Neptune's deep atmosphere revealed

The brightness temperature of Uranus at 20 cm is 260 + or - 10K, while Neptune it is 318 + or - 16K. Since NH3 is the dominant absorber at this wavelength the microwave spectra of Neptune have been modeled based upon an assumed deep gaseous mixing ratio of NH3 and subsequent loss into clouds. The difference between the two brightness temperatures implies that the NH3 mixing ratio below the level of cloud formation on Neptune compared to Uranus is lower by nearly two orders of magnitude. An alternative explanation is that the 20 cm radiation from Neptune is a combination of thermal plus synchrotron emission as proposed by de Pater and Goertz (1989).