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

Abstract

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.

Simulating Radiative Heat Transfer in Multi‐Scattering Irregular Surfaces: Application to Snow and Ice Morphologies on Europa

“…penitente size. The key quantity of interest is the valley-peak temperature difference δT since warmer troughs promote penitente growth (Macias et al, 2023).…”

“…Penitente growth primarily requires higher sublimation rates at the valleys compared to those at the peaks (Claudin et al, 2015), which occurs when valleys are sufficiently warmer than the peaks. In Macias et al (2023), the required valley-peak temperature difference, δT, is a function of penitente shape and peak temperature. Using Equation 10 from Macias et al (2023), as well as penitente heights and corresponding noontime peak temperatures presented in Figure 8, the required δT values are approximately 0.6 K (H p = 0.05 m), 1.7 K (H p = 0.05 m), and 2.6 K (H p = 0.05 m).…”

“…In Macias et al (2023), the required valley-peak temperature difference, δT, is a function of penitente shape and peak temperature. Using Equation 10 from Macias et al (2023), as well as penitente heights and corresponding noontime peak temperatures presented in Figure 8, the required δT values are approximately 0.6 K (H p = 0.05 m), 1.7 K (H p = 0.05 m), and 2.6 K (H p = 0.05 m). For all penitente heights, the simulated δT values exceed the threshold δT values, signifying potential penitente growth.…”

“…δT increases with penitente size, suggesting that penitente growth may accelerate as the penitente size increases. However, according to Macias et al (2023), the required δT for penitente growth increases with penitente size. To further extrapolate the effects of penitente size on δT, we must consider dynamical surface morphologies, peculiar temperature profiles such as the "W" temperature profile, and sublimation effects not present in this study.…”

“…The model can simulate RHT in irregular surfaces due to incident radiation by a moving direct light source, incorporating wavelength-dependent multi-scattering of photons in the granular interior and accounting for internal heat conduction. A model developed by Macias et al (2023) complements our RHT model…”