Non-synchronous rotation on Europa driven by ocean currents

TL;DR

This study models how ocean currents beneath Europa's ice shell can cause non-synchronous rotation, providing insights into ice shell dynamics and constraining its physical properties through high-resolution ocean simulations.

Contribution

It introduces a novel model linking ocean-driven torques to ice shell rotation, incorporating viscoelastic deformation and using high-resolution ocean circulation data.

Findings

Ice shell may exhibit negligible, constant, or oscillatory drift.

Expected rotation rate exceeds approximately 30 meters per year.

Future spacecraft observations can validate the model predictions.

Abstract

It has been suggested that the ice shell of Jupiter's moon Europa may drift non-synchronously due to tidal torques. Here we argue that torques applied by the underlying ocean are also important and can result in non-synchronous rotation (NSR). The resulting spin rate can be slightly slower than the synchronous angular rate that would have kept the same point of the ice shell facing Jupiter. We develop an ice shell rotation model, driven by ocean stress calculated using a high-resolution state-of-the-art ocean general circulation model, and take into account the viscoelastic deformation of the ice shell. We use the ice shell model results together with observed limits on the ice shell drift speed to constrain ice shell parameters such as effective viscosity, which is currently uncertain by at least four orders of magnitude. Our results suggest, at best, sluggish ice shell convection. Depending on the relaxation time scale of the ice shell and on the ocean currents, the ice shell may exhibit negligible drift, constant drift, or oscillatory drift superimposed on random fluctuations. The expected rotation rate exceeds $\sim$30~m/yr; future spacecraft observations can be used to test these predictions and yield insight into the properties of the ice shell and underlying ocean.