1987/12/30 by F. Herbert, B. R. Sandel, R. V. Yelle +5 · 117 citations
Physics and Astronomy · Earth and Planetary Sciences · #Astro and Planetary Science #Planetary Science and Exploration #Geology and Paleoclimatology Research #Uranus #Atmosphere (unit) #Mixing ratio #Physics #Saturn #Jupiter (rocket family) #Occultation #Geometric albedo #Opacity #Atmosphere of Jupiter #Astronomy #Wavelength #Brightness #Astrophysics #Atmospheric sciences #Planet #Optics #Meteorology #Jovian
paper · doi:10.1029/ja092ia13p15093
published in Journal of Geophysical Research Atmospheres 92(A13), 15093-15109 (American Geophysical Union)
openalex publication_date 1987/12/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/23
Occultation observations of the upper atmosphere of Uranus by the Voyager 2 ultraviolet spectrometer are analyzed. The measurements extend from 0.5 mbar to about 10 −6 µbar using the EUV wavelengths 520 ≤ λ ≤ 1700 Å. H 2 dominates the atmosphere (the approximately 15% He content deduced by the Voyager 2 infrared spectrometer cannot be seen in occultation at these wavelengths) up to the vicinity of the exobase near 1.25 R U , where atomic H becomes the major constituent. Apparently because of weak eddy mixing, the hydrocarbon mixing ratios are quite small in the measured pressure range, so that the atmosphere is more transparent than those of Jupiter and Saturn. Thus H 2 Rayleigh scattering is the dominant source of opacity in the lower portion of the observed pressure range. The mixing ratio of C 2 H 2 is on the order of 10 −8 there, while only an upper limit (≤ 10 −7 ) is available for CH 4 . Also, some evidence exists for the possible presence of C 2 H 6 at a mixing ratio of several × 10 −8 . The value of the eddy diffusion coefficient at the homopause is much lower than at Jupiter and Saturn; the best fitting of several photochemical models which were matched to the observations assumed a value of 10 4 cm² s −1 . This may represent an upper limit. In addition, the two high‐latitude occultations indicate little difference in upper atmospheric structure between the day and night hemispheres, despite the constancy of the illumination geometry over recent decades. The atmospheric temperature above about 0.01 to 0.001 µbar is 800 ± 100 K. Because of this high temperature the thermal component of the H exosphere extends to great altitude, with number densities of several hundred cm −3 at 2 R U . This high gas density has important implications for ring dynamics, possibly being responsible for the extreme narrowness and isolation of the visible Uranian rings. The extent and density of the H exosphere and the nonthermal corona (which has an even larger scale height) will also strongly affect the origin and maintenance of the unusual plasma populations observed at Uranus by the Voyager 2 plasma science and low‐energy charged particle experiments.