Did Galileo Pass Through a Plume on E26?

Abstract:

On eleven of its orbits around Jupiter, the Galileo spacecraft passed close to Europa. Recently, Huybrighs et al. (2020, https://doi.org/10.1029/2020GL087806) suggested that data acquired by the Energetic Particle Detector (EPD) on pass E26 demonstrate that the spacecraft encountered a vapor plume shortly after closest approach (near 400 km altitude). We believe a more comprehensive analysis of the data is needed to draw this conclusion and we will present another interpretation. Huybrighs et al. used Monte Carlo simulations of the trajectories of ~100 keV protons in a magnetic field extracted from an MHD model to account for features present in the relevant EPD channel. The results of the analysis depend strongly on the form of the modeled magnetic field, which differs significantly from the measured field data. Although we agree that data showing the loss of protons below about 100 keV over a limited range of pitch angles could indicate an interaction with neutral material, we would not expect more energetic protons to be lost at the exact same locations and by close to the same fraction in a pass over a localized plume (since their charge-exchange cross-sections with H and O drop precipitously above 100 keV). The loss of protons across a wide energy range (maybe up to 1 MeV) is likely to have resulted from another mechanism; probably the EPD detector was viewing forbidden particle trajectories in the vicinity of Europa (i.e., those that, when traced backward in time, intersect the moon) and not trajectories of particles that had charge exchanged with plume neutrals. We conclude that the presence of a plume on the E26 pass remains unproven.