Molecular Transport Conditions Required for the Formation of Penitentes on Airless, Ice‐Covered Worlds, With Specific Application to Europa, Enceladus, and Callisto.

The surface morphology of airless, ice‐covered moons of the outer solar system, such as Europa, Enceladus, and Callisto, is not well known at centimeter‐ to meter‐scales. Ice and snow erode differently on such worlds in part because sublimation is the dominant process. On Earth, ice penitentes have been observed in sublimation‐driven environments, and may provide a guide for similar formations on ice‐covered worlds. Penitentes are blade‐like snow features observed on Earth in high‐altitude, low‐latitude snowfields. Models of penitente formation on Earth break down within the free‐molecular regime of airless bodies, leaving a major gap in understanding whether such morphologies can form on their surfaces. To investigate the morphologic evolution of icy bodies, we developed a Sublimation Monte Carlo (SMC) model that enables a numerical approach to modeling exosphere‐surface interactions at free‐molecular conditions. The SMC model uses Monte Carlo tracking of molecules emitted from the surface to determine the net molecular interchange that drives surface changes. We validated results against experiments, matching the evolution of pre‐formed penitentes as they receded in height and became less pronounced. Our results reveal the importance of molecular redeposition on topology, indicating that the stable morphology of isothermal topographies is a planar morphology on regions of net sublimation, regardless of initial surface shape. A study of parametrically varying temperature profiles for sinusoidal penitentes resulted in the following requirement for penitente growth: the trough temperature must exceed the peak temperature by a threshold value, which notably depends on the surface aspect ratio and peak temperature. Plain Language Summary: The surfaces of airless, ice‐covered moons of the outer solar system are not well known at the centimeter‐ to meter‐scale because no spacecraft has yet imaged these worlds at that scale. Ice and snow evolve and erode differently on such worlds in large part because sublimation is the dominant process. On Earth, ice penitentes, which are blade‐like structures, have been observed in sublimation‐driven environments, and as such may provide a guide for similar formations on ice‐covered worlds. Our modeling results, however, show that icy surfaces on worlds like Jupiter's moons Europa and Callisto, and Saturn's moon Enceladus, do not easily form structures analogous to penitentes. Rather, the sublimation of water molecules off a rough isothermal surface leads to a flat, icy surface on regions of net sublimation. Under the conditions we studied, in order for penitentes or related blade‐like structures to form, we find that the temperature of the trough regions in the ice must be a few degrees Celsius higher than the temperature of the blade tips. This condition is hard to generate naturally on pure ice patches of icy bodies. Consequently, we predict that penitentes will not be found in pure ice regions of these distant worlds. Key Points: We present a sublimation model capable of simulating surface morphology changes over geologic time scales on airless worldsFor regions of net sublimation, the stable morphology of isothermal topographies is a flat surface regardless of initial geometryOur model shows good agreement with a cryogenic vacuum chamber experiment, showing that pre‐formed snow ridges decay with time [ABSTRACT FROM AUTHOR]