The Blinking Crystallinity of Europa: A Competition between Irradiation and Thermal Alteration

TL;DR

This study models the competition between irradiation-induced amorphization and thermal crystallization on Europa's surface, revealing depth-dependent crystallinity profiles and seasonal fluctuations that can inform future spectroscopic observations.

Contribution

First to simulate depth-dependent crystallinity profiles on icy moons, integrating non-linear, temperature-dependent processes into a comprehensive multiphysics model.

Findings

Surface is mainly amorphous at the top layer, crystalline below 1 mm.

Crystallinity varies with latitude, highest at equator, lowest at poles.

Seasonal fluctuations in crystalline fraction can reach 35%.

Abstract

The surface of Europa experiences a competition between thermally-induced crystallization and radiation-induced amorphization processes, leading to changes of its crystalline structure. The non-linear crystallization and temperature-dependent amorphization rate, incorporating ions, electrons and UV doses, are integrated into our multiphysics surface model (MSM) LunaIcy, enabling simulations of these coupled processes on icy moons. Thirty simulations spanning 100 000 years, covering the full ranges of albedo and latitude values on Europa, explore the competition between crystallization and irradiation. This is the first modeling of depth-dependent crystallinity profiles on icy moons. The results of our simulations are coherent with existing spectroscopic studies of Europa, both methods showing a primarily amorphous phase at the surface, followed by a crystalline phase after the first millimeter depth. Our method provides quantitative insights into how various parameters found on Europa can influence the subsurface crystallinity profiles. Interpolating upon our simulations, we have generated a crystallinity map of Europa showing, within the top millimeter, highly crystalline ice near the equator, amorphous ice at the poles, and a mix of the two at mid-latitudes. Regions/depths with balanced competition between crystallization and amorphization rates are of high interest due to their periodic fluctuations in crystalline fraction. Our interpolated map reveals periodic variations, with seasonal amplitudes reaching up to 35% of crystalline fraction. These variations could be detected through spectroscopy, and we propose a plan to observe them in forthcoming missions.