The prevalence and settings of sedimentary iron oxides on Mars

Abstract:

Iron oxides and oxyhydroxides are important minerals in the sedimentary rock record because they can act as tracers of changing aqueous geochemistry and atmospheric composition over time, providing key insight into the history of water, atmospheric evolution, and habitability of terrestrial planets. For example, evidence of iron mobilization linked to reducing atmospheres on early Earth can be seen in iron formations (IFs) that formed exclusively in the Precambrian (~2.5 – 4 Ga old). In contrast, the majority of iron oxide occurrences that have been detected in Martian sedimentary rocks appear to have formed by diagenetic groundwaters and later alteration, and are therefore not analogs to IFs. We performed a systematic search for iron oxides associated with Noachian- and Hesperian-aged stratified (candidate sedimentary) rocks outcrops on Mars using high resolution visible-shortwave infrared spectral datasets from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). We used a semi-automated technique known as factor analysis and target transformation to examine hundreds of scenes. To date, only 3% of the images we surveyed with stratified outcrops contain ferric absorptions from crystalline iron oxides and oxyhydroxides. Further analyses of the geologic settings and mineral assemblages in these scenes suggest all detections likely precipitated from upwelling groundwaters or surface alteration, and do not represent transported iron that precipitated directly in the water column. We will discuss how current evidence for a lack of IFs on Mars provides constraints on the evolution of Mars’ atmosphere, hydrosphere, and biosphere compared with Earth. Perhaps the geologic settings needed to produce terrestrial IFs (deep, standing bodies of Fe²⁺-rich waters) were never present on Mars or, if present, existed only at smaller spatial/temporal scales not viewable from orbit. Alternatively, if these settings did exist on Mars and ferric phases did not precipitate, then a role for biologic activity may have been necessary to precipitate ferric phases. Government sponsorship acknowledged.