Exobiology

Galileo data provides strong evidence that a subsurface ocean may exist beneath Europa's icy surface (Pappalardo et al., 1999). Surface features offer important clues regarding the character of Europa's icy shell and the potential for biological niches. We conclude that warm ice plumes may induce partial melting within Europa's ice shell, potentially creating accessible intra-ice biological niches.

Lenticulae are pits, domes, and dark spots ~10 km in diameter, inferred to be the surface manifestation of diapirs, warm ice masses that have risen buoyantly through the icy shell (Pappalardo et al., 1998; Rathbun et al., 1998). Indeed, subsolidus convection and diapirism is predicted if the ice shell lies above an ocean and is greater than ~10-25 km thick (Pappalardo et al., 1998; McKinnon, 1999). Theoretical modelling of radiogenic and tidal heating predicts that Europa's equilibrium ice shell thickness is ~20 km (Ojakangas and Stevenson, 1989).

Lenticulae and chaos share many characteristics, and lenticulae can merge to form larger chaos regions (Spaun et al., 1999). Chaos blocks can translate and rotate within a mobile matrix, and block characteristics imply that matrix material was softer than pure warm ice (Collins et al., 2000); however, thermal constraints and ice flow properties preclude complete melt-through of Europa's ice shell (Collins et al., 2000; Stevenson, 2000). A self-consistent model of Europa's thermal state and surface geology is that lenticulae and chaos represent partial melting of a salt-rich ice shell in response to upwelling warm ice diapirs.

Theoretical modeling and Galileo NIMS compositional data suggest that Europa's ocean and ice shell are salt-rich (McCord et al., 1998; Kargel et al., 1999). Analogous to terrestrial ice, salt-rich Europan ice should partially melt when contacted by a heat pulse (i.e., a warm ice diapir), disrupting and mobilizing surface blocks (Head and Pappalardo, 1999). Theoretical modeling shows that runaway tidal heating and melting can indeed occur within Europa's ice shell (Wang and Stevenson, 2000), consistent with this model. Theoretical predictions and geological observations support a Europan ice shell that is >10 km thick, warm, salt-rich, convecting, and situated above liquid water. Warm ice diapirs could circulate subsurface Europan material (including potential nutrients and organisms) between a subsurface ocean and the near surface. We conclude that the most accessible near-surface niches in which to find extent or dormant Europan biology are brine-rich liquid inclusions within the ice, analogous to those recently identified in terrestrial sea ice (Eicken et al., 1999).