Radio emission from the planets

The present state of knowledge of the radio emission of the planets is reviewed. Only in the case of Mars and Saturn can simple heating of the planet by the incident solar radiation explain the observed radio emission. Mercury, Venus and Jupiter all have more radio emission at some wavelengths than would be expected on this model. For Mercury an internal source of heat (radioactive heating) has been suggested, while for Venus there are a variety of theories in which the energy derives indirectly from radiation or corpuscular bombardment from the Sun. In the case of Jupiter the excess microwave emission is almost certainly synchrotron emission from relativistic electrons in a radiation belt of magnetically trapped particles (Van Allen Belt). The origin of the decametric bursts of radiation from Jupiter is less certain but may be associated with "dumping" of particles from the Van Allen Belt. The survey reveals a great need for more observations. The major areas where further data are required are indicated in the Conclusion.