Interpretation of the 6818.9-Å methane feature observed on Jupiter, Saturn, and Uranus

Abstract

High-resolution (0.1-Å) spectra of the 6818.9-Å methane feature obtained for Jupiter, Saturn, and Uranus by K. H. Baines, W. V. Schempp, and W. H. Smith ((1983). Icarus 56, 534–542) are modeled using a doubling and adding code after J. H. Hansen ((1969). Astrophys. J. 155, 565–573). The feature's rotational quantum number is estimated using the relatively homogeneous atmosphere of Saturn, with only J = 0 and J = 1 fitting the observational constraints. The aerosol content within Saturn's northern temperate region is shown to be substantially less than at the equator, indicating a haze only half as optically thick. Models of Jupiter's atmosphere are consistent with the rotational quantum-number assignment. Synthetic line profiles of the 6818.9-Å feature observed on Uranus reveal that a substantial haze exists at or above the methane condensation region with an optical depth eight times greater than previously reported. Seasonal effects are indicated. The methane column abundance is 5 ± 1 km-am. The mixing ratio of methane to hydrogen within the deep unsaturated region of the planet is 0.045 ± 0.025, based on an H 2 column abundance of 240 ± 60 km-am ( W. H. Smith, W. Macy, and C. B. Pilcher (1980). Icarus 43, 153–160), thus indicating that the methane comprises between one-sixth and one-half of the planet's mass. However, proper reevaluation of H 2 quadrupole features accounting for the haze reported here may significantly reduce the relative methane abundance.