The mid-UV spectrum of irradiated NaCl at Europa-like conditions

TL;DR

This study confirms that irradiated NaCl on Europa's surface produces a mid-UV absorption feature, supporting the hypothesis that NaCl is a key component of Europa's subsurface ocean, based on laboratory experiments and Hubble observations.

Contribution

The paper provides laboratory evidence linking irradiated NaCl to the mid-UV absorption feature observed on Europa, confirming its presence in the planet's surface and implications for its ocean composition.

Findings

Irradiated NaCl develops a 220 nm absorption feature.

Laboratory results match Europa's mid-UV spectrum.

NaCl presence suggests sodium and chlorine in Europa's ocean.

Abstract

Recent observations from the Hubble Space Telescope show a mid-UV absorption feature localized to leading hemisphere chaos regions on Europa. The same regions were previously found to have a visible absorption at 450 nm that was attributed to the presence of irradiated NaCl. The lack of any additional diagnostic absorptions for NaCl in the visible spectrum of these terrains made confirmation of this identification difficult. Here we use laboratory experiments to show that NaCl irradiated at Europa's surface temperatures develops an absorption at $\sim$220 nm consistent with the new detection in Europa's mid-UV spectrum, strongly supporting the NaCl identification. Irradiated NaCl in leading-hemisphere chaos terrain would suggest that sodium and chlorine are important components of Europa's subsurface ocean.