Geodynamics of the High Pressure Ice Layer on Ganymede

Jupiter’s moon Ganymede, the largest moon in the Solar System, is the main focus of the JUICE mission, which will observe its surface and measure its interior with unprecedented detail (Van Hoolst et al., 2024). In contrast with smaller moons such as Europa or Enceladus where an ocean is in contact with the silicate interior, Ganymede contains a high pressure (HP) ice layer between its ocean and the rocky core. Thus, on Ganymede, the dynamics in the high pressure ice layer control the exchange of heat and chemical species between the ocean and rocky interior.

 

The thickness of the HP ice layer is not well constrained, and interior structure models suggest thicknesses around 400 km, with values as low as 100 km (Kalousova et al., 2018) and as high as 700 km (Vance et al., 2018). Depending on the thickness of this layer, various polymorphs of HP ice might appear (Hussmann et al., 2015), such as ice V and ice VI, and for a sufficiently cold ocean also ice III (Journaux et al., 2020). Here we focus on ice V and ice VI, as they might exhibit different viscosities that in turn can substantially affect the convective behavior of the HP ice layer. Rheological experiments of ice V and ice VI are rare, but existing studies (Sotin & Poirier, 1987) indicate that ice V can be harder to deform than ice VI, and the viscosity ratio can reach up to three orders of magnitude.

 

We investigate the dynamics of Ganymede’s HP ice layer using the geodynamical code GAIA (Hüttig et al., 2013). Our models use the viscosity formulation of Kalousová et al. (2018) that has been derived from rheological experiments (Sotin et al., 1985; Durham et al., 1996). We test models where the HP ice layer of Ganymede is subdivided into ice V and ice VI layers. Our models vary the reference viscosity of the ice VI layer between 10¹⁵ and 10¹⁸ Pa s and apply a viscosity contrast between the ice V and ice VI layers of up to 1000. Similar to Choblet et al. (2017), we limit the temperature to the melting temperature of the HP ice layers and compute the amount of melt produced throughout the evolution. Our models consider a decaying heat flow boundary condition at the ice-rock boundary using values from Choblet et al. (2017), and assuming that the heat flow exponentially decreases from 40, 20 or 10 mW/m² at 4.5 Gyr ago to a present-day value of 5 mW/m².

 

Our models show that the ice shell dynamics substantially change with the increase of viscosity contrast between ice V and ice VI, leading eventually to a two layered convection structure. Heat and material transport from the ice-rock interface to the ocean occurs in pulses, when convective plumes can penetrate through the upper, high-viscosity ice V layer. Future models will include the effects of tidal heating and track the redistribution of impurities, i.e., salts, through the high pressure ice layers of Ganymede.