The tropospheric methane molar fraction (f(CH4,t)) and the ortho/para hydrogen ratio are derived for Uranus and Neptune based on new determinations of spectroscopic parameters for key hydrogen features as reported by D. W. Ferguson et al. (1993, J. Mol. Spec. 160, 315-325). For each planet, the relatively weak laboratory linestrengths (approximately 30 and 15% less than the theoretical 4-0 S(0) and S(1) linestrengths, respectively) results, when compared to analyses adopting theoretical values, in a similar to 30% decrease in the tropospheric methane ratio and a comparable increase in the pressure level of the optically thick cloudtop marking the bottom of the visible atmosphere (P-cld). The increase in the ratio of S(1)/S(0) linestrengths from 4.4 (theoretical) to similar to 5.9 (measured) results in a decrease in the range of viable ortho/para ratios; an equilibrium hydrogen distribution is now the best fit for both planets. For Uranus, we find f(CH4,t) = 0.016(-0.005)(+0.007), P-cld = 3.1(-0.2)(+1.1) bars, and 0.85 < f(eH2) <less than or equal to> 1.00 (where f(eH2) and 1 - f(eH2) denote the fraction of H-2 in the equilibrium and normal state, respectively) compared to the K. H. Baines and J. T. Bergstralh (1986, Icarus 56, 543-559) values of f(CH4,t) = 0.030(-0.010)(+0.016), P-cld = 2.7(-0.4)(+0.5) bars, and 0.63 < f(eH2) < 0.95. For Neptune, we find f(CH4,t) = 0.022(-0.006)(+0.005), P-cld 3.8(-0.3)(+0.7) bars, and 0.89 < f(eH2) less than or equal to 1.0 compared to the K. H. Baines and W. H. Smith (1990, Icarus 85, 65-108) values of f(CH4,t) = 0.03 +/- 0.007, P-cld = 3.3(-0.1)(+0.5) bars, and 0.85 < f(eH2) less than or equal to 1.0. The methane mixing ratios reported here are in agreement with the value of 0.023 derived by the Voyager Radio Occulatation Experiment (G. F. Lindal 1992, Astron. J. 103, 967-982) for Neptune, but slightly lower than the Voyager Uranus measurement of 0.023 reported by G. F. Lindal et al. (1987, J. Geophys. Res. 99, 14987-15001). The relative carbon-to-hydrogen abundances of 20(-6)(+9) and 28(-8)(+7) times the solar value of 4.7 x 10(-4) derived by D. L. Lambert (1978, Mon. Not. R. Astron. Soc. 192, 249-272) for Uranus and Neptune support planetary formation mechanisms involving the dissolution of carbon-bearing planetesimals in the atmospheres of both planets during their early stages of formation (e.g., J. B. Pollack et al., 1986, Icarus 67, 409-443). (C) 1995 Academic Press, Inc.
