The Physical Nature of Regolith on Icy Moons

Estimating surface properties such as porosity and grain sizes is key for planning lander missions and landing site selection on icy moons.

However, spaceborne instruments do not measure the regolith properties directly: instead, they record proxy measurements such as thermal flux, which are then interpreted through modeling to estimate thermal inertia, porosity, grain size, etc.

A striking conclusion from all thermal measurements that probed the uppermost surface (first millimeters) of icy moons is they all show an exceptionally low thermal inertia, ranging from 9 to 20 J.m-2.K-1.s-0.5. This value is orders of magnitude lower than that of bulk hexagonal water ice (2000 J.m-2.K-1.s-0.5) at these temperatures.

We demonstrate that a regolith thermally dominated by hexagonal water ice may only achieve such thermal inertia through a combination of extremely high porosity (>80%), small grain radii (<1 mm), and an unconsolidated regolith (minimal contact area between grains), consistent with previous photometry and spectroscopy studies. For the Galilean moons, deeper thermal observations (>1 cm) have revealed higher thermal inertia (>~50 J.m-2.K-1.s-0.5), indicating that the regolith compacts over centimeter scales.

Since gravity has no effect on compaction on such scale, we propose three formation scenarios to account for vertical layering: deposition cover, degradation by impactors, and temperature gradient metamorphism.

We discuss how monodisperse grains can reach such extreme porosities and provide examples of experimental analogs that could best represent the regolith. We propose that high porosity regolith are favored on icy moons due to the adhesive nature of water ice and their low-gravity environment.

Cyril Mergny, Thomas Cornet, Alice Le Gall, Guillaume Cruz-Mermy, Lucas Lange, Tina Rückriemen-Bez, Bastian Gundlach, Paula Heitmann, Moritz Goldmann, Paul O. Hayne, Apurva Oza

Comments: Submitted to JGR:Planets on May 11, 2026. Please refer to the published version when available
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.27048 [astro-ph.EP] (or arXiv:2605.27048v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.27048
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Submission history
From: Cyril Mergny
[v1] Tue, 26 May 2026 14:02:01 UTC (17,493 KB)
https://arxiv.org/abs/2605.27048

Astrobiology, Astrogeology,