Geologic context of proposed chloride-bearing materials on Mars

We use Thermal Emission Imaging System (THEMIS) data to identify and characterize the global distribution of distinct materials interpreted to contain chloride salts on the Martian surface. Previously mapped global geochemical and physical properties are used in concert with thermophysical and morphological observations to assess the materials' local and regional characteristics. The results of our survey have expanded the characterization of the materials from similar to 200 to similar to 640 distinct sites dispersed throughout low-albedo Noachian- and Hesperian-aged terrains. Our survey also shows that the materials are detected in locally thermophysically distinct terrains and display a range of morphologies. Topography indicates that the majority of the materials occur in local lows, although crosscutting relationships indicate that some sites are located in geologic windows implying that the materials may be older than the terrains in which they are situated. Once exposed, the materials appear to undergo erosion, which may be the reason we do not observe large laterally extensive materials at the surface. The materials are predominantly local in nature, yet their prevalence across the southern highlands suggests that they represent one or more globally ubiquitous processes. We consider a number of formation hypotheses but find that most observations are consistent with formation via ponding of surface runoff or groundwater upwelling, although efflorescence and hydrothermal activity may also be possible in some locales. The materials' inferred ages suggest that the conditions that enabled the deposition of the materials persisted for up to 1 billion years.