Oxychlorine Species on Mars: A Review

Abstract

Oxychlorine species (mainly perchlorate and chlorate) have been identified at multiple locations on the surface of Mars by both orbiter and in situ rovers. They have also been found in martian meteorites. Cl-isotopes in meteoritic minerals suggest that an oxychlorine cycle has been operating on the martian surface for the last ∼4 billion years. The present surface conditions are more favorable for their formation than the past and a multitude of formation pathways are likely responsible for their accumulation on Mars. Isotopic analysis of Cl and O can help constrain oxychlorine formation processes. Once formed, oxychlorine species accumulate on the surface as salts or brines, and drive critical geochemical processes. Oxychlorine salts can absorb water from the thin martian atmosphere to form transient brines and percolate into the subsurface. Chlorate anion is an effective oxidizing agent and likely contributes to oxidizing organic matter, iron and manganese minerals on Mars. Given their detection at multiple locations coupled with their ability to stabilize liquid water, oxidize redox-sensitive elements, and promote anaerobic respiration for certain terrestrial microorganisms, oxychlorine compounds have important implications for martian geochemistry and astrobiology, both in the past and in the present. Their propensity to form highly oxidizing brines in closed system environments makes them a critical compound under consideration during the Mars Sample Return mission. This article reviews oxychlorine detection, formation, destruction, and implications on Mars, and identifies potential areas of future research.