The atmosphere of Uranus: Results of radio occultation measurements with Voyager 2

During Voyager 2's occultation by Uranus the radio link from the spacecraft probed the atmosphere of the planet at latitudes ranging from 2° to 7° south. The measurements, which were conducted at two coherently related wavelengths, namely, 13 cm (S band) and 3.6 cm (X band), did not show any clear signs of microwave absorption. However, the Doppler frequency perturbations observed on the radio link have provided new data on the equatorial radius and atmosphere of the planet. From integral inversion of the Doppler data, profiles in height of the electron number density in the ionosphere and the gas refractivity, number density, pressure, temperature, and methane abundance in the troposphere and stratosphere have been determined. The gas data were acquired in the pressure range from about 0.3 mbar to 2.3 bars over an altitude interval of approximately 250 km. At the 2.3-bar level the nominal model has a temperature of 101 K with a 1σ uncertainty of 2 K when the uncertainty in the composition is assumed to be negligible.

The corresponding temperature lapse rate is 0.95+/-0.1 K/km. A 2- to 4-km-thick layer with a small refractivity scale height was detected during ingress and egress, which is consistent with the presence of a methane cloud layer centered at the 1.2-bar level. For the nominal model the methane mixing ratio below the base of the cloud is 2.3% by number density. At the tropopause, which was observed near the 100-mbar level, the temperature is 53+/-1 K. A comparison with infrared data acquired with the infrared interferometer spectrometer instrument on board the Voyager spacecraft indicates that the gas in this region consists of 85+/-3% hydrogen with the remainder being mostly helium. Above the tropopause the gas temperature increases with increasing altitude, reaching 114+/-10 K near the 0.5-mbar level. Several warm layers, which may be produced by absorption of solar radiation by hydrocarbon aerosols, were detected in the stratosphere.

From the data acquired at ingress and egress the shape and size of the isobaric surfaces of Uranus have been determined. The shape indicates that the gas in the region probed by the link rotates with an average period of about 18 hours, which corresponds to a zonal wind velocity of 110 m/s relative to the magnetic field. This implies that the equatorial atmosphere rotates slower than the interior, in contrast to the situation at Jupiter and Saturn. The 1-bar isobaric surface has an equatorial radius of 25,559+/-4 km. Extrapolating from the equator to the south pole by using available data on the gravity field and the zonal wind velocities gives a polar radius of 24,973+/-20 km. The corresponding oblateness, (R_eq-R_p)/R_eq, is 0.02293+/-0.00080.