ALMA Spectroscopy of Europa: A Search for Active Plumes

TL;DR

This study used ALMA to search for molecular emissions from Europa's potential plumes, setting upper limits on their composition and suggesting differences from Enceladus's plumes, advancing understanding of Europa's subsurface activity.

Contribution

First ALMA spectral line observations of Europa's atmosphere to constrain plume compositions and compare with other icy moons, providing new upper limits on molecular abundances.

Findings

No active plumes detected with significant molecular emission.

Upper limits on HCN, H$_2$CO, SO$_2$, CH$_3$OH abundances.

Possible chemical differences between Europa and Enceladus plumes.

Abstract

The subsurface ocean of Europa is a high priority target in the search for extraterrestrial life, but direct investigations are hindered by the presence of a thick, exterior ice shell. Here we present spectral line and continuum maps of Europa obtained over four epochs in May-June 2021 using the Atacama Large Millimeter/submillimeter Array (ALMA), to search for molecular emission from atmospheric plumes, with the aim of investigating subsurface processes. Using a 3D physical model, we obtained upper limits for the plume abundances of HCN, H$_2$CO, SO$_2$ and CH$_3$OH. If active plume(s) were present, they contained very low abundances of these molecules. Assuming a total gas production rate of $10^{29}$ s$^{-1}$, our H$_2$CO abundance upper limit of $<0.016$\% is more than an order of magnitude less than measured in the Enceladus plume by the Cassini spacecraft, implying a possible chemical difference between the plume source materials for these two icy moons.