Searching for Pedogenic Phyllosilicates in Ancient Soils on Mars

On Earth, vast deposits rich in phyllosilicates are commonly created during soil formation, or pedogenesis. When soils are preserved in the stratigraphic record as paleosols, they become valuable resources for terrestrial geologists because paleosol isotopes, mineralogy, and chemical weathering profiles can be used to reconstruct ancient surface environments and to provide quantitative constraints on regional paleo-climate. Thus, paleosol sequences developed in sedimentary settings can record millions of years of surface and climatic evolution. Ancient paleosols on Earth also have excellent organic and biosignature preservation potential, and therefore harbor some of the oldest known (2-3 Ga) non-marine organics, biosignatures, and fossils. On Mars, pedogenesis in the ancient past may be responsible for some of the phyllosilicate-bearing units observed today, especially for regionally extensive deposits and those in clear sedimentary settings (e.g., Arabia Terra/Mawrth Vallis, Gale Crater, Noctis Labyrinthus). Many of these possibly pedogenic deposits exhibit compositional layering (e.g., interbedded kaolinites, smectites, and sulfates), which may have formed due to episodic sediment deposition under changing environmental conditions. Such deposits represent excellent targets for in situ investigation, as finding and characterizing paleosols on Mars would allow us to place constraints on the extent and duration of past surface and near-surface habitability, and may even provide preserved samples of ancient martian organics. We are currently investigating a broad range of methods for identifying and characterizing paleosols on Mars from orbit and in situ with Mars Science Laboratory, based on analysis of phyllosilicate-rich (30-95 wt.%) Eocene-Oligocene paleosols in the Painted Hills of the John Day Fossil Beds National Monument in Eastern Oregon. These paleosols were formed under a wide range of environmental conditions, and include highly weathered soils rich in well-crystalline oxides and kaolinites, moderately weathered soils rich in smectites, and minimally weathered soils rich in poorly-crystalline allophanes and ferrihydrite. Here we present (1) an overview of the climatic regimes that lead to the pedogenesis of specific phyllosilicate minerals, (2) the near and mid-infrared spectral properties and interpreted mineral assemblages of these terrestrial paleosols, and (3) an evaluation of a pedogenic origin for phyllosilicates at several sites on Mars, including those listed above. Preliminary results from near-infrared spectral analysis of our terrestrial paleosols indicate that paleo-environment can be constrained based on mineral assemblages interpreted from spectral properties, including phyllosilicate composition (constrains water availability), the presence of allophane and ferrihydrite (indicating a cool climate), and the strength of oxide absorptions (constrains soil maturity). Mineral assemblages can also be used to detect burial diagenesis by the presence of diagenetic minerals, including celadonite, illite, and hematite (in the presence of phyllosilicates indicating less mature soils). Our results also indicate that poorly crystalline minerals (allophane and ferrihydrite) can be spectrally dominant in these soils even after burial and diagenesis.