PARTIALLY MOLTEN MAGMA OCEANS ON IO AND EUROPA

Partial melting in a silicate mantle affects the thermal evolution, surface processes, and ultimately the habitability of a planetary body. On Earth, magma forms only in localized regions within the mantle, but on Io, tidal dissipation is expected to lead to a global partially molten magma ‘ocean’. Partial melting in Europa’s silicate mantle and subsequent volcanic eruptions on its seafloor has been suggested as a possible mechanism to maintain chemical disequilibrium in Europa’s ocean providing an energy source for life [e.g. 1]. We investigate the generation of magma on Io and Europa.

A partially molten magma ‘ocean’ beneath a solid lithosphere has significant implications for heat transport in the mantle. It increases the density contrast between the lithosphere and the mantle, decreases the average viscosity of the mantle, and provides an alternative to conductive heat loss: advective heat loss (volcanic eruptions). We derive scaling laws for mantle convection that include these effects (analogous to solid-state scaling laws [e.g. 3]). To determine the amount and distribution of magma, we adapt a one-dimensional melt migration model developed for decompression melting beneath seafloor spreading zones on Earth [2].

We find that Io has a partially molten layer over 200 km thick, which is 5% molten. The heat loss due to volcanic eruptions exceeds the heat loss due to conduction through a stagnant lithosphere by two orders of magnitude, which is consistent with Io’s high observed eruption rate. The internal heating rate in Europa’s silicate mantle is less certain. Tidal dissipation is expected to be low in comparison to radiogenic heating [4], so we expect an internal heating rate <10¹² W but consider a wider range from 10¹¹ to 10¹⁴ W. We find that for internal heating rates <10¹² W, little to no melting occurs in Europa’s silicate mantle. However, for higher heating rates, Europa could also have a thick partially molten layer. Further work is required to assess the likelihood of eruption, but internal heating rates >10¹² W in Europa’s silicate mantle will lead to magma generation.

[1] Gaidos, E.J. et al. (1999) Science 284, 1631-1633. [2] Hewitt, I.J. and Fowler, A.C. (2008) Proc. R. Soc. A. 464, 2467-2491. [3] Solomatov, V.S. and Moresi, L.N. (2000) JGR 105, 21795-21818. [4] Tobie, G. et al. (2003) Icarus 177, 534-549.