An unusual fate for Ganymede’s silicate crust?

Ganymede is an emblematic moon: the largest in the solar system, it possesses an intrinsic magnetic field [1] and likely harbors an internal ocean. As such, it is the main planetary object that underlies the ESA JUICE mission, to be launched in 2023. Models of Ganymede’s internal structure have been developed and revived prior to the mission in order to propose plausible end-members that JUICE may discriminate. These include focus on the possibly exotic dynamo[2–4], the interaction of its thermal history with its orbital evolution involving a Laplace-like resonance [5], an allegedly complex sandwiched structure for the hydrosphere [6] or the specific heat and mass transfer that may occur within a thick “mantle” of warm high-pressure ice polymorphs [7–9].

In this context, a possibly overlooked portion of the interior is the crust, i.e. the layer that forms as the result of silicate mantle melting. Crustal rocks have a distinct composition as, upon melting, specific minerals enter the liquid phase preferentially. While the liquid phase is generally buoyant, the density of volcanic rocks that solidify from such melts depends on pressure (and temperature) [10] with potentially major consequences on interior dynamics: for example, the negative buoyancy of basaltic rocks relative to the ambiant mantle above a given pressure is proposed as the main contributor to the formation of Earth’s continental crust whose thickness is remarkably uniform (≃ 40 km) besides tectonically active regions[11]. Using numerical models for mantle convection, we assess the melting potential of Ganymede’s rock mantle. Magma emplacement occurs either directly at the base of Ganymede”s hydrosphere or, deeper, as intrusions beneath a crust that already formed. We investigate the stability of the crust in light of expected density variations: the downward mass transfer associated to foundering of mafic cumulates potentially enhances chemical exchanges between the rock layer and the hydrosphere.

References

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