The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (<1%) of acetylene and propane also appear to be present. Ammonia is present at a level that does not exceed 0.5%. The radial and angular distributions of the gas density near the closest approach, as well as other independent evidence, suggest a significant contribution to the plume from a source centered near the south polar cap, as distinct from a separately measured more uniform and possibly global source observed on the outbound leg of the flyby.

https://science.sciencemag.org/content/sci/311/5766/1419.full.pdf

Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure.

The Cassini spacecraft flew close to Saturn's small moon Enceladus three times in 2005. Cassini's UltraViolet Imaging Spectrograph observed stellar occultations on two flybys and confirmed the existence, composition, and regionally confined nature of a water vapor plume in the south polar region of Enceladus. This plume provides an adequate amount of water to resupply losses from Saturn's E ring and to be the dominant source of the neutral OH and atomic oxygen that fill the Saturnian system.

/pdf/enceladus-water-vapor-plume-48vc5itqkm.pdf

Enceladus' Water Vapor Plume

The Cassini magnetometer has detected the interaction of the magnetospheric plasma of Saturn with an atmospheric plume at the icy moon Enceladus. This unanticipated finding, made on a distant flyby, was subsequently confirmed during two follow-on flybys, one very close to Enceladus. The magnetometer data are consistent with local outgassing activity via a plume from the surface of the moon near its south pole, as confirmed by other Cassini instruments.

http://www.igpp.ucla.edu/public/mkivelso/refs/PUBLICATIONS/1120985Dough.pdf

Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer

We show that the plasma and magnetic fields in the inner region of Saturn's plasma disk rotate in synchronism with the time-variable modulation period of Saturn's kilometric radio emission. This relation suggests that the radio modulation has its origins in the inner region of the plasma disk, most likely from a centrifugally driven convective instability and an associated plasma outflow that slowly slips in phase relative to Saturn's internal rotation. The slippage rate is determined by the electrodynamic coupling of the plasma disk to Saturn and by the drag force exerted by its interaction with the Enceladus neutral gas torus.

https://science.sciencemag.org/content/sci/316/5823/442.full.pdf

The Variable Rotation Period of the Inner Region of Saturn's Plasma Disk

The Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft observed the saturnian magnetosphere from January 2004 until Saturn orbit insertion (SOI) on 1 July 2004. The MIMI sensors observed frequent energetic particle activity in interplanetary space for several months before SOI. When the imaging sensor was switched to its energetic neutral atom (ENA) operating mode on 20 February 2004, at approximately 10(3) times Saturn's radius RS (0.43 astronomical units), a weak but persistent signal was observed from the magnetosphere. About 10 days before SOI, the magnetosphere exhibited a day-night asymmetry that varied with an approximately 11-hour periodicity. Once Cassini entered the magnetosphere, in situ measurements showed high concentrations of H+, H2+, O+, OH+, and H2O+ and low concentrations of N+. The radial dependence of ion intensity profiles implies neutral gas densities sufficient to produce high loss rates of trapped ions from the middle and inner magnetosphere. ENA imaging has revealed a radiation belt that resides inward of the D ring and is probably the result of double charge exchange between the main radiation belt and the upper layers of Saturn's exosphere.

https://science.sciencemag.org/content/sci/307/5713/1270.full.pdf

Dynamics of Saturn's magnetosphere from MIMI during Cassini's orbital insertion.

Neutral oxygen in the saturnian system shows variability, and the total number of
oxygen atoms peaks at 4 x 10^(34). Saturn’s aurora brightens in response to solar-wind
forcing, and the auroral spectrum resembles Jupiter’s. Phoebe’s surface shows variable
water-ice content, and the data indicate it originated in the outer solar system.
Saturn’s rings also show variable water abundance, with the purest ice in the
outermost A ring. This radial variation is consistent with initially pure water ice
bombarded by meteors, but smaller radial structures may indicate collisional
transport and recent renewal events in the past 10^7 to 10^8 years.

https://science.sciencemag.org/content/sci/307/5713/1251.full.pdf

Willian E. McClintock

Papers

Ultraviolet Imaging Spectroscopy Shows an Active Saturnian System