Retrieval of Water Vapor Column Abundance and Aerosol Properties from ChemCam Passive Sky Spectroscopy

TL;DR

This study develops a method to derive water vapor and aerosol properties on Mars from ChemCam passive sky spectra, revealing seasonal patterns and diurnal interactions of water vapor with the surface.

Contribution

It introduces a novel retrieval technique combining radiative transfer modeling with ChemCam data to analyze Martian atmospheric composition and aerosols.

Findings

ChemCam water vapor data aligns with other orbital and surface measurements in seasonal trends.

ChemCam water vapor abundances are generally lower than orbital data but higher than pre-dawn REMS-H measurements.

Aerosol analysis shows seasonal dust size variation and inter-annual increase in water-ice cloud opacities.

Abstract

We derive water vapor column abundances and aerosol properties from Mars Science Laboratory (MSL) ChemCam passive mode observations of scattered sky light. 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. 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.