Orbital remote sensing of impact-induced hydrothermal systems on Mars

Suites of hydrous and hydrated minerals have been detected on Mars through space-borne remote-sensing instruments. These minerals are often found associated with impact craters, where their origin is attributed to (1) exhumation of pre-existing ancient hydrothermal systems, and (2) impact-related hydrothermal activity. This paper provides a review of the latter, that is, impact-induced hydrothermalism on Mars detected and mapped using orbital imaging spectroscopic as well as high-resolution panchromatic and digital elevation data. The major alteration mineral assemblages that have been detected include phyllosilicates such as smectites, kaolinites, prehnites, chlorites, and mica, and some tectosilicates such as hydrated silica. The most common minerals are chlorites, Fe/Mg smectites, with a few instances of prehnite, which indicate sub-surface alteration. Al-phyllosilicates are also commonly observed suggesting near-surface liquid water activity for extended periods. The observed hydrothermal alteration assemblages are used to get insights into the nature of circulating hydrothermal fluids, including the fluid/rock ratios and pH. Spatial patterns in the distribution of the minerals is not discernible and these impact-induced hydrothermal systems may suggest local events, which are limited within individual craters.