Stability of Ice/Rock Mixtures with Application to a Partially Differentiated Titan

TL;DR

This study models the thermal evolution of Titan's ice/rock interior, showing that compositional gradients delay but do not prevent differentiation, challenging the idea of a partially differentiated Titan.

Contribution

It introduces a parameterized model of double-diffusive convection to assess Titan's internal differentiation, considering compositional heterogeneity and heat removal over time.

Findings

Double-diffusive convection delays ice melting in Titan's interior.

Realistic density gradients cannot prevent complete differentiation.

Titan is likely fully differentiated despite compositional heterogeneity.

Abstract

Titan's moment of inertia, calculated assuming hydrostatic equilibrium from gravity field data obtained during the Cassini-Huygens mission, implies an internal mass distribution that may be incompatible with complete differentiation. This suggests that Titan may have a mixed ice/rock core, possibly consistent with slow accretion in a gas-starved disk, which may initially spare Titan from widespread ice melting and subsequent differentiation. A partially differentiated Titan, however, must still efficiently remove radiogenic heat over geologic time. We argue that compositional heterogeneity in the major Saturnian satellites indicates that Titan formed from planetesimals with disparate densities. The resulting compositional anomalies would quickly redistribute to form a vertical density gradient that would oppose thermal convection. We use elements of the theory of double-diffusive convection to create a parameterized model for the thermal evolution of ice/rock mixtures with a stabilizing compositional gradient. Simulations are performed for a wide range of initial conditions to account for large uncertainties in material properties and accretionary processes. Ultimately, for realistic density gradients, double-diffusive convection in the ice/rock interior can delay, but not prevent, ice melting and differentiation, even if a substantial fraction of potassium is leached from the rock component. Consequently, Titan is not partially differentiated.