Global modelling of the early Martian climate under a denser CO2 atmosphere: Water cycle and ice evolution

We discuss 3D global simulations of the early Martian climate that we have performed assuming a faint young Sun and denser CO2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere-surface interactions, atmospheric transport, and the radiative effects of CO2 and H2O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, `icy highlands’ scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.

💡 Research Summary

The authors present a suite of three‑dimensional global climate model (GCM) simulations designed to explore the early Martian environment under a faint young Sun and a denser CO₂ atmosphere. By incorporating a fully coupled water cycle—covering surface‑atmosphere exchange, atmospheric transport, and the radiative effects of CO₂, H₂O gases and clouds—the study evaluates how pressure, orbital obliquity, and topography influence temperature, ice distribution, and the potential for liquid water.

Four atmospheric pressure cases (0.1, 0.5, 1, and 2 bar) were examined, each combined with four obliquity values (15°, 25°, 35°, 45°). The simulations reveal a robust adiabatic cooling effect once the pressure exceeds roughly half a bar. Because pressure drops with altitude, highland regions—especially the southern Noachian valley‑network terrain—experience temperatures 20–30 K lower than the global mean. This vertical temperature gradient drives a persistent migration of water ice from low‑lying basins to the highlands, a process that occurs across the full range of obliquities and is largely insensitive to the presence of the Tharsis bulge.

A thick, persistent ice cap forms on the southern pole, spatially coincident with the Noachian/Hesperian Dorsa Argentea Formation. The cap can reach several hundred meters in thickness and undergoes seasonal sublimation‑recondensation cycles, but the planet remains globally sub‑zero. Surface melting is limited to brief summer intervals in isolated locations where the model permits unusually low albedo and thermal inertia; such conditions are considered unrealistic for pure water ice. Consequently, long‑term stable liquid water is absent even in multi‑bar CO₂ atmospheres.

The authors argue that episodic heating events—impacts or volcanic eruptions—could generate intense, short‑lived melting, producing transient runoff capable of carving the observed fluvial features. Crucially, after such events the same adiabatic cooling mechanism would again drive ice back to the highlands, replenishing the water source. This “icy highlands” scenario therefore offers a self‑consistent explanation for the majority of Noachian fluvial geology without invoking a sustained warm, wet climate.

In summary, the paper demonstrates that a dense CO₂ atmosphere on early Mars leads to a globally cold climate in which highland ice accumulation, polar cap formation, and occasional impact‑ or volcanism‑driven melt events together can account for the planet’s ancient river valleys. The findings challenge the necessity of long‑term greenhouse warming and suggest that Mars may have been predominantly icy, with brief, localized melting episodes shaping its early surface.