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.

Tall, spiky snow structures (penitentes) occur high in subtropical mountains, in the form of blades oriented east-west and tilted toward the noontime sun. By trapping sunlight, they cause a reduction of albedo by similar to 0.3 relative to flat snow. The formation of penitentes, explained by Lliboutry in 1954, requires weather conditions allowing the troughs to deepen rapidly by melting while the peaks remain dry and cold by sublimation, losing little mass, because of the 8.5-fold difference in latent heats. Lliboutry's explanation has been misrepresented in some recent publications. A concern has been raised that in the low latitudes of Jupiter's moon Europa, the ice surface may have developed penitentes, which would pose a hazard to a lander. They would require a different mechanism of formation, because Europa is too cold for melting to occur. If penitentes are present on Europa, they cannot be resolved by the coarse-resolution satellite images available now, but the high albedo of Europa (similar to 0.7 at visible wavelengths) argues against the existence of such extreme roughness.