High-energy ion impacts into the sulfur-bearing ice surface of Europa: an atomistic study of chemical transformations

Physics Department and Research Center OPTIMAS, University Kaiserslautern, Erwin-Schrödinger-Straße, 67663 Kaiserslautern, Germany
e-mail: urbassek@rhrk.uni-kl.de

Received: 26 February 2019
Accepted: 19 April 2019

Abstract

Context. The ice surface of Europa is unique due to its high concentration of sulfur compounds such as SO₄, SO₂, and H₂S. Energetic ion impacts originating from the magnetosphere of Jupiter may alter the composition of the ice surface.

Aims. We explore the chemical alteration of the surface due to a 20 MeV sulfur ion impact, for which the most pronounced effects are expected, and monitor the chemical transformations occurring inside the ice.

Methods. Molecular dynamics simulations are used based on a reactive (REAX) potential, which allows for the molecular breakups and the ensuing reactions to be followed on an atomistic scale.

Results. We observe dissociation of SO₄ and also a loss of SO₂, while SO₃ is created; this is in qualitative agreement with laboratory experiments. Hydrolysis of water leads to abundant formation of H⁺, H₃O⁺ and OH⁻; in addition, we predict the presence of both sulfurous acid, H₂SO₃, and sulfuric acid, H₂SO₄, as well as traces of carbonic acid, H₂CO₃. The irradiation produces H₂ and O₂, which are free to escape from the surface, in agreement with the tenuous Europa atmosphere detected.

Conclusions. Since magnetospheric sulfur ions have a high mass and may possess large energies, they provide a unique source of high energy deposition in the ice surface of Europa leading to abundant radiolysis fragments and products. In addition, sulfur compounds existing in the ice are chemically transformed, for example, by sulfite formation.