ICARUS 65, 406-441 (1986) The Structure of the Uranian Atmosphere: Constraints from the Geometric Albedo Spectrum and H2 and CH4 Line Profiles KEVIN H. BAINES AND JAY T. BERGSTRALH Jet Propulsion Laboratory, ('alifi,'nia Institute of "l'echmdo~,v. 4800 Oak (;rot, e Drive, Pasadena. ('al(fin'nia 91109 Received July 8. 1985: revised November 22, 1985 Constraints on the atmospheric structure of Uranus are derived rrom recently acquired, high- quality spectral observations [J. S. Neff, D. C. Humm. J, T. Bergstralh, A. L. Cochran, W. D. Cochran, E. S. Barker, and R. G. Tult (19841 lcaru.s 60, 221-235; J. T. Yrauger and J. q'. Bergstralh (1981) Bull. Amer. Astron. Soc. 13, 732; K. H, Baines, W. V. Schempp, and W. H. Smith (1983) Icarus 56, 534-542]. The analysis, based on delailed modeling of a broadband 17 A) geometric albedo spectrum from 3500 to 10,500 A, and high-resolution (30 and 100 m,~, respectively) observa- tions of H: 4-0 quadrupole and 6818.9-A CH4 features, yields a family of models which parameter- izes an upper tropospheric haze layer, a lower optically infinite cloud at a pressure level PcLa, the cloudqevel methane molar fraction,,/~Ha, and the mean ortho/para ratio in the visible atmosphere. Limits include 2.40 < PCld < 3.2 bars, 0.020 ./tHa "" (I.046, and 0.63 <" .I~H: < 0.95, where f.H: denotes the fraction of He in the equilibrium slate. The haze optical deplh at 6435 A is found to be 0.4 < rH (6435 A) < 1.0, in reasonable agreement with 1,. M. Traflon's [(19761 Astrophys. J. 207, 1007-1024] determination but significantly less than that reported by K. H. Baines 111983) Icarus 56, 543-559]. The single-scattering albedo of atmospheric aerosols exhibits a sleep darkening between 5890 and 6040 A, reminiscent of UV-irradiated H_,S ice crystals. These constraints are consistent with recent infrared and submillimeter and millimeter analyses. The analysis also agrees with the theoretical He quadrupole line strengths but conflicts with a number of reported laboratory measurements. ~, 1986Academic P~ess, Inc 1. INTRODUCTION The atmospheric structure of Uranus has not been well constrained. As one example, recent studies of the appearance of meth- ane absorption in the planet's geometric al- bedo spectrum have led to deep-atmo- sphere CH4 molar fraction ranging from 3 x 10 3 (Teifel, 1983) to as much as 0.10 (Wal- lace, 1980). Such discrepancies can be traced largely to assumptions about crucial model parameters which are not con- strained by the data under analysis. For ex- ample, the depth of the visible atmosphere, which cannot be derived uniquely from ex- isting geometric albedo data, is assumed by Teifel (1983) to be limited by Rayleigh scat- tering in a semi-infinite, aerosol-free atmo- sphere. On the other hand, Wallace (1980) adopts a shallower optical lower boundary from complementary data sets analyzed by 0019- I (135/86 $3.00 Cop~,right 1986by AcademicPress, lnc All righls of reproduclion in an}' form re~er~cd other investigators. Specifically, a visible depth of 427 km-am of He is assumed on the basis of Trafton's (19761 analysis of 3-0 and 4-0 quadrupole equivalent width mea- surements. In addition to Wallace (1980) and Trafton (19761, a number of other investigators [no- tably Belton et al. (1971) and Danielson el a/. (1977)] have attempted a methodical, in- tegrated analysis of complementary data sets to establish a system of mutually con- sistent model constraints. Other research- ers have examined more limited data sets closely (e.g., Benner and Fink, 1980: Atai et al., 1981; Teifel, 19831. However, all have been forced to combine observational data acquired over intervals of a decade or more, Many of these data are characterized by poor resolution, poor signal-to-noise ra- tio, and uncertain calibration. Further- more, evidence of secular changes in 4()6