Title: Formation of Metre-Scale Bladed Roughness on Europa’s Surface by Ablation of Ice —- Short Story: Strategic Plan Core Capability/Objective Reference: 1.3 (Landform Evolution Modeling) The journal Nature Geoscience has accepted a manuscript that includes co-authors Jeff Moore of NASA ARC and Ames-based Orkan Umurhan. The paper describes research to model the formation of jagged, sublimation-sculpted ice blades known as penitentes on Jupiter’s moon Europa. On Earth, the sublimation of massive ice deposits at equatorial latitudes under cold and dry conditions in the absence of any liquid melt leads to the formation of these spiked and bladed textures eroded into the surface of the ice. For this process to take place on another planet, the ice must be sufficiently volatile to sublimate under surface conditions and diffusive processes that act to smooth the topography must operate more slowly. The paper presents calculations of sublimation rates of water ice across the surface of Europa, finding that surface sublimation rates exceed those of erosion by space weathering processes in Europa’s equatorial belt (latitudes below 23°), and that conditions in this region therefore favour penitente growth. The authors estimate that penitentes on Europa could reach 15 m in depth with a spacing of 7.5 m near the equator, on average, if they were to have developed across the interval permitted by Europa’s mean surface age. Although available images of Europa have insufficient resolution to detect surface roughness at the multi-metre scale, radar and thermal data are consistent with the authors’ interpretation that the equatorial regions of Europa are systematically different to those at higher latitudes as shown in the accompanying figure. (a) shows the instantaneous total power radar albedo, M, returned from 12.6 cm radar sounding of Europa using the Arecibo telescope. A striking equatorial minimum is revealed, which can be explained by a jagged surface effectively absorbing wavelengths shorter than the scale of its structures. (b) shows the instantaneous nighttime brightness temperatures of Europa’s surface as inferred from Galileo spacecraft data, revealing a very similar equatorial minimum. This may be a consequence of viewing angle effects, whereby due to the radiative scattering occurring within the penitentes, the tips of their blades cool significantly faster than the pits between them; oblique viewing angles will obstruct views of the pit interiors and so proportionately cooler temperatures would be presented to the observer. The results of this study suggest that penitentes could pose a hazard to a future lander on Europa.