An estimate of the temperature and abundance of CH 4 and other molecules in the atmosphere of Uranus

Abstract

We present a preliminary analysis of CH 4 absorptions near 6800 Å in new high resolution spectra of Uranus. A curve of growth analysis of the data yields a rotational temperature near 100 K and a CH 4/H 2 ratio that is 1 to 3 times that expected for a solar type composition. The long pathlengths of CH 4, apparently demanded by absorptions near 4700 Å, are qualitatively shown to be the result of line formation in a deep, predominantly Rayleigh scattering atmosphere in which continuum absorption is a strong function of wavelength. The analysis of the CH 4 also yields a minimum value for the effective pressure of line formation (∼ 2 atm). This value is shown to be twice that expected on Uranus if the atmosphere were predominantly H 2. It is speculated that large amounts of some otherwise optically inert gas is present in the Uranus atmosphere. N 2 is suggested as a possible candidate since there are cosmogonic reasons why Uranus should contain large amounts of N relative to C, He, and H, and also because the pressure-induced pure rotation spectrum of N 2 could possibly account for the low brightness temperatures that have recently been observed at 33 and 350 μm. If N 2 is present the planet probably possesses a surface at the 10–100 atmosphere level.