Episodic warm climates on early Mars primed by crustal hydration

Abstract

Geological records indicate that the surface of ancient Mars harboured substantial volumes of liquid water, a resource gradually diminished by processes such as the chemical alteration of crustal materials by hydration and atmospheric escape. However, how a relatively warm climate existed on early Mars to support liquid water under a fainter young Sun is debated. Greenhouse gases such as H₂ in a CO₂-rich atmosphere could have contributed to warming through collision-induced absorption, but whether sufficient H₂ was available to sustain warming remains unclear. Here we use a combined climate and photochemical model to simulate how atmospheric chemistry on early Mars responded to water–rock reactions and climate variations, as constrained by existing observations. We find that H₂ outgassing from crustal hydration and oxidation, supplemented by transient volcanic activity, could have generated sufficient H₂ fluxes to transiently foster warm, humid climates. We estimate that Mars experienced episodic warm periods of an integrated duration of ~40 million years, with each event lasting ≥10⁵ years, consistent with the formation timescale of valley networks. Declining atmospheric CO₂ via surface oxidant sinks or variations in the planet’s axial tilt could have led to abrupt shifts in the planet’s redox state and transition to a CO-dominated atmosphere and cold climate.