Saturn's upper atmosphere during the Voyager era: Reanalysis and modeling of the UVS occultations

TL;DR

This study reanalyzed Voyager UVS occultation data to produce updated profiles of Saturn's upper atmosphere, clarifying temperature discrepancies and exploring atmospheric variability, serving as a baseline for future Cassini data interpretation.

Contribution

It provides a comprehensive reanalysis of all six Voyager UVS occultations, offering new vertical profiles and insights into Saturn's thermospheric structure and variability.

Findings

Revised temperature and density profiles of Saturn's upper atmosphere.

Identification of variability in methane profiles indicating atmospheric dynamics.

Explanation of previous temperature discrepancies and density profile shapes.

Abstract

The Voyager 1 and 2 Ultraviolet Spectrometer (UVS) solar and stellar occultation dataset represents one of the primary, pre-Cassini sources of information that we have on the neutral upper atmosphere of Saturn. Despite its importance, however, the full set of occultations has never received a consistent, nor complete, analysis, and the results derived from the initial analyses over thirty years ago left questions about the temperature and density profiles unanswered. We have reanalyzed all six of the UVS occultations (three solar and three stellar) to provide an up-to-date, pre-Cassini view of Saturn's upper atmosphere. From the Voyager UVS data, we have determined vertical profiles for H2, H, CH4, C2H2 , C2H4, and C2H6, as well as temperature. Our analysis also provides explanations for the two different thermospheric temperatures derived in earlier analyses (400-450 K versus 800 K) and for the unusual shape of the total density profile noted by Hubbard et al. (1997). Aside from inverting the occultation data to retrieve densities and temperatures, we have investigated the atmospheric structure through a series of photochemical models to infer the strength of atmospheric mixing and other physical and chemical properties of Saturn's mesopause region during the Voyager flybys. We find that the data exhibit considerable variability in the vertical profiles for methane, suggesting variations in vertical winds or the eddy diffusion coefficient as a function of latitude and/or time in Saturn's upper atmosphere. The results of our reanalysis will provide a useful baseline for interpreting new data from Cassini, particularly in the context of change over the past three decades.