JADE Observations of the Water Group Gas Torus in the Proximity of Europa

Abstract

Water group gas continuously escapes from the surface and atmosphere of Jupiter’s icy moon Europa to form a co-orbiting population of particles, or neutral torus. These neutral tori are of particular interest in the context of astrobiological research, as they provide valuable insights into their source moons—revealing the loss processes and compositions of their parent bodies, altering their local plasma composition, and acting as a sink for particles. Though indirect observations and atmospheric constraints have long been used to infer the physical nature of Europa's neutral torus, direct observations and measurements have largely been limited in both number and scope. As such, this study seeks to expand on previous findings by using (\textit{in situ}) ion measurements from NASA's Jovian Auroral Distributions Experiment (JADE) apparatus in order to further explore and quantify the composition, structure, and interactions of Europa's neutral torus. More specifically, we apply a combination of data fitting and subtraction techniques to produce in-flight estimates of the (\text{H}^+) and (\text{H}_2^+) partial densities in the proximity of Europa's orbit. We then use these values to calculate the overall loss and production rates of the predominantly-(\text{H}_2) torus. We find the ion densities to decrease monotonically with radial distance from 13 to 18 Jovian radii, at a density ratio of (\text{H}_2^+/\text{H}^+ = 7.2 \pm 1.6 %). From these observations, we derive a total (\text{H}_2^+) production rate of (0.6\pm0.1) kg/s and a total (\text{H}_2) loss rate from Europa of (1.0\pm0.4) kg/s.