Reactive Iron Mineral Phases on Mars: Implications for Organic Carbon Preservation

It is estimated that ∼20% of organic carbon stored in sediments on Earth is bound to reactive iron minerals. They often occur as nanoparticulate or X-ray amorphous iron (hydr)oxides, which are challenging to identify with mineralogical techniques—especially those that require any type of heating, which also makes them susceptible to alteration in response to diagenetic processes. Reactive iron species have been identified on Mars in the form of nanophase ferric oxides in Gusev crater and at Meridiani Planum with Spirit’s and Opportunity’s Mössbauer spectrometers, respectively, and as Fe-rich amorphous material in Gale crater with Curiosity’s CheMin X-ray diffraction channel. Here, we use the amount of reactive iron minerals and geochemical evidence for diagenetic processes to assess the relative preservation potential of sedimentary rocks at these three landing sites. Sedimentary rocks in Gale crater show the highest preservation potential, and organic carbon compounds have been identified in these rocks. Sedimentary rocks in Gusev crater show equally high preservation potential. However, reactive Fe minerals can also react with organic carbon when heat is added during pyrolysis and laser desorption. These effects need to be considered to determine accurately the organic carbon inventory measured during current and future missions as well as in returned samples.