Saturn in hot water: viscous evolution of the Enceladus torus

TL;DR

This paper explores the neutral-neutral interactions in Enceladus's water vapor torus, showing that such interactions can cause rapid viscous spreading and heating, which can be observed with millimeter-wave astronomy.

Contribution

It introduces a model for the neutral water vapor torus around Enceladus, emphasizing viscous evolution driven by neutral interactions and predicting observable signatures.

Findings

Neutral torus undergoes rapid viscous heating and spreading.

The torus's rotational lines can distinguish physical models.

The torus is observable in the millimeter band with Herschel.

Abstract

The detection of outgassing water vapor from Enceladus is one of the great breakthroughs of the Cassini mission. The fate of this water once ionized has been widely studied; here we investigate the effects of purely neutral-neutral interactions within the Enceladus torus. We find that, thanks in part to the polar nature of the water molecule, a cold (~180 K) neutral torus would undergo rapid viscous heating and spread to the extent of the observed hydroxyl cloud, before plasma effects become important. We investigate the physics behind the spreading of the torus, paying particular attention to the competition between heating and rotational line cooling. A steady-state torus model is constructed, and it is demonstrated that the torus will be observable in the millimeter band with the upcoming Herschel satellite. The relative strength of rotational lines could be used to distinguish between physical models for the neutral cloud.