A recent study led by Adrian Brown of Plancius Research has found a region of the Nili Fossae olivine-carbonate lithology on Mars that displays no carbonate or phyllosilicate spectral signatures and may represent the least altered or “population zero” part of the unit. This lithology also outcrops at the Mars 2020 landing site in Jezero Crater, where it has been linked to aqueous conditions conducive to habitability. The new population zero findings should enable further work to determine the conditions under which the olivine was formed around 3.81 billion years ago and then altered over the millennia.
The study used CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) spectral data from Mars Reconnaissance Orbiter to map the position of the 1 micron band of olivine and compare it to laboratory olivine signatures, taking into account the effects of dust, mixing with pyroxene and siderite, and comparison with synthetic olivines. This work also develops a new method for determining the grain size and composition range. The composition is likely intermediate olivine (Fo#44-66) rather than forsteritic olivine, as had been previously suggested, and a minimum grain size of 500 microns was estimated for the population zero subunit of the lithology. These results suggest a new quandary for the martian thermophysical community: there was no reliable correlation between the thermal inertia of the unit and the apparent grain size of the olivine unit, a phenomenon that might be addressed in situ with the Mars 2020 rover. READ MORE