Why is Enceladus' surface so radar-bright?

Abstract

International audienceThe recent Cassini RADAR Enceladus E16 data reveal a tectonically-complex surface near the South Pole, including areas of different apparent backscatter characteristics. These appear to be tectonically-delimited, implying that the backscatter intensity is the result of either tectonic resurfacing or surface maturity. Previous studies using Earth-based and distant Cassini observations (Ostro et al., 2006, Icarus 183, 479-490, and references within) revealed that many icy satellites, particularly the most active, exhibit unusually low emissivity and high albedo at and near Ku-band (2.17 cm). However, the extreme nature of Enceladus' brightest surfaces are difficult to account for, and appear to be the most intensely backscattering non-specular surfaces in the solar system: up to ~6 dB sigma0 at 48-57 deg incidence angle. We have developed a 2-dimensional finite element model (Khankhoje et al., 2012, IGARSS) of Ku-band radar interactions with ice in order to explore what surface configurations could produce the observed backscatter, including large ice crystals, fracture planes and liquid films formed, under geologically plausible thermochemical conditions. Different structures and mechanisms that could produce the brightness will then be considered in the context of Enceladus' evolution, with due consideration given to whether these same factors could also apply on other icy worlds. Our preliminary results show that rough surfaces are insufficient. We speculate after Ostro et al. (2006) that these surfaces are the result of coherent backscatter from geologically immature surfaces, resulting in a phenomenon similar to cats eyes. The precise structures and their forming mechanisms (space weathering, deposition, or thermal or mechanical modification) are not yet understood, but several types of organized structure in the 10s of cms range are being considered: (1) quasi-spherical surface scatterers cobbles and small bounders, already observed at 10s of metres scales by the Imaging Science System; (2) complex but regular fractures; and (3) unusually large monocrystals. We will continue to investigate these, whittling them down by a process of deduction