Recovering the hydrocarbon distributions in Saturn's upper atmosphere through mathematical inversion.

Abstract

The spacecraft Voyager 2 UVS occultation experiments measured the ultraviolet absorption properties of the upper atmosphere of Saturn. In the extreme-ultraviolet wavelength region from 1300 Å to 1700 Å, CH₄, C₂H₂, C₂H₄, C₂H₆ and C₄H₂ are the major absorbers in the Saturnian upper atmosphere. In this dissertation, using the linear constrained matrix method, the Saturnian stellar EUV occultation data has been inverted. This results in, for the first time, the number density distributions of the 5 major hydrocarbons over an altitude range from 1030 km to 630 km. The synthetic transmission curves based on these inverted distributions exhibit excellent agreement with the observed transmission curves in all usable wavelength channels. There are two major findings in the Saturnian upper atmosphere from the inverted hydrocarbon profiles: (1) The number densities of CH₄ and C₂H₆ are comparable. It is even likely that there is more C₂H₆ than CH₄ in Saturn's upper atmosphere between 1000 km and 800 km. (2) C₂H₄, rather than C₂H₂, is the 3rd most abundant hydrocarbon. From 1000 km down to 600 km, the number density of C₂H₄ is greater than the number density of C₂H₂. These two findings are generally in conflict with the expectations from photochemical models for the atmospheres of the giant planets.