Observation of the giant planets (Jupiter, Saturn, Uranus, and Neptune) at radio wavelengths have added significantly to our knowledge of their atmospheres. For Jupiter, radio observations have also revealed details of the magnetic field topology and of the distribution of relativistic electrons in the inner magnetosphere. Detailed images of the Jupiter and Saturn have revealed a wealth of phenomena. However, some critical problems can be addressed only with by the enhanced VLA.
Jupiter, Saturn, Uranus and Neptune are gaseous planets primarily composed of molecular hydrogen and helium. The intermediate elements such as C, N, O and S are present in molecular forms in abundances which are crudely solar. Important molecules include NH, HO, CH, CO, and possibly HS and NHSH. These molecules exist in gaseous forms at large atmospheric pressures and condense out into clouds at points determined by specific thermodynamic conditions along the vertical adiabatic profiles of the various planets. In particular, ammonia becomes saturated and condenses to ice crystals near the 150 K level at about the 1 bar pressure level for all of the giant planets. The water cloud forms near the 270 K levels, which occurs at pressure levels from a few bars on Jupiter but down to 10 to 20 bars on the other planets. The consequence of this is that NH is the major opacity source at centimeter wavelengths on all giant planets and thermally emitted radiation from pressures greater than 1 or 2 bars can only be observed at frequencies significantly less than the broad NH absorption band near 24 GHz. In effect, the major planets have an ``ammonia lid'' on then which is nearly impenetrable in the wavelength range from about a millimeter to roughly 6 centimeters!
The VLA wavelength gap from 6 to 20 cm greatly limits our ability to map the giant planets in the deep atmospheres, e.g., at the levels of the expected water clouds. A striking example of this limitation is in interpretation of the Comet Shoemaker - Levy 9 impact with Jupiter in July 1994: quantitative analysis of the event depends on the existence or non - existence of the presently unseen water clouds. VLA observation at 20 cm may be sensing emission at the level of the water clouds but the maximum spatial resolution of limits the quality of the maps, severely so for Saturn, Uranus and Neptune (which has a maximum diameter of ). The water vapor below the putative water clouds is another major opacity source that can't be penetrated at centimeter wavelengths. A VLA capability near 13cm (2.4 GHz band) would significantly improve our ability to study the deep atmospheres of Saturn and Uranus, in particular. Jupiter is a very important special case since radio maps of that planet at 20cm and longer are completely dominated by non-thermal emission from the radiation belts. The 2.4 GHz window is ideal for escaping the effects of the ammonia clouds, yet not be completely dominated by the non-thermal emission.
While a 2.4 GHz capability will allow us to significantly improve our understanding of Uranus and Neptune, improvements in spatial resolution are obviously needed. The addition of at least one VLBA antenna into the phase coherent A configuration is vital for real progress and the A+ configuration would be nearly perfect.