Evidence for Recent Solid-State Convection on Europa: The Nature of Pits, Domes, Spots, and Ridges

J. W. Head, R. T. Pappalardo (Brown Univ.), R. Greeley, R. J. Sullivan (ASU), C. Pilcher (NASA HQ), G. Schubert, W. Moore (UCLA), M. Carr (USGS Menlo Park), J. Moore (NASA Ames), M. Belton (NOAO)

At high resolution Galileo imaging of a portion of Jupiter's satellite Europa reveals that its late-stage geology is dominated by circular to elliptical pits, domes, and dark spots about 7-15 km across, spaced from 5 to 20 km apart, as well as a larger region of chaotic terrain. A sequence of increasing surface disruption appeas to be represented by these features. Domes provide evidence for upwarping and deformation of the preexisting surface, pits commonly show chaotic texture suggestive of collapse, and spots are suggestive of localized extrusion and/or thermal alteration of the surface. The morphology and structure of these features, their areal density, and the interpreted associated vertical motion, localized heat, and magmatism suggests that they are the manifestation of relatively warm subsurface diapirs. Ridges and triple bands on Europa also show evidence for formation and modification by upbowing of material from below, and the array of ridge morphologies can be arranged in a possible evolutionary sequence. A possible mechanism for their formation is solid-state convection of Europa's icy shell, which is predicted theoretically if the ice is sufficiently thick and underlain by a warm layer, such as liquid water. Estimates for likely effective ice viscosity suggest that solid-state convection would initiate in a Europan ice shell of about 7 km thickness. This scale is similar to the observed spacing of pits, domes, and spots, consistent with a thermal convection origin for these surface features. Longer fractures related to tidal deformation may also initiate linear diapirism.