Liquid water and active resurfacing on Europa

Abstract

Voyager images of the jovian moon Europa show a bright icy surface transected by a complex pattern of long, linear fracture-like markings with a slightly lower albedo1. Very few impact craters are observed, with sizes generally hi the range 10–20 km. Indicators of surface composition and texture include deep IR H2O absorption features2, UV absorption features characteristic of sulphur in a water matrix on the trailing hemisphere3, and a photometric function indicating much more homogeneous scattering than areas of equal albedo on Ganymede and Callisto4. Europe's mean density is 3.03 g cm−3, indicating a primarily silicate composition. However, if the density of the silicates is the same as for Io, the H2O mass fraction would be ∼6%, enough to form an outer layer of H2O > 100 km thick if completely differentiated. Several models have been suggested for the evolution and present state of Europa5–8. We present here arguments for recent resurfacing by H2O from a liquid layer, based on new interpretations of recent spacecraft and Earth-based observations and revised theoretical calculations.