Latitudinal variations of ammonia in the atmosphere of Uranus: An analysis of microwave observations

Abstract

A radio map of Uranus made with the Very Large Array at a wavelength of 2 cm is analyzed. The main features of the data are that the planet appear strongly limb darkened, and that the image is not symmetric about the sub-Earth point. These features are interpreted as being due to a latitudinally varying ammonia mixing ratio. Assuming NH 3 to be the only parameter that varies, it appears that the south polar region is depleted in ammonia by about a factor of 3 relative to midlatitudes. The change in NH 3 abundance occurs near −45° lat. The calculated NH 3 molar mixing ratios in the 5 to 20 bar region average 4.9 ± 0.7 × 10 −7 for latitudes from −90° to −45°, and 1.3 ± 0.4 × 10 −6 from −45° to −15°. Equatorward of this, the model is poorly constrained due to the observing geometry and abundances cannot be accurately estimated. The quoted error bars represent noise in the data. Other sources of error, such as uncertainty in the NH 3 absorption coefficient and in the dat calibration, can change the absolute value of calculated ammonia abundances, but are unlikely to affect the relative variations across the disk of the planet. This model is in agreement with previous work that indicated ammonia is depleted relative to a solar abundance and also is consistent with a preliminary analysis of 6-cm observations. The relatively ammonia rich region in our model includes a region where an analysis of Voyager IRIS data indicates there is an upwelling in the atmosphere.