E. C. Sittler

The Neutral Cloud and Heavy Ion Inner Torus at Saturn

Abstract Saturn has a torus of neutral water molecules and dissociated water molecule products (OH and O) which coexists with the icy satellites and the E-ring. Here we calculate the morphology of this cloud using Voyager plasma data and recent laboratory data on water molecule sputter yields and energy distributions. In this model the icy satellites are sources of sputtered water vapor. The molecules so produced co-orbit with the satellites, forming a cloud of neutrals. This cloud can act to limit the amount of H + close to the orbital plane, via charge exchange, and it is the source of the heavy ion plasma via ionization. The plasma production rates are also calculated and show a structure with distance from Saturn and with distance from the orbit plane which suggests that the heavy ion plasma is not equilibrated inside ∼7 R s . The source rates are consistent with ion lifetimes of ∼10 6 –10 7 sec near Dione and Tethys; however, they clearly indicate that additional sources of plasma are required inside ∼4 R S . Finally, we examine the possibility that the E-ring may be a precipitate of this neutral cloud initiated by low-energy ion-molecule reactions.

Mass Loss Processes in Titan's Upper Atmosphere

Although Titans atmospheric column density is about ten times that of the Earths, its measured 15N/14N ratio suggests that considerable escape has occurred or that Titans original material had a ratio closer to that of cometary materials. A number of active escape processes have been proposed: thermal escape, chemical-induced escape, slow hydrodynamic escape, pick-up ion loss, ionospheric outflow and plasma-ion-induced atmospheric sputtering. These loss processes and relevant simulations are reviewed in light of recent Cassini data.

Energetic nitrogen ions within the inner magnetosphere of Saturn

[1] We investigate the importance of nitrogen ions within Saturn’s magnetosphere and their contribution to the energetic charged particle population within Saturn’s inner magnetosphere. This study is based on the Voyager observations of Saturn’s magnetosphere and Cassini observations. The latter have shown that water group ions dominate both the plasma and energetic particle populations but that nitrogen ions over a broad range of energies were observed at � 5% abundance level. In the outer magnetosphere, methane ions were predicted to be an important pickup ion at Titan and were detected at significant levels in the outer magnetosphere and at Titan. O + ions were found to be the dominant heavy ion in the outer magnetosphere, � 60%, with methane ions being � 30% of the heavy ions and N + being a few percent. The two major sources of nitrogen ions within Saturn’s magnetosphere are Titan’s atmosphere and primordial nitrogen trapped in the icy crust of Saturn’s moons and its ring particles deep within the magnetosphere. It is important to understand the source, transport, and sinks of nitrogen in ordertodeterminewhethertheyhaveaprimordialoriginorarefromTitan’satmosphere.The energetic component is important, since it can come from Titan, be implanted into the surfaces of the icy moons, and reappear at plasma energies via sputtering obfuscating the ultimatesource.Aswewillshow,suchimplantationofnitrogenionscanproduceinteresting chemistry within the ice of Saturn’s moons. The emphasis will be on the nitrogen, but the oxygenandotherwatergroupionsarealsoconsidered.Wearguethatneutralcloudsofheavy atoms and molecules within Saturn’s outer magnetosphere may be the dominant source of energetic heavy ions observed within the inner magnetosphere. Pickup heavy ions in the outer magnetosphere have energies � 1–4 keV when born. If they diffuse radially inward, whileconservingthefirstandsecondadiabaticinvariants,theycanhaveenergiesgreaterthan several hundred keVinside of Dione’s L shell. We will show how observations relate to the various sources and acceleration processes such as ionization, collisions, wave-particle interactions, and radial diffusion.

Cassini Plasma Spectrometer Investigation

Cassini/Huygens is a joint project of NASA and the European Space Agency designed to explore the Saturnian system in depth during its four-year mission. Cassini, the orbiter spacecraft, will carry twelve hardware investigations while Huygens, the Titan atmospheric probe, will carry an additional six. The Cassini Plasma Spectrometer (CAPS), one of 12 orbiter investigations, includes 3 plasma sensors designed to cover the broadest possible range of plasma energy, composition, and temporal variation. It is conservatively estimated that CAPS will provide a factor of ten or more improvement in measurement capabilities over those of the Voyager spacecraft at Saturn.