Habitability and preservation from source to sink: Evidence for habitable surface environments in soils on early Mars and their possible contribution to fluvial deposits

One of the most widespread habitable environments on Earth lurks just under our feet. Soils, which are created by precipitation-induced chemical weathering of rocks and sediments, provide abundant geochemical sources of energy for microbes, and form even under water-limited or snow-dominated climates. The mineralogy of soils is directly related to climatic and environmental conditions. Typical neutral, well-drained soils are dominated by phyllosilicates produced through hydrolysis by carbonic acid, but the additional presence of sulfates, oxides, reduced iron, and carbonates can indicate other conditions, including variations in water saturation, redox state, and pH. The high clay content of soils enhances preservation, and reducing soils, like those formed in wetlands, can preserve high concentrations of organics in the form of coal precursors.

On Mars, deposits with mineralogies consistent with paleosols are observed in several locations, but are best exemplified by clay-bearing deposits on the plateau surrounding Mawrth Vallis. Widespread smectites at Mawrth are consistent with regional paleosols formed under a temperate climate, and detections of leached Al-rich minerals, acid sulfates, and reduced iron-bearing phases are consistent with localized modification of these soils by fluctuating ground water tables. Together, this suggests a wetlands-like surface environment with diverse chemical gradients providing many energy pathways for life and locally high preservation potential.

While sites like Mawrth could be excellent targets for organics and biosignatures on Mars, one limitation of such dispersed sites is that finding high concentrations of organics can be challenging. An alternative would be to utilize deltas and other fluvial deposits as a geological filter. Deltas concentrate the often organic-rich clay size fraction of detrital sediments in well-defined locations. On Earth, the majority of deltaic organics and clays are sourced from soils upstream, and have been shown to host some of the oldest known non-marine organics. Thus, while deltas can be a powerful tool for searching for organics and biosignatures, their utility is greatly enhanced if the sediment source region also exhibits clear evidence for habitability, in the form of soils or other environmental indicators.