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On the Possibility of Melting Water Ice during the Recent Past of Mars. Application to the Formation of Gullies

Lucas Lange, Francois Forget

TL;DR

The paper reassesses the feasibility of melting water ice on Mars in the last $4$ million years (obliquity $\le 35^\circ$) to explain gullies. Using the Mars PCM with slope microclimates and explicit sublimation/latent-heat physics, it shows surface ice cannot melt due to sublimation cooling, and subsurface ice at equilibrium remains too deep to melt; even after regolith destabilization, melting requires unrealistic conditions. The study identifies three limited scenarios where melting might occur: near-surface ice exposed by CO$_2$-driven erosion, extremely favorable diffusion and ice-inertia parameters, or dust-induced solid-state greenhouse heating, but none emerges as robust or likely. It emphasizes that, overall, liquid water formation in the recent Martian past is unlikely, though dusty ice–related solid-state greenhouse effects remain a possible but uncertain mechanism.

Abstract

The formation of gullies on Mars has often been attributed to the melting of (sub)surface water ice. However, melting-based hypotheses generally overlook key processes: (1) sublimation cooling by latent heat absorption, (2) the non-stability of ice where melting conditions can be reached, and (3) the particular microclimates of gullied slopes. Using state-of-the-art climate simulations, we reassess ice melting scenarios over the past four million years (obliquity $\le$35\textdegree)), beyond the estimated period of gully formation. We find that surface melting is impossible anywhere due to sublimation cooling, while (quasi-) stable subsurface ice is typically too deep to reach melting temperatures. We propose an alternative mechanism in which seasonal CO$_2$ frost sublimation destabilizes the regolith and brings the underlying water ice close to the surface, allowing rapid heating. Even under these optimal conditions, melting requires unrealistic assumptions. The only remaining possibility is solar absorption in dusty ice, though its occurrence remains uncertain.

On the Possibility of Melting Water Ice during the Recent Past of Mars. Application to the Formation of Gullies

TL;DR

The paper reassesses the feasibility of melting water ice on Mars in the last million years (obliquity ) to explain gullies. Using the Mars PCM with slope microclimates and explicit sublimation/latent-heat physics, it shows surface ice cannot melt due to sublimation cooling, and subsurface ice at equilibrium remains too deep to melt; even after regolith destabilization, melting requires unrealistic conditions. The study identifies three limited scenarios where melting might occur: near-surface ice exposed by CO-driven erosion, extremely favorable diffusion and ice-inertia parameters, or dust-induced solid-state greenhouse heating, but none emerges as robust or likely. It emphasizes that, overall, liquid water formation in the recent Martian past is unlikely, though dusty ice–related solid-state greenhouse effects remain a possible but uncertain mechanism.

Abstract

The formation of gullies on Mars has often been attributed to the melting of (sub)surface water ice. However, melting-based hypotheses generally overlook key processes: (1) sublimation cooling by latent heat absorption, (2) the non-stability of ice where melting conditions can be reached, and (3) the particular microclimates of gullied slopes. Using state-of-the-art climate simulations, we reassess ice melting scenarios over the past four million years (obliquity 35\textdegree)), beyond the estimated period of gully formation. We find that surface melting is impossible anywhere due to sublimation cooling, while (quasi-) stable subsurface ice is typically too deep to reach melting temperatures. We propose an alternative mechanism in which seasonal CO frost sublimation destabilizes the regolith and brings the underlying water ice close to the surface, allowing rapid heating. Even under these optimal conditions, melting requires unrealistic assumptions. The only remaining possibility is solar absorption in dusty ice, though its occurrence remains uncertain.

Paper Structure

This paper contains 17 sections, 4 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Methods
  3. Surface-Atmosphere model
  4. Surface ice/frost sublimation model
  5. Subsurface ice sublimation model
  6. Icy subsurface thermal model
  7. Results
  8. Melting frost on the surface
  9. Melting subsurface water ice in equilibrium with the atmosphere
  10. Melting suddenly warmed subsurface water ice
  11. Present-day:
  12. Recent paleoclimates:
  13. Melting during the formation of a lag-deposit:
  14. Discussion
  15. Effects of salts on the melting of water ice
  16. ...and 2 more sections

Figures (3)

  • Figure 1: Left column: Maximum thickness of seasonal frost on 30° pole-facing slopes for an obliquity of 35° and L$_\text{sp}$ = 90° (a), and L$_\text{sp}$ = 270° (c) Gullies locations from Noblet2024 are reported by black dots. Right column: Maximum temperatures of seasonal frost on 30° pole-facing slopes for an obliquity of 35° and L$_\text{sp}$ = 90° (b), and L$_\text{sp}$ = 270° (d). The red line is the melting point of water ice (273.15 K).
  • Figure 2: Left column: Depth at which ice table beneath a 30° pole-facing slope for present-day (a), an obliquity of 35° and L$_\text{sp}$ = 90° (c), and L$_\text{sp}$ = 270° (e). Gullies' location from Noblet2024 is reported by black dots. Right column: Maximum temperatures of the upper surface of the ice table for present-day (b), an obliquity of 35° and L$_\text{sp}$ = 90° (d), and L$_\text{sp}$ = 270° (f). The red line is the melting point of water ice (273.15 K).
  • Figure 3: Distribution of maximum subsurface ice temperatures simulated after surface erosion. Each count represents a simulation with parameters detailed in Table S1, normalized by the total number of simulations (288). Top panels (a–c) correspond to present-day obliquity and water vapor content, while bottom panels (d–f) correspond to 35° obliquity. Panels a and c illustrate sensitivity to the dry soil diffusion coefficient; b and e to ice thermal inertia; c and f to dry soil thermal inertia. Only ice temperatures beneath 30° slopes are shown, as this configuration produces the highest ice temperatures during summer. Temperatures may exceed the melting point, as melting and latent heat are not included in the model.