Absorption coefficients for the 6190-Å CH₄ band between 290 and 100°K with application to Uranus' atmosphere

Absorption coefficients for the CH₄ 6190-Å band as a function of temperature from 290 to 100°K are reported. We have developed and used a laser intracavity photoacoustic spectroscopy method which permits accurate control of the sample temperature due to the small sample volume required. Over the temperature interval from 290 to 100°K, we found the peak absorption coefficient to increase from 0.6 to 1.0 cm^-1 (km-am)^-1. The peak absorption increase was accompanied by significant band shape changes. The low-temperature data were then used to constrain further the Baines and Bergstralh (1986, Icarus 65, 406-441) standard model atmosphere for Uranus and found to yield an excellent fit to the bandshape near the minimum of the 6190-Å band. The new low-temperature coefficients reduce the pressure level needed to fit the Neff et al. (1984, Icarus 60, 221-235) photometric data, as well as the Voyager Uranus observations, to ∼3 bar, rather than the 7-10 bar required by the room temperature CH₄ absorption coefficients, thus producing agreement between models derived from individual H₂ quadrupole lines and CH₄ lines. The limitations of the present data are discussed in the context of the observations.