CARBON SEQUESTRATION ON MARS: CONSTRAINTS FROM THE MORPHOLOGY, COMPOSITION AND THERMOPHYSICAL PROPERTIES OF THE NILI FOSSAE CARBONATE PLAINS

Martian carbonates have been observed telescopically, from orbit, in situ and in martian meteorites; however, a long-postulated geologic reservoir that accounts for the proposed thinning of a multi-bar early Mars atmosphere by CO₂ sequestration has not been identified. The question remains, where are the atmospheric constituents required for a thicker early atmosphere, or was a thicker atmosphere present at all? Recent results from MAVEN suggest that >500 mbar of CO₂ was lost to space. Here, we use morphological, spectral, and thermophysical datasets from orbital assets at Mars to consider the geologic context and reservoir of atmospheric CO₂­ sequestered in the Nili Fossae carbonate plains (21.5˚N, 78.5˚E) in the context of past atmospheric drawdown.

We find that the Nili Fossae rocks are olivine-enriched (~20%–25) basalts that have been variably altered by a near-surface, low-temperature, in-situ carbonation process to at most ~20% Fe-Mg carbonate. By using the areal extent of this deposit, and extrapolating to extents of unaltered precursor rocks (conservative estimate), the atmospheric carbon sequestration potential in the Nili Fossae region is limited to at most 50 mbar during the late Noachian/early Hesperian, and occurred before or concurrent with valley network formation.

While other atmospherically connected present day reservoirs exist, such as polar CO₂ ice (~15 mbar) or adsorbed CO₂ on regolith grains (<40 mbar), the amount that has escaped to space over martian history is significant (>500 mbar). Any large, geologic CO₂ reservoirs, such as the “deep carbonates” are likely not atmospherically sourced and are inaccessible. Ultimately, we find that atmospheric sequestration of CO₂ in rocks is likely ineffective on Mars and thus most of the CO₂ Mars formed with is either inaccessible or lost making recent discussions of “terraforming” via release of CO₂ from currently available reservoirs on Mars implausible.