Groundbased Microwave Observatories

The submillimeter to microwave part of the giant planets' thermal emission spectra is a very interesting one since it allows for the probing of deep-pressure levels of these planets with weighting functions extending down to almost 100 bar for Uranus and Neptune. The spectral range contains absorption features of ammonia, CO, HCN and other constituents, and thus the deep abundance of these molecules may be

Figure 7.18. The KAO

telescope looking through the aperture in the aircraft's side. Courtesy of NASA.

Figure 7.18. The KAO

telescope looking through the aperture in the aircraft's side. Courtesy of NASA.

determined. The transmission of the Earth's atmosphere at microwave wavelengths was shown in Figure 7.3, and ground-based microwave observations are thus restricted to the spectral windows between the main absorption bands. Since the main absorber is once again water vapor, microwave observatories are, like visible/IR observatories, preferentially located at high altitude in dry regions of the world. Observing the giant planets at microwave wavelengths poses considerable problems, not least of which is the very low power of microwave emission radiated by the giant planets, which means that antennas must be very large and detectors must be very sensitive. A second problem, for Jupiter observations, is that synchrotron emission is also observed from the radiation belts, which needs to be subtracted for wavelengths longer than about 4 cm. The final problem is that at these long wavelengths, it is technically very difficult to make antennas large enough to resolve structure in the atmospheres of these planets. For example, an antenna able to resolve down to 1 arcsec at the wavelength of the inversion absorption band of ammonia at 1.3 cm needs to be 3.2km across! For this very reason, microwave and radio observatories have led the way in developing interferometric arrays to increase their angular resolution. There are currently several large millimeter arrays in the world that have been used for giant planet observations, which will now be reviewed.

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