Earth-Based Remote Sensing of Planetary Surfaces And Atmospheres at Radio Wavelengths

Radio observations of the planets are providing much valuable information about these remote environments. Most of these objects are black bodies emitting thermal radiation which appears brighter at short wavelengths but we generally wish to investigate the effects of atmospheric absorption and subsurface heat conduction on them which requires observations using a wide variety of astronomical techniques over a large range of wavelengths. Most parameters change only slowly with wavelength so that continuum radiometry at selected wavelengths is sufficient in most cases. Many of the planetary signals are weak and it is very important to accurately separate their emission from that of the background sky and nearby confusing sources. This involves continuous comparison with the nearby background by various switching, on-off, and scanning techniques plus reobservation of the same spot in the sky after the planet has moved away. Often, over 2/3 of the time required for an observation is actually spent on comparison rather than looking at the actual target planet. Recently, aperture synthesis observations using interferometers have provided very-high-resolution pictures of some of the planets and removed many of the background problems. Another important concern is calibration of the results so we can have accurate values of the absolute temperatures of these bodies. This requires measurement of the antenna parameters and also the effects of the Earth´s atmosphere upon the incoming signals.