Disambiguating the soils of Mars

Anticipated human missions to Mars require a methodical understanding of the unconsolidated bulk sediment that mantles its surface, given its role as an accessible resource for water and as a probable substrate for food production. However, classifying martian sediment as soil has been pursued in an ad hoc fashion, despite emerging evidence from in situ missions for current and paleo-pedological processes. Here we find that in situ sediment at Gusev, Meridiani and Gale are consistent with pedogenesis related to comminuted basalts mixing with older phyllosilicates – perhaps of pluvial origin – and sulfates. Furthermore, a notable presence of hydrated amorphous phases indicates significant chemical weathering that mirrors pedogenesis at extreme environments on Earth. Effects of radiation and reactive oxygen species are also reminiscent of such soils at Atacama and Mojave deserts. Some related phases, like perchlorates and Fe-sulfates, may sustain brine-driven weathering in modern martian soils. Meanwhile, chemical diversity across in situ and regional soils suggests many different soil types and processes. But the two main soil classification systems – the World Reference Base for Soil Resources (WRB) and the U.S. Soil Taxonomy – only inadequately account for such variability. While WRB provides more process insight, it needs refinement to represent variability of martian soils even at the first level of categorical detail. That will provide a necessary reference for future missions when identifying optimal pedological protocols to systematically survey martian soil. Updating Earth-based soil classification systems for this purpose will also advance soil taxonomy as a research field.