Constraints on the composition and particle size of chloride salt‐bearing deposits on Mars

Abstract

Chloride salt‐bearing deposits on Mars were discovered using the Mars Odyssey Thermal Emission Imaging System (THEMIS) and have been characterized by both mid‐infrared (MIR) and visible‐to‐near‐infrared (VNIR) remote sensing instruments. The chloride salt‐bearing deposits exhibit a blue slope at MIR wavelengths and a featureless red slope at VNIR wavelengths. These deposits also lack strong 3 µm bands in VNIR spectra, indicating that they are desiccated compared to the surrounding regolith. The lack of VNIR spectral features suggests that an anhydrous chloride salt, the most likely of which is halite, is responsible for the observed spectral slope. In this work, we use laboratory spectra and a hybrid T‐matrix/Hapke light scattering model to constrain the particle sizes and salt abundances of the Martian chloride salt‐bearing deposits. Our work shows that the two broad spectral classes of these deposits observed by THEMIS can be explained by a difference in the particle size of the admixed silicate regolith. In all cases, chloride salt abundances of 10–25% are required to match the THEMIS data. The chloride salt abundances determined in this work suggest deposition in a lacustrine/playa setting or in association with late‐stage groundwater upwelling.

T‐matrix and radiative transfer hybrid models for densely packed particulates at mid‐infrared wavelengths

“…This increased fidelity to reality increases the likelihood that such models can be used to determine the compositions of finely particulate surfaces like Martian dusty regions or lunar regolith from MIR spectra. The effectiveness of the T‐matrix/RT hybrid models has already allowed researchers to constrain mineral percentages in fine particulate mixtures [ Glotch et al ., ]. Mie‐based approaches would likely have a tremendous difficulty completing a similar task due to very poor modeling of spectral features (e.g., transparency features, Reststrahlen bands) that make the Mie/RT modeled spectra mostly impractical for such applications.…”

“… γ T and ζ T are dependent on single scattering albedo ( ω T ) and hemispherical asymmetry parameter ( β T ). For ease of calculation, the asymmetry parameter β T can be substituted with the cosine asymmetry parameter, g , derived from either the Mie or T‐matrix model [ Arnold , ; Glotch et al , ; Hardgrove et al , ].…”

Recurring slope lineae and chlorides on the surface of Mars

“…The estimated porosity of this surface is 45%, based on previous studies of terrestrial playa such as Lake Bonneville [ Turk et al ., ]. The areal fraction of chloride required to produce spectral slopes in the TIR is estimated to have a range of 10–25% [ Glotch et al ., ]. The optical depth of TIR in chloride minerals is approximately ~150 µm [ Berger et al ., ].…”

“…Additional laboratory studies indicate that Martian chloride spectra are best replicated by a mixture of chloride grains with the remainder best represented by surface type 1 (ST1) basalts [ Osterloo et al ., ; Jensen and Glotch , ; Glotch et al ., ]. The work of Glotch et al .…”

“…The work of Glotch et al . [] replicated the observed THEMIS spectral slope with mixtures of flood basalt and halite with 10–25% chloride abundances at 63–180‐µm size fractions of halite for coarse particulate surfaces and <10‐µm size fractions of halite for fine particulate surfaces.…”