Geological evidence for solid-state convection in Europa's ice shell

Abstract

The ice-rich surface of the jovian satellite Europa is sparsely cratered, suggesting that this moon might be geologically active today. Moreover, models of the satellite's interior indicate that tidal interactions with Jupiter might produce enough heat to maintain a subsurface liquid water layer. But the mechanisms of interior heat loss and resurfacing are currently unclear, as is the question of whether Europa has (or had at one time) a liquid water ocean. Here we report on the morphology and geological interpretation of distinct surface features-pits, domes and spots-discovered in high-resolution images of Europa obtained by the Galileo spacecraft. The features are interpreted as the surface manifestation of diapirs, relatively warm localized ice masses that have risen buoyantly through the subsurface. We find that the formation of the features can be explained by thermally induced solid-state convection within an ice shell, possibly overlying a liquid water layer. Our results are consistent with the possibility that Europa has a liquid water ocean beneath a surface layer of ice, but further tests and observations are needed to demonstrate this conclusively.

Effects of insoluble particles on grain growth in polycrystalline ice: Implications for rheology of ice shells of icy satellites

“…Recent studies have revealed that low viscosities due to small grain sizes are required for convective instability in ice shells. Convection can start in an ice shell above an internal ocean if the ice grain size is less than a few milli meters for medium and large sized icy satellites such as Europa, Ganymede, and Callisto (Pappalardo et al, 1998;McKinnon, 1999;Barr et al, 2004;Barr and Pappalardo, 2005;McKinnon, 2006) and if the ice grain size is less than a few hundred microns for the smaller icy satellite Enceladus (Barr and McKinnon, 2007a). Our preliminary results suggest that such small grain sizes are achieved and kept constant by the presence of very small fractions of insoluble dust particles.…”

“…Because the ice viscosity in the grain size sensitive creep regime strongly depends on grain size, many stud ies have examined the critical grain size required for con vective instability in ice shells above internal oceans for various icy satellites (Pappalardo et al, 1998;McKinnon, 1999;Barr et al, 2004;Barr and Pappalardo, 2005;McK innon, 2006;Barr and McKinnon, 2007a). In a convection current, grain size is controlled by several kinetic process es such as grain growth, dynamic recrystallization, and high pressure transformation (Karato, 2008).…”

Probing for Evidence of Plumes on Europa With HST/Stis

“…The combination of geological, compositional, gravity, and induced magnetic field measurements all indicate a contemporary global saline liquid water ocean of ~100 km in depth (Anderson et al, 1998;Pappalardo et al, 1998;Kivelson et al, 2000;Zimmer et al, 2000;Hand & Chyba, 2007). Cycling of the ocean with a silicate seafloor and oxidant laden surface ice could yield an ocean rich with the elements and chemical energy needed to sustain life (Chyba 2000;Chyba & Hand, 2001;Hand et al, 2007;Hand et al, 2009).…”

Methanogenesis as a potential source of chemical energy for primary biomass production by autotrophic organisms in hydrothermal systems on Europa

“…Remote observations of the jovian satellite Europa indicate that a liquid water ocean may lie beneath its solid ice surface [Khurana et al, 1998;Cart et al, 1998;Pappalardo et al, 1998]. Furthermore, the ocean appears to be underlain by a differentiated, rocky interior [Anderson et al, 1998].…”