The Soil of Mars.

Mars and Earth are terrestrial planets that have been altered by aqueous processes. There is a continuum of soil-forming environments that grade from warm and wet extremes of Earth, to the cold and dry climate of Mars. The present climatic points of connection between the two planets are the warm and cold hyperarid deserts of Earth. I will outline the similarities between ancient soils of the Atacama Desert and those of Meridiani Planum, and discuss what this suggests about the long-term climate history of Mars.

A fundamental scientific barrier exists in this field of research: the concept of “soil” in planetary geology implies no assumption of in situ geochemical alteration. In Mars research, the definition is further compounded by the implicit assumption by some researchers that observable evidence of in situ soil alteration is, at best, improbable. This is not just an issue of semantics, because if mission payloads and science teams are not prepared to search for evidence of pedogenic processes, it is unlikely they will be discovered. The geological principle of Uniformitarianism implies that processes that produce features in observable situations (like soils in deserts on Earth of today) should be among the first considered when similar features are found on planets also known to have once had water. Yet, rare and highly-localized weathering processes on Earth have been invoked to explain Mars soil chemistry, at the near exclusion of what is arguably the most common chemical reaction on Earth: pedogenisis.

The opportunities ahead focus not on terminology, but on arriving at a consensus that the pedogenic alteration of Mars by water may span both space and time. The conceptual and instrumental toolkits required to exploit this information should be a first order science priority on future missions.