Magnetite authigenesis and the warming of early Mars

Abstract

The <i>Curiosity</i> rover has documented lacustrine sediments at Gale Crater, but how liquid water became physically stable on the early Martian surface is a matter of significant debate. To constrain the composition of the early Martian atmosphere during sediment deposition, we experimentally investigated the nucleation and growth kinetics of authigenic Fe-minerals in Gale Crater mudstones. Experiments show that pH variations within anoxic basaltic waters trigger a series of mineral transformations that rapidly generate magnetite and H₂(<i>aq</i>). Magnetite continues to form through this mechanism despite high P_CO2 and supersaturation with respect to Fe-carbonate minerals. Reactive transport simulations that incorporate these experimental data show that groundwater infiltration into a lake equilibrated with a CO₂-rich atmosphere can trigger the production of both magnetite and H₂(<i>aq</i>) in the mudstones. H₂(<i>aq</i>), generated at concentrations that would readily exsolve from solution, is capable of increasing annual mean surface temperatures above freezing in CO₂-dominated atmospheres. We therefore suggest that magnetite authigenesis could have provided a short-term feedback for stabilizing liquid water, as well as a principal feedstock for biologically relevant chemical reactions, at the early Martian surface.