Geochemistry constrains global hydrology on Early Mars

TL;DR

This study uses geochemical modeling to evaluate whether global groundwater circulation could explain sulfate mineral formation on early Mars, finding that the required CO2 sequestration levels challenge the hypothesis of a warm, habitable early Mars.

Contribution

The paper provides a quantitative assessment of CO2 sequestration needed for global groundwater to produce sulfate minerals, constraining the plausibility of a warm early Mars with active hydrology.

Findings

High CO2 sequestration (30-5000 bars) would bury the atmosphere, challenging the groundwater hypothesis.

If sulfate rocks are Mg-rich, CO2 sequestration is lower (0.3-400 bars), consistent with deep groundwater origin.

Global groundwater circulation likely terminated surface habitability by sequestering CO2.

Abstract

Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks that formed during Mars' Hesperian period are sulfate-rich rocks, explored by the $Opportunity$ rover from 2004-2012 and soon to be investigated by the $Curiosity$ rover at Gale crater. A leading hypothesis for the origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, as part of a global circulation of groundwater. Global groundwater circulation would imply sustained warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water initially in equilibrium with Mars' CO$_2$ atmosphere implies subsurface formation of carbonate. We find that the CO$_2$ sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich rocks on Mars is 30-5000 bars if the $Opportunity$ data are representative of Hesperian sulfate-rich rocks, which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This disfavors the hypothesis that the cations for Mars' Hesperian sulfates were derived from upwelling of deep sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO$_2$ sequestration is 0.3-400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars' Hesperian sulfates were derived from upwelling of deep sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. $Curiosity$ will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change.