Motional Induction in Ganymede’s Ocean

We investigate the magnetic signature of oceanic circulation in Ganymede’s subsurface ocean using kinematic induction modeling. Our approach couples zonal jet flows from rotating thermal convection simulations with magnetic field models incorporating Ganymede’s internal dynamo and external contributions from Jupiter.

We solve the induction equation in spherical geometry for deep-ocean (493 km) and shallow-ocean (287 km) scenarios with varying magnetic Reynolds numbers. Ocean flows generate a predominantly toroidal magnetic field through the omega-effect, with a weaker poloidal component pervading beyond the conductive ocean layer.

For some, but not all, induction configurations, analysis of the time-averaged Lowes-Mauersberger spectra reveals that ocean-induced signals dominate at spherical harmonic degrees. Deep ocean scenarios with magnetic Reynolds numbers above unity produce surface magnetic signals up to 9 nT. Our results demonstrate that Ganymede’s intrinsic magnetic field creates favorable conditions for detecting subsurface ocean dynamics, thus emphasizing the need for low-altitude orbits for the Juice probe.

Plain Language Summary

Jupiter’s moon Ganymede harbors a vast subsurface ocean. Understanding its dynamics is key to assessing how its circulation may facilitate the exchange of heat and materials essential for life between the moon’s interior and the overlying ice shell. However, since this ocean is buried under kilometers of ice, direct observation of this circulation appears to be impossible.

Our study shows that Ganymede’s ocean currents might actually reveal themselves through magnetic signals. Ganymede is unique among icy moons because it has its own magnetic field, generated deep in its metallic core. When electrically conducting ocean water flows through this magnetic field, it creates additional magnetic signatures that could be detected by spacecraft magnetometers.

Using computer simulations, we find that strong east-west ocean currents may generate magnetic signals up to 9 nT at Ganymede’s surface. The European Space Agency’s Juice mission, en route to study Jupiter’s moons, should be capable of detecting these signals when it orbits Ganymede, with enhanced sensitivity if low-altitude orbits are performed.

This research suggests that magnetic measurements could provide a welcome glimpse into the hidden dynamics of an extraterrestrial ocean, opening a new window into worlds that might support life.

Motional Induction in Ganymede’s Ocean, Geophysical Research Letters (open access)

Astrobiology,