Interpretation of the 6818.9-Å methane line in terms of inhomogeneous scattering models for Uranus and Neptune

We have obtained high-resolution spectra of Uranus and Neptune in the methane transition near 6800 Å, and in particular, the 6818.9Å feature. Calculated equivalent widths for this line using recently proposed models of the atmospheres of these two planets indicate that the C/H ratio is greater than or equal to 5 × 10^-3 below the CH₄ saturation level. This value is 12 times the solar mixing ratio. The half-widths of the computed line profiles are in agreement with the observed half-widths. Therefore, it is unnecessary to introduce an unidentified constituent with an abundance comparable to H₂, postulated recently by Belton and Hayes, and by Bergstrahl, to account for the observed line broadening.