The Low Thermal Inertia of Icy Moons: Implications on Surface Porosity, Grain Size, and Regolith Structure

Surface properties like porosity and grain sizes on icy moons remain poorly understood, despite decades of remote sensing observations. Spaceborne instruments do not measure the ice microstructure directly: instead, they record proxy measurements (such as thermal flux or reflectance spectra) which are then interpreted through modeling to estimate thermal inertia, porosity, grain size, etc... However, from data acquisition to parameter inversion, these models rely on various assumptions and simplifications. As a result, different studies using the same raw data but distinct modeling approaches can give different estimates for the same physical parameters, making it difficult to place a robust constraint on the true surface characteristics of the ice.

This study takes a different approach by exploring the parameter space of values that are physically incompatible with icy moon conditions. Notably, the consistently low thermal inertia (<20 SI from Howett et al. 2010) measured at the very top surface of all icy moons is far below that of bulk crystalline water ice (~2000 SI). While a lower thermal conductivity could be attributed to the mix of insulating materials (e.g., dust or amorphous ice phases) regions of pure crystalline ice also exist on these bodies and yet they still present such low thermal inertia near the surface.

Through physics-based reasoning on this data, we demonstrate that pure crystalline water ice can only achieve such low thermal inertia through a combination of very high porosity (>80%), small grain sizes (<1 mm) and an unconsolidated regolith (minimal bond sizes).Tighter or looser constraints can be derived depending on the assumptions underlying the various models found in the literature, which are also discussed. By defining the range of allowed porosities and grain sizes, these constraints will help Bayesian inversion modeling in spectroscopy (Cruz-Mermy et al. 2025) including for future MAJIS (JUICE) and MISE (Europa Clipper) spectrometers, as well as for the planning of rover operations and landing site selection on such highly porous surfaces (e.g., Voyager2050, ESA’s L4 mission).