Simulations of Europa’s vapor plumes and ice-grain fallout

Abstract

Jupiter’s moon, Europa, is believed to host an ocean of liquid water under a km-thick shell of ice. Water inclusions within the ice or fractures of this solid shell, generated by Jupiter-induced tidal forces, have been proposed as possible mechanism of water-vapor plume generation [1]. Plume activity on Europa was observed in 2012 with the Hubble Space Telescope (HST) [2], from the magnetometer onboard the Galileo probe [3], and later from the Keck Observatory [4]. However these findings are challenged by a number of successive no-detections [5,6]. Europa Clipper, set to launch in 2024, hosts a suite of science  instruments that will search for signs of plume activity and hopefully settle the debate. Plume measurements via the onboard mass spectrometers would probe, indirectly, Europa’s sub-surface ocean. Additionally, on-board imagers will study the composition of surface ice and search for cryovolcanic fallout from potential plumes. In this work, we intend to perform numerical simulations of a plume and estimate the lateral extent of ice grain fallout. We will consider a vapor mass-flow-rate compatible with the HST observations, and a simplified vent geometry, selected compatibly with the 7-hour period of continued observed plume activity [7]. For such parameters, we use the Direct Simulation Monte Carlo (DSMC) method to obtain density, velocity and temperature maps within and near the vent aperture. These simulations include Europa’s gravity and will also indicate the amount of surface accretion due to molecular deposition. As a second step, the density, velocity and temperature fields are fed to a gas-drag model, to study the transport and deposition of ice grains. We compare our results with the findings of [8], that predicted grains to be localized within some tens of meters, vertically, from the vent. Our result will provide maps of lateral resurfacing that can be, as a future activity, converted into synthetic images, useful for a direct comparison with Clipper’s measurements.