Mapping of a Widespread Olivine-Rich Layer on Mars: Identification of a Global Impact Ejecta Deposit?

Exposures of the most ancient materials on Mars, much like on Earth, have the possibility to illuminate a period of planetary surface evolution that is difficult to constrain. However, these exposures on planetary surfaces are rare and often difficult to positively identify without in situ measurements. Identification of these materials is especially difficult on Mars, as it is a largely volcanic planet dominated by basaltic magmatism and the ability to distinguish ancient materials from more recent deposits is primarily reliant on convincing stratigraphic relationships. Important but relatively minor variations in the basaltic composition of the planet are observed on the global scale; however, the majority of the planet's mineralogical variability and diversity occurs at small scales, for example the identification of in place phyllosilicates, carbonates, olivine-rich basalts, sulfate deposits, and opaline silica. It is these local-scale deposits that have fundamentally refined and shaped our current understanding of the evolutionary history and geology of Mars. Olivine-rich basalts have been characterized and mapped extensively, both globally and locally. These units have been associated with a variety of formation mechanisms, including volcanism, impact ejecta material, and lag deposits. Following the work of Edwards et al. (2008), we have mapped and characterized a compositionally distinct olivine-rich layer using TES, THEMIS, and CRISM spectral data. This thin and flat lying (~200m thick), continuous, rocky (TI >600-800 J K^-1m^-2s^-1/2), olivine-rich (>15% areal abundance of ~Fo₅₈-Fo₇₄) basalt layer extends for a minimum of thousands of kilometers and may be global in scale. Additionally, the composition of this material is consistent with the crystallization of a melt derived from the martian mantle that underwent little fractional crystallization. The stratigraphic location of this layer, which was first identified in the walls of Valles Marineris in Ganges and Eos Chasmata, necessitates that it formed early in martian history. There are several possible mechanisms that could have formed the observed laterally extensive, olivine-rich layer on early Mars. These mechanisms include the overturn of a magma ocean, voluminous volcanism related to remnant accretionary heat and planetary differentiation, or the ejecta deposit of a mega-impact event. Regardless of the absolute extent of this layer, we propose that the most likely scenario for the formation of this layer with the aforementioned characteristics and extent is a mega-impact event. If the northern lowlands were created as a result of a giant impact, it is likely that the martian mantle would be included as ejecta from this event and dispersed over the planetary surface as a relatively thin continuous layer. The identification of this layer and its possible formation mechanisms has far-reaching implications and could help place constraints on planetary evolution, early Mars topography, and basin forming impact processes. Edwards, C. S., P. R. Christensen, V. E. Hamilton, (2008), J. Geophys Res.