Upper atomosphere of Uranus: mean temperature and temperature variations

The number-density, pressure, and temperature profile of the Uranian atmosphere in the pressure interval from 0.3 to 30 dynes cm-2 are derived from observations of the occultation of SAO 158687 by Uranus on 1977 March 10, observations made from the Kuiper Airborne Observatory and the Cape Town station of the South African Astronomical Observatory. The mean temperature is found to be about 95 K, but peak-to-peak variations from 10 K to 20 K or more exist on a scale of 150 km or 3 scale heights. The existence of a thermal inversion is established, but the inversion is much weaker than the analogous inversion on Neptune. The mean temperature can be explained by solar heating in the 3.3 μm methane band with a methane mixing ratio of 4 x 10-6 combined with the cooling effect of ethane with a mixing ratio of < or approx. =4 x 10-6. This explanation does not violate the existing upper limit on the infrared emission of ethane. The temperature variations are probably due to a photochemical process that has formed a Chapman layer. Further infrared observations and detailed aeronomical calculations are needed to elucidate further the unexpected structure of the Uranian upper atmosphere