Understanding the escape of water from Enceladus

Abstract

On 14 July 2005, Cassini passed within 175 km of Enceladus’ surface enabling a direct in situ measurement of water escaping from the surface by the Ion and Neutral Mass Spectrometer (INMS) and the observation of a stellar occultation by the Ultraviolet Spectrometer (UVIS). We have developed a three‐dimensional, Monte Carlo neutral model to simultaneously model the INMS and UVIS measurements of water gas density and column density, respectively. The data are consistent with a two‐component atmosphere; the first with a weak, distributed source on the surface which, if global, has a source rate of ∼8 × 1025 H2O/s, and the second with a much larger source localized at the south pole with a source rate ∼1028 H2O/s. This latter source is possibly coincident with the “tiger stripe” series of fractures revealed by the Imaging Science Subsystem instrument where the ice was measured to be warmer than the surrounding regions by the Composite Infrared Spectrometer instrument. We estimate the plasma mass loading rate due to interaction between the plume and magnetospheric plasma is 2–3 kg/s for a plume source of 1028 H2O/s. Pickup of water group ions in the plume slows down the plasma to ∼10 km/s relative to Enceladus in the region of, and downstream of, the south polar plume. This is consistent with the mass loading rate inferred from magnetic field perturbations detected during the Cassini flyby and suggests an additional source may be needed to explain the plasma flow deflections detected by the Cassini Plasma Spectrometer.

Mass loading of Saturn's magnetosphere near Enceladus

“…[25] More recently, Burger et al [2007] have developed a three-dimensional Monte Carlo neutral cloud model of the Enceladus plume to simultaneously model the INMS water-vapor density profile along Cassini's trajectory and the UVIS measurements of the column density of the plume. They show that the observations are consistent with a two-component source model of water.…”

“…This process leads to the generation of fast neutrals (which escape) and new ions which begin with Keplerian velocities and extract momentum from the flow as they are accelerated up to corotation velocities. Recent analysis by Burger et al [2007] suggests that in the vicinity of Enceladus, charge exchange dominates over electron impact ionization and photoionization by a factor of $100 in ''mass loading'' the flow. In this work, we use the term ''mass loading'' loosely to denote all newly born ions (whether from photo or electron impact ionization or from charge exchange) that extract momentum from the background flow.…”

Science goals and mission concept for the future exploration of Titan and Enceladus

“…The deflected plasma forms a system of currents that lead to measurable deviations in the planetary dipolar magnetic field and the corotational electric field (Dougherty et al, 2006;Kriegel et al, 2009Kriegel et al, , 2011Jia et al, 2010) and charge exchange collisions lead to an effective deceleration of the corotational plasma. On the other hand, the plume gas feeds a neutral torus around the orbit of Enceladus (Burger et al, 2007;Fleshman et al, 2010). Electron impacts and photoionization ionize neutrals in the plume and torus, thus replenishing the magnetospheric plasma (Tokar et al, 2006(Tokar et al, , 2008Fleshman et al, 2010).…”

Saturn Plasma Sources and Associated Transport Processes

“…All the events in Figure 19 had an energy signal, so both M/Q and M are determined, and species such as H 2 + and He ++ , which have the same value of M/Q and coincide in Figure 19, can be separated. (Burger et al, 2007;Jia et al, 2010;Dong et al, 2011). The H + has several potential sources including the solar wind and dissociation of the Enceladus water, but the largest source appears to be ionization of the extensive neutral H cloud arising from Saturn's atmosphere (e.g., Melin et al, 2009).…”