of Navcam calibrated images with the radiometrically calibrated data of MSL Mastcam (Bell et al, 2017). The retrievals of Mastcam-Left Filter number 4 were used, as the effective wavelength of this filter (674 nm) is the closest one to the effective wavelength of the navigation cameras (~ 650 nm) (Maki et al, 2012).…”