Inferring Io's interior from tidal monitoring

Io, the innermost of Jupiter’s satellites, is the most volcanically active, and probably one of the most remarkable body in the outer Solar System [1]. The total power emitted from Io's surface is estimated to about 100 TW at present [2,3], which is several orders of magnitude greater than can be explained by radiogenic heating alone. Io’s interior process manifests on the surface as extreme volcanic activity, providing some clues about the thermal state of its interior through the eruption properties and global distribution of volcanism [e.g. 4,5].

A variety of models have been proposed to determine the mechanism at the origin of the huge tidal dissipation in Io’s interior [e.g. 6,7,8,9]. The presence of a partially molten layer in the upper mantle of Io is broadly consistent with these interior models prediction, as well as with magnetic induction measurements [10], although this interpretation has recently been questioned [11]. However, while a high concentration of melts below the lithosphere is in line to explain the heat production and the heat released by volcanic activity, the degree of melting of this subsurface layer, going from moderately molten mantle to fully liquid subsurface ocean [7], is still largely debated.

The distribution of melt within Io is a key question driving future mission concept dedicated to Io’s investigation (IVO), planning to test these interior models via a set of geophysical measurements. One way to characterize it is through the tides. The measurement of the tidal deformation (either via the potential Love number k2 or the phase lag) can provide information about the internal structure of planetary bodies [e.g. 12].

The total amount of heat produced by tidal friction and its distribution in the interior is intimately linked to the structure and thermal state of Io's interior, especially the distribution of temperature and melt fraction [8,9]. Describing the mechanical response of partially molten rocks on a wide range of melt fraction is essential to correctly describe the tidal friction in Io, partial melt severely affecting viscoelastic properties of rocks. Understanding the retroaction between melt distribution and heat production is thus crucial to explain the heat budget of Io and to understand its tidally-induced volcanism.

In this context, the goal of the study is to calculate the tidal response of Io’s interior [13] for various distribution of melt within the mantle, to discriminate them in future tidal monitoring. A coherent melt profile between the sublithospheric partially molten layer and the underlying mantle is considered following petrological and two-phase flow arguments [4,14]. A rheological parameterization is developed in order to take into account the role of melt fraction on the elastic and viscous parameters of Io's partially molten interior. The results are analyzed in terms of tidal Love number k₂, taking into account a re-evaluation of the heat production by tidal friction in Io's partially molten interiors, by quantifying the role of melt fraction on both shear and bulk dissipation.

Our calculations show that the amount of melt fraction within the 100-km thick asthenospheric partially molten layer could be discriminated from tidal k₂ measurements and heat flow patterns. Depending on the assumed value of viscosity at the melting point and extent of melting beneath the lithosphere, three groups of internal structure models able to reproduce Io’s heat budget of 100 TW can be distinguished : (1) low viscosity mantle (<10¹⁷ Pa s) and moderately molten mantle and asthenosphere (<7%-<20% respectively) ; (2) high viscosity mantle (>10¹⁸ Pa s) with high melt content in the asthenosphere (>30%) ; (3) fully liquid magma ocean beneath a highly dissipative crust. Both mantle dissipation (1) and crust dissipation (2) models result in a comparable heat flow pattern, with maximal dissipation at the poles but can be distinguished by k₂ measurements. The asthenospheric dissipation (2) model has a k₂ Love number only slightly higher than the mantle dissipation model, but results in a totally different heat flow pattern. Note that our computed dissipation included bulk dissipation in addition to shear dissipation, which further enhances dissipation in the equatorial region for the asthenospheric dissipation (2) model. For the two other dissipation model, bulk dissipation has a negligible effect. 

Acknowledgements

The present work received financial supports from the ANR OASIS project and from CNES (Europa Clipper/SUDA, JUICE).

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