Laboratory band strengths of methane and their application to the atmospheres of Jupiter, Saturn, Uranus, Neptune, and Titan

Abstract

This paper reports laboratory studies of the visible spectrum of methane at column densities between 0.4 and 5 km-am and confirms the identification of bands at 4410, 4590, 4860, 5090, 5430, 5760, and 5970 A as caused by methane. Detailed equivalent-width measurements at 15 different pressure path lengths are employed to determine curves of growth and band strengths for the bands at 4410, 4860, 5430, and 5760 A. Using the curve-of-growth measurements in the reduction of planetary observations, the methane abundances in the atmospheres of Jupiter and Saturn are found to be between a factor of 3 and 4 larger than previously accepted values based on the analysis of the 3 nu(3) band at 1.1 microns, while the amount on Titan is significantly less than that obtained from an analysis of the same band with the assumption of a pure methane atmosphere. The present results, when combined with the band analysis, suggest a surface pressure on Titan of at least 0.4 atm. Extrapolation of these laboratory data to observations of Uranus and Neptune lead to single-air-mass column densities of 5.8 and 7.6 km-am of methane, respectively.