Constraining Time Variations in Enceladus' Water-Vapor Plume With Near-Infrared Spectra from Cassini-VIMS

TL;DR

This study uses Cassini-VIMS near-infrared spectra to analyze the variability of water vapor in Enceladus' plume, confirming emission features and examining their correlation with orbital phase.

Contribution

It provides the first detailed analysis of water vapor emission variability in Enceladus' plume using Cassini-VIMS data, complementing previous UV observations.

Findings

Water vapor emission line strength corresponds to a column density of ~10^20 molecules/m^2.

Water vapor flux does not systematically vary with Enceladus' orbital phase.

Column density variations on different timescales are possible and warrant further study.

Abstract

Water vapor produces a series of diagnostic emission lines in the near infrared between 2.60 and 2.75 microns. The Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft detected this emission signal from Enceladus' plume, and so VIMS observations provide information about the variability of the plume's water vapor content. Using a data set of 249 spectral cubes with relatively high signal-to-noise ratios, we confirmed the strength of this water-vapor emission feature corresponds to a line-of-sight column density of order 10^20 molecules/m^2, which is consistent with previous measurements from Cassini's Ultraviolet Imaging Spectrograph (UVIS). Comparing observations made at different times indicates that the water-vapor flux is unlikely to vary systematically with Enceladus' orbital phase, unlike the particle flux, which does vary with orbital phase. However, variations in the column density on longer and shorter timescales cannot be ruled out and merit further investigation.