Penitente Formation is Unlikely on Europa

Europa’s icy surface is unknown at the meter scale. The airless surface of Europa yields a large mean-free path for water (~10⁵ m), leading to efficient sublimation, instead of the differential sublimation modulated by self-illumination that is required for penitentes [1]. Conditions for penitentes include: (1) low humidity, (2) strong insolation, and (3) low temperature. A fourth parameter, sublimation rate, is an important factor in the speed at which the penitentes form. Conditions (1) and (3) can be combined into a requirement that the snow or ice resides in a temperature-pressure regime close to the vapor-pressure curve. If it deviates too far from this curve, then either liquid phase processes dominate, or there is too little vapor for a temperature differential to arise across the surface, which is required to initiate the feedback instability that leads to penitente formation. Another important aspect of conditions (1) and (2) is that the surface cannot be treated simply as a reflecting surface. Self-illumination is insufficient; vapor diffusion above, and heat conduction within the snow is required. Differential sublimation is ultimately what makes penitentes possible. The geometry of self-illumination is an important component of the sublimation differential, but the temperature gradient within the snow or ice, and the vapor diffusion at the solid-gas interface, are critical. This is significant as the model cited by Hobley et al. (2018) was a surface self-illumination model. The authors of that work acknowledged the limitations of their model, but Hobley et al. (2018) extrapolated nevertheless. Where the physics of penitentes fails on Europa is in the requirement for sufficient vapor to sustain periodic temperature differentials. Simply put, penitentes cannot form when sublimation dominates and vapor diffusion is non-existent. Efficient sublimation leads to loss of the entire surface. On airless bodies like Europa, the physical concepts of fluid boundary layers and diffusion break down, Europa’s surface operates in the free-molecular flow regime, where particle mean free paths are of order 10⁵ m. The pressure and temperature regime is too low to form a boundary layer capable of modulating penitente formation.

1. Hand, K. P., Berisford, D., Daimaru, T., Foster, J., Hofmann, A. E., & Furst, B. (2020). Penitente formation is unlikely on Europa. Nature Geoscience, 13(1), 17-19.