How to determine if Io has a magma ocean

Abstract:

Peale et al. [1979, Science 203] predicted that Io would have a thin lithosphere over a magma ocean, with heat loss via conduction, but Voyager 1 revealed tall mountains that appear to be uplifted blocks of crust, requiring a thick, rigid lithosphere. The tidal heat from within Io is hypothesized to instead come to the surface via the advection (bulk transport) of magma through the lithosphere in a “heat-pipe” mechanism; this same process is expected on any planetary body with a cooling magma ocean. The Galileo mission and telescopic results suggest that some of Io’s lavas are hotter than any recent eruptions on Earth, although likely prevalent on the early terrestrial planets. Such high-temperature eruptions are expected if the mantle undergoes significant melting, consistent with a magma ocean. The magnetic induction interpretation of Khurana et al. [2011, Science 332] requires at least 20% melt in Io’s upper mantle, but uncertainty persists about whether plasma interactions in an asymmetric atmosphere could mimic the induction signature. The leading alternative is partial melt in Io’s mantle, which rises and erupts rather than forming a magma ocean. An intermediate degree of melting would need to overcome the expected rapid segregation of the liquid and solid phases.

If Io has a detached lithosphere over a global magma ocean, then two independent measurements should provide definitive evidence. One of these is tidal k2, the ratio of imposed gravitational potential from Jupiter (which is well known) and the induced gravitational potential from the deformation of Io. The second measurement is libration amplitude of Io. Also needed are new magnetic induction measurements with better data on Io’s plasma environment and flybys optimized to the best times and places for measuring variations in the magnetic field, plus new laboratory measurements of electrical conductivities of relevant planetary materials. Furthermore, these measurements together provide a strong constraint on lithospheric thickness and rigidity, a key test of the heat pipe model. Global maps of heat flow and volcanic and tectonic features on Io, and measurement of lava temperatures and compositions, will provide additional data to interpret whether Io has a magma ocean. A future Io mission such as the Io Volcano Observer (IVO) can provide these measurements.