THE ABUNDANCES OF METHANE AND ORTHO PARA HYDROGEN ON URANUS AND NEPTUNE - IMPLICATIONS OF NEW LABORATORY 4-0 H-2 QUADRUPOLE LINE PARAMETERS/

The tropospheric methane molar fraction (f(CH4,t)) and the ortho/para hydrogen ratio are derived for Uranus and Neptune based on new determi nations of spectroscopic parameters for key hydrogen features as repor ted by D. W. Ferguson et al. (1993, J. Mol. Spec. 160, 315-325). For e ach planet, the relatively weak laboratory linestrengths (approximatel y 30 and 15% less than the theoretical 4-0 S(0) and S(1) linestrengths , respectively) results, when compared to analyses adopting theoretica l values, in a similar to 30% decrease in the tropospheric methane rat io and a comparable increase in the pressure level of the optically th ick cloudtop marking the bottom of the visible atmosphere (P-cld). The increase in the ratio of S(1)/S(0) linestrengths from 4.4 (theoretica l) 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 t han 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) valu es 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, Icaru s 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 mi xing ratios reported here are in agreement with the value of 0.023 der ived by the Voyager Radio Occulatation Experiment (G. F. Lindal 1992, Astron. J. 103, 967-982) for Neptune, but slightly lower than the Voya ger Uranus measurement of 0.023 reported by G. F. Lindal et al. (1987, J. Geophys. Res. 99, 14987-15001). The relative carbon-to-hydrogen ab undances of 20(-6)(+9) and 28(-8)(+7) times the solar value of 4.7 x 1 0(-4) derived by D. L. Lambert (1978, Mon. Not. R. Astron. Soc. 192, 2 49-272) for Uranus and Neptune support planetary formation mechanisms involving the dissolution of carbon-bearing planetesimals in the atmos pheres of both planets during their early stages of formation (e.g., J . B. Pollack et al., 1986, Icarus 67, 409-443). (C) 1995 Academic Pres s, Inc.