Of the remaining planets, only Mars and Saturn have been observed as radio sources, and not very much information is available. Mars has been observed twice at about 3 -- cm wave length, and the intensity of the observed radiation is in reasonable agreement with the thermal radiation which might be predicted on the basis of the known temperature of Mars. The low intensity of the radiation from Saturn has limited observations, but again the measured radiation seems to be consistent with a thermal origin. No attempts to measure the radio emission of the remaining planets have been reported, and, because of their distances, small diameters, or low temperatures, the thermal radiation at radio wave lengths reaching the earth from these sources is expected to be of very low intensity. In spite of this, the very large radio reflectors and improved amplifying techniques which are now becoming available should make it possible to observe the radio emission of most of the planets in a few years.

The study of the radio emission of the moon and planets began with the detection of the thermal radiation of the moon at 1.25 -- cm wave length by Dicke and Beringer (1946). This was followed by a comprehensive series of observations of the 1.25 -- cm emission of the moon over three lunar cycles by Piddington and Minnett (1949). They deduced from their measurements that the radio emission from the whole disk of the moon varied during a lunation in a roughly sinusoidal fashion; that the amplitude of the variation was considerably less than the amplitude of the variation in the infrared emission as measured by Pettit and Nicholson (1930) and Pettit (1935); and that the maximum of the radio emission came about 3 -- 1 2 days after Full Moon, which is again in contrast to the infrared emission, which reaches its maximum at Full Moon. Piddington and Minnett explained their observations by pointing out that rocklike materials which are likely to make up the surface of the moon would be partially transparent to radio waves, although opaque to infrared radiation. The infrared emission could then be assumed to originate at the surface of the moon, while the radio emission originates at some depth beneath the surface, where the temperature variation due to solar radiation is reduced in amplitude and shifted in phase. Since the absorption of radio waves in rocklike material varies with wave length, it should be possible to sample the temperature variation at different depths beneath the surface and possibly detect changes in the structure or composition of the lunar surface material.