Could Mars Have Melted Ice Recently?
Martian gullies, small channels carved into crater walls, look strikingly similar to meltwater-carved features on Earth. Because of this resemblance, scientists have long wondered whether liquid water might have existed on Mars within the last few million years. In this paper, Lange et al. investigate that possibility using advanced climate models, taking into account several physical processes that earlier studies either simplified or omitted. The authors argue that understanding sublimation, the stability of water ice, and the microclimates of gully-bearing slopes is essential for evaluating whether melting could truly have occurred.
Background: The Challenge of Liquid Water on Mars
The authors begin by describing why liquid water is difficult to sustain on Mars today. Even though temperatures and pressures can occasionally exceed water’s triple point, Mars’ atmosphere is extremely dry, holding only about 0.01% relative humidity. Under such conditions, liquid water would rapidly boil or evaporate away. Yet, gullies appear geologically young (younger than ~4 million years) and resemble terrestrial features shaped by flowing water. This contrast motivates the central question of the paper: could melting have happened during Mars’ recent climate cycles, especially during periods of higher obliquity when the planet’s tilt increases sunlight at mid-latitudes?
Surface Ice: Sublimation Prevents Melting
To explore this, the authors apply both one-dimensional and three-dimensional climate models that include detailed physics, such as latent-heat cooling from sublimation. When testing whether surface frost or snow could melt, they find that sublimation cooling is so strong that the ice never warms enough to reach 273 K (water’s melting point). Even under the most favorable orbital conditions in the last 4 million years, high obliquity and increased atmospheric humidity, the maximum frost temperatures remain several degrees too cold. This directly challenges previous studies that suggested meltwater could form on pole-facing slopes.
Subsurface Ice: Too Deep and Too Cold to Melt
The team next examines subsurface ice, a potential source of meltwater if heated from above. Using their models, they calculate how deep such ice would sit if in equilibrium with the atmosphere. They find that, under both modern and past climates, the ice table lies far below the zone where seasonal heating can reach melting temperatures. Even when subsurface ice sits unusually shallow due to local conditions, it still remains too cold; sublimation would remove it before warming could occur. This means that stable, long-lived subsurface ice is generally too deep to melt, consistent with earlier theoretical predictions.
Sudden Exposure Scenarios: Still Unfavorable for Melting
A more intriguing scenario arises from observed gully activity: seasonal CO₂ frost can destabilize the soil, exposing subsurface water ice or leaving only a thin layer of regolith above it. Lange and collaborators simulate this rapid exposure to determine whether a sudden pulse of springtime sunlight could heat the ice enough to melt. Even then, melting is extremely limited and requires unrealistic combinations of soil diffusion properties, unusually low ice thermal inertia, and specific slope geometries. In the few simulations where temperatures exceed the melting point, the amount of meltwater produced is tiny, far below that needed to mobilize debris flows like the observed gullies.
Additional Mechanisms: Salts and Solar Heating at Depth
Finally, the authors consider two additional effects: salt-induced melting and the solid-state greenhouse effect. While salts can lower water’s melting point, the concentrations required (up to ~40% by weight) far exceed what is typically found in Martian soils. Likewise, transparent or dust-containing ice could, in principle, trap sunlight and warm at depth, but the conditions needed for this mechanism remain uncertain and would require future study. As a whole, their findings imply that recent melting on Mars is theoretically possible only under very rare and highly specific circumstances, and likely insufficient to explain the majority of Martian gullies.
Conclusion: A Dry Explanation for a Watery Mystery
In summary, Lange et al. conclude that melting of surface frost, equilibrium subsurface ice, or freshly exposed ice is highly unlikely on Mars during the past 4 million years. Instead, present-day gully activity is more plausibly explained by CO₂-driven dry processes rather than flowing water. While melting cannot be ruled out entirely, the authors show that it would require conditions that Mars’ recent climate probably did not provide, reshaping our understanding of these mysterious Martian landforms.
Source: Lange
