Understanding the escape of water from Enceladus

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 × 10 25 H 2 O/s, and the second with a much larger source localized at the south pole with a source rate ∼10 28 H 2 O/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 10 28 H 2 O/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.