Surface Warming Induced by the Radiative Effect of Water Ice Clouds in the Late Amazonian: Implications for (Sub)-Surface Activity

The present-day Martian climate is governed by three major cycles: CO₂, water, and dust. Every winter, approximately 30% of the atmospheric condenses to form the seasonal CO₂ caps [1]. In spring, these caps disappear, triggering surface activity such as CO₂ geysers and contributing to the formation of gullies [2,3]. In late spring, the perennial North pole ice cap begins to sublimate, injecting water vapor into the dry atmosphere, forming thin clouds at mid and low latitudes, and millimetric night frost forms at the surface that vanishes by early morning [4].

This climate may have been more pronounced in the past due to the planet's orbital variations, and in particular its obliquity, which varied by as ±15° in the recent past [5]. Climate models show that during high-obliquity periods the polar night would be more extensive, intensifying the CO₂ cycle, and that the amount of water vapor injected from the North Polar cap would be greater due to higher summer insolation [6]. Recent studies [e.g, 7] have notably shown that water ice clouds forming following this summer injection play a key role in Martian paleoclimates: by absorbing solar radiation, they warm the atmosphere, which can then hold more water vapor, producing larger clouds and creating feedback mechanisms that make the Martian atmosphere far more humid than it is today. While most models suggest that the CO₂ cycle was more pronounced than today, a recent study by [8] shows that higher atmospheric humidity could promote the formation of pore-filling ground ice at mid- to low latitudes, thereby limiting the amplification of the CO₂ cycle.

A second effect of these clouds is the warming of the surface. These clouds emit infrared radiation toward the surface, which can be significant during periods of low solar insolation such as winter or nighttime. On present-day Mars, this warming can reach 10 to 15 K at night in equatorial regions [9] Given how significant this effect already is today, the question of its impact on Martian paleoclimates naturally arises given the greater cloud abundance expected under past conditions. Yet it has never been studied to date.

In this presentation, we will discuss the surface warming induced by the downwelling infrared flux emitted by these clouds. We will show in particular that for obliquities above 35°, this warming is large enough that surface temperatures can no longer reach the CO₂ condensation point over most of the Martian surface. As a result, the CO₂ cycle, a defining feature of the present Martian climate, nearly disappears entirely under past high-obliquity conditions. Surface activity driven by CO₂ condensation and sublimation is therefore not representative of recent surface history. In particular, we will show that if CO₂ is indeed the agent responsible for forming the enigmatic Martian gullies, then these features could only have formed during low-obliquity periods below 30°, which is consistent with their young age [10]. Finally, while previous studies suggest that water ice could form within the pore spaces of dusty equatorial regions during high-obliquity periods [e.g., 8,11], we will show that this is unlikely due to surface warming. The presence of water-ice deposits within the pores of the upper equatorial regolith is therefore unlikely, although remnants of past massive ice sheets may still persist today [12,13].

References :

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[11] Aharonson et al., 2026: Milankovitch forcing of equilibrium ground-ice on Mars, Icarus, 444, 116772. DOI: 10.1016/j.icarus.2025.116772

[12] Forget et al,  2006: Formation of Glaciers on Mars by Atmospheric Precipitation at High Obliquity, Science, 311(5759), 368–371. DOI: 10.1126/science.1120335

[13] Vos et al., 2026, The Martian mid-latitude subsurface ice is the remnant of a past ice sheet, Communications Earth & Environment. DOI: 10.1038/s43247-026-03418-x