The microwave spectra of planets

This paper reviews our current knowledge of the spectra of planets, from the centimeter to the submillimeter range, both from an observational and a theoretical point of view. In the case of Venus, due to its very thick ${\mathrm{CO}}_2$ atmosphere, the spectrum is dominated by the pressure-induced ${\mathrm{CO}}_2$ absorption, with some other contributions from minor constituents $({\mathrm{SO}}_2,{\mathrm{H}}_2\mathrm{O}\mathrm{).}$ In the case of Mars, which has a very tenuous atmosphere, the microwave spectrum is composed of discrete narrow molecular lines of CO, ${\mathrm{H}}_2\mathrm{O}$ and their isotopes, superimposed over a continuum defined by the surface emissivity and temperature. The microwave spectra of the giant planets probe their tropospheres. They are dominated by collision-induced absorption, mostly due to ${\mathrm{H}}_2-{\mathrm{H}}_2$ and ${\mathrm{H}}_2-\mathrm{He}$ collisions, with, in the case of Jupiter and Saturn, additional features due to ${\mathrm{NH}}_3$ and ${\mathrm{PH}}_3.$ For all cases, observed and synthetic spectra are described, and the various sources of uncertainty are discussed.