Presentation #109.02 in the session “Europa”.
With liquid water in direct contact with a rock mantle, Europa’s subsurface ocean contains both a solvent (water) and the elemental building blocks necessary for life. However, in the absence of sunlight, life in Europa’s ocean must extract energy from chemical disequilibrium in the environment, which must be maintained by an influx of reactants that prevent the ocean from reaching chemical equilibrium and “entropic death” [Chyba and Hand, 2001]. Recently, Elder and Bland  and Behounkova et al.  showed that tidal dissipation within Europa’s silicate interior can generate substantial melt, which migrates upward over relatively short timescales, and may provide reactants to the ocean via seafloor volcanism. However, neither study evaluated the feasibility of magma actually leaving the mantle and erupting on the seafloor. On the Earth, propagation of melt through the crust is complex and the ratio of intrusive volcanism (magma injected into the crust rather than erupted on the surface) to extrusive eruption may be as high as 10:1 [White et al., 2006]. Intrusion may be even more favored in the absence of plate tectonics (i.e., on Europa). Melt production in Europa’s mantle should not, therefore, be equated with volcanism at the seafloor. Here we apply two different models of dike propagation to Europa’s silicate crust to constrain the conditions under which mantle melt can erupt on the seafloor. The fracture toughness (kc) of the crust, a parameter that is poorly known even in terrestrial settings, determines which model is the most relevant to Europa. If kc is large (model 1), dike propagation is governed by a balance between magma buoyancy and rock strength [Weertman, 1971]. The resulting dikes are long (~1 km), narrow (~10 cm), and freeze quickly, limiting their ability to propagate through a crust ~100 km thick. If kc is small (model 2), dike propagation is governed by a balance between magma buoyancy and viscous resistance [Lister and Kerr, 1991]. The resulting dikes are thicker (~1 m) and propagate fast enough that magma can reach the seafloor if the volumetric flux into the dike is ≥0.1 m2 s-1. Using the results of Elder and Bland , we find that such a flux is plausible if mantle magma is drawn from an area beneath the dike with radius ≥5.5 km. Isolated dikes with large volumetric fluxes are thus more likely to reach the seafloor than numerous small dikes, each with smaller volumetric fluxes. Although dike propagation to the surface is far from inevitable, seafloor volcanism remains a plausible mechanism for sustaining a habitable seafloor environment if the rocks of Europa’s crust are mechanically weak.