The Diversity and Implications of Unusual Chloride-Bearing Deposits on Mars

Abstract:

Although nearly all planetary surface materials have characteristic vibrations in the thermal infrared wavelengths, one class of notable exceptions are chloride salts. Despite this, chlorides salts have been previously identified on Mars using THEMIS thermal infrared observations, based on this lack of spectral features and the fact that their emissivity is significantly less than 1.0. The recent development of a THEMIS quasi-spectral index for identifying chloride salts enabled an improved global survey of chloride-bearing deposits, which increased the known inventory from 642 to 1,605 distinct deposits. By further investigating and classifying the geologic context of each deposit, we have identified multiple classes of unusual chloride-bearing material that greatly complicate our current understanding of the origin, deposition and modification of chloride salts throughout Martian history. These include: chloride-bearing deposits on regional topographic highs, small crater ejecta composed entirely of chloride-bearing material, large crater ejecta containing discrete clumps of chloride-bearing material, occurrences of chloride-bearing deposits in Late Hesperian-aged terrains, subsurface chloride-bearing material exposed in channels, and channel-shaped chloride-bearing deposits that do not follow local topography. Since chloride salts are so easily re-mobilized, especially in the presence of liquid water, each and every one of these unusual occurrences represents a unique probe into the climate history of Mars stretching from the pre-Noachian to the Late Hesperian, and likely deserve their own dedicated investigations to reveal what exactly they are trying to tell us. In this presentation, we will provide an overview of the unusual chloride-bearing deposits we have identified, our initial interpretations of them, and a discussion of how they may reveal unique details about a complicated Martian climate history.