Nature 301, 225 - 226 (20 January 1983); doi:10.1038/301225a0

Liquid water and active resurfacing on Europa

Steven W. Squyres¹, Ray T. Reynolds¹, Patrick M. Cassen¹ & Stanton J. Peale²

¹NASA Ames Research Center, Moffett Field, California 94035, USA
²Department of Physics, University of California, Santa Barbara, California 93106, USA

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.

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