THE EFFECTS OF CLIMATE, ENVIRONMENT, AND DIAGENESIS ON THE SPECTRAL PROPERTIES OF VOLCANIC SOILS

Visible/near-infrared (VNIR) spectral signatures consistent with smectite and kaolinite have been observed from orbit in many regions across Mars, and large stratigraphic horizons associated with a few of these clay-bearing deposits exhibit spectral signatures that have been interpreted as evidence for reduced Fe-bearing clays. Examples of these units occur together in Gale crater, and are future targets of exploration for the Curiosity rover. One hypothesis for the origin of these clays is that they represent ancient soils or leaching profiles formed in environments with different redox conditions. To better interpret these potentially pedogenic spectral signatures observed on Mars, here we evaluate the effects of climate, redox conditions, and diagenesis on the spectral properties and mineralogy of Mars-relevant mafic soils.

We compare VNIR reflectance spectra, thermal-infrared emission spectra, and XRD of Hawaiian basaltic soils with various precipitation rates, soil ages, and drainage conditions, to basaltic andesite paleosols from the John Day Fossil Beds, OR. Spectral results show the progression of smectite and/or allophane to kaolinite and then gibbsite with age in soils, and that these clays are preserved in paleosols after post-burial zeolitization and oxidation. Redox state of the soils during formation is also discernable after diagenesis based on iron absorptions in VNIR spectra, and in fact, reduced soils exhibit much stronger iron absorptions after diagenesis than their modern counterparts. Modern reduced soils exhibit only weak broad VNIR absorptions that we attribute to green rust or other poorly crystalline Fe(II)-bearing phases, while paleosols exhibit complex strong absorptions that we attribute to iron in crystalline clays produced during diagenesis of the poorly crystalline phases. The intensity of these absorptions in the paleosols appears to increase with the degree of saturation of the original soil, where seasonally wet paleosols exhibit markedly weaker Fe(II) absorptions than perennially saturated paleosols. The spectral signature attributed to Fe(II) in clays on Mars may be consistent with these latter analogs, suggesting that these units may have been formed in a perennially saturated environment such as a lakeshore, wetlands, or near-surface aquifer.