title: Uranus: variability of the microwave spectrum.
abstract: Radio astronomical observations of Uranus show that the radio emission spectrum is evolving in time. Ammonia vapor must be depleted in the Uranian atmosphere as Gulkis and his co-workers previously suggested. Since 1965, ammonia either has been decreasing in time or is a decreasing function of latitude, or both, provided that the radio emission is atmospheric in origin. If Uranus has an observable low-emissivity "surface," these trends may be reversed. The microwave observations made in 1965, at the time when the spin axis of Uranus was nearly perpendicular to the sun-Uranus line, are consistent with an atmospheric opacity profile that would be produced by saturated ammonia vapor in a predominantly hydrogen atmosphere. At the present time, when the spin axis of Uranus is nearly aligned with the sun-Uranus line, the measurements require an opacity that would be produced by saturated water vapor. A large thermal gradient between the pole and equator is ruled out.
authors.forename: S
authors.initials: S
authors.lastname: Gulkis
authors.forename: E T
authors.initials: ET
authors.lastname: Olsen
authors.forename: M J
authors.initials: MJ
authors.lastname: Klein
authors.forename: T J
authors.initials: TJ
authors.lastname: Thompson