Don't give up on the Europa Lander

Abstract:

There is a concern that in the low latitudes of Europa, the ice surface has developed meter-scale bladed roughness, which would pose a hazard to a lander. That concern, by Hobley et al. (Nature Geoscience, 2018), was inspired by the presence of such structures on Earth’s subtropical mountains. However, the formation of these structures (“penitents”, or “penitentes” in Spanish), which is driven by differential absorption of solar radiation, requires melting along with sublimation, which cannot occur on Europa. The troughs 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 of sublimation versus melting. In the Andes of central Chile, penitents of heights 1-4 meters occur in an intermediate altitude range (4000-5200 m). Below 4000 m, the snow instead develops small “suncups” of centimeter-scale amplitude, with melting at all parts of the surface. Above 5200 m, only sublimation occurs; the dry snow surface remains nearly flat. Quoting Lliboutry’s classic paper (Journal of Glaciology, 1954), “Although sublimation is indispensable for penitent formation, in a field of penitents most of the ablation proceeds from melting.” Lliboutry’s mechanism has been confirmed by subsequent studies in the Argentinian Andes, the Pakistani Karakoram, and in the East Pamirs of Tajikistan.

Melting cannot occur on Europa, so the terrestrial mechanism for formation of penitents is ruled out for Europa. Hobley et al. proposed that penitents could form on Europa by means of sublimation only, because of the long time available. But visible-wavelength remote sensing provides a test. Penitents in the Andes are observed to cause a reduction of albedo by about 30% relative to a flat surface, by trapping sunlight. The high area-averaged albedo of Europa’s leading hemisphere (0.72 at 550-750 nm) therefore argues against the existence of extreme surface roughness.