The roles of charge exchange and dissociation in spreading Saturn's neutral clouds

TL;DR

This study investigates the processes of charge exchange and dissociation in shaping Saturn's neutral clouds, introducing new modeling approaches and considering ion gyrophase effects, leading to refined understanding of neutral cloud distributions.

Contribution

It presents a novel modeling approach for charge exchange that tracks ions within a neutral background and examines the impact of ion gyrophase on neutral cloud densities.

Findings

High-speed dissociation extends OH influence from 9 to 15 Rs.

Gyrophase significantly affects OH and oxygen densities but not H2O.

Charge exchange produces more extended oxygen clouds than H2O, with OH being the most confined.

Abstract

Neutrals sourced directly from Enceladus's plumes are initially confined to a dense neutral torus in Enceladus's orbit around Saturn. This neutral torus is redistributed by charge exchange, impact/photodissociation, and neutral-neutral collisions to produce Saturn's neutral clouds. Here we consider the former processes in greater detail than in previous studies. In the case of dissociation, models have assumed that OH is produced with a single speed of 1 km/s, whereas laboratory measurements suggest a range of speeds between 1 and 1.6 km/s. We show that the high-speed case increases dissociation's range of influence from 9 to 15 Rs. For charge exchange, we present a new modeling approach, where the ions are followed within a neutral background, whereas neutral cloud models are conventionally constructed from the neutrals' point of view. This approach allows us to comment on the significance of the ions' gyrophase at the moment charge exchange occurs. Accounting for gyrophase: (1) has no consequence on the H2O cloud; (2) doubles the local density of OH at the orbit of Enceladus; and (3) decreases the oxygen densities at Enceladus's orbit by less than 10%. Finally, we consider velocity-dependent, as well as species-dependent cross sections and find that the oxygen cloud produced from charge exchange is spread out more than H2O, whereas the OH cloud is the most confined.