Rotational Raman Scattering in Planetary Atmospheres

When spectra of deep solar Fraunhofer lines recorded in sunlight scattered by the Earth's atmosphere are compared with similar spectra of direct, unscattered sunlight, it is found that the scattered line profiles are systematically less deep (relative to the continuum) than the direct profiles by a few per cent. This has been taken to indicate the presence of an extra, inelastic component of the scattered radiation fie'd. Its nature has remained unexplained. In this paper it is pointed out that rotational Raman scattering in the atmosphere can be expected to produce just such an extra component. Previous observational work is reviewed and interpreted in light of this explanation. The magnitude of the effect in the atmospheres of other planets is also briefly explored