ALMA Spectroscopy of Europa: A Search for Active Plumes

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, H₂CO, SO₂ and CH₃OH. 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 H₂CO 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.