Spatial Variations in the Dust-to-Gas Ratio of Enceladus' Plume

TL;DR

This study combines remote sensing and in-situ measurements to reveal significant spatial variations in the dust-to-gas ratio of Enceladus' plume, indicating diverse subsurface conditions across its fissures.

Contribution

It provides the first detailed comparison of dust and gas ratios in Enceladus' plume using multiple instruments and observations, highlighting regional differences.

Findings

Dust-to-gas ratio varies by an order of magnitude across fissures.

Plume composition differences suggest diverse subsurface environments.

Consistent trends observed in remote sensing and in-situ data.

Abstract

On day 138 of 2010, the plume of dust and gas emerging from Enceladus' South Polar Terrain passed between the Sun and the Cassini spacecraft. This solar occultation enabled Cassini's Ultraviolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS) to obtain simultaneous measurements of the plume's gas and dust components along the same lines of sight. The UVIS measurements of the plume's gas content are described in Hansen et al. (2011, GRL 38:11202) , while this paper describes the VIMS data and the information they provide about the plume's particle content. Together, the VIMS and UVIS measurements reveal that the plume material above Baghdad and Damascus sulci has a dust-to-gas mass ratio that is roughly an order of magnitude higher than the material above Alexandria and Cairo sulci. Similar trends in the plume's dust-to-gas ratio are also found in data obtained when Cassini flew through the plume in 2009, during which time the Ion and Neutral Mass Spectrometer (INMS), Radio and Plasma Wave Science instrument (RPWS) and Cosmic Dust Analyzer (CDA) instruments made in-situ measurements of the plume's gas and dust densities (Dong et al. 2015 JGR 120:915-937). These and other previously-published systematic differences in the material erupting from different fissures likely reflect variations in subsurface conditions across Encealdus' South Polar Terrain.