Methane absorbtion in the jovian atmosphere. II. Absorption line formation

Abstract

The methane 3v3 band R-branch has been observed at high dispersion in the spectrum of Jupiter, at seven points along the planet's equator, and the equivalent widths of four lines, R(0), R(2), R(3) and R(5), have been measured. Three models for spectral line formation in the Jovian atmosphere have been tested by comparing variations in the equivalent widths of the methane lines predicted by the models with the observed variations. The reflecting-layer model (RLM), i.e., transparent gas above a diffusely reflecting opaque surface, has been used by almost all investigators to date to analyze molecular absorption bands in the Jovian spectrum quantitatively. It can be made to reproduce the observed behavior of the methane absorption lines across the Jovian disk only if rather artificial assumptions are made about the geometry of the reflecting surface, i.e., the Jovian cloud tops. The Chamberlain homogeneous scattering layer model (HSL) can not be ruled out on the basis of the observational material but does not reproduce the latter convincingly. A simple inhomogeneous scattering model (IHSL) developed by R. E. Danielson and M. G. Tomasko to describe line formation in an atmosphere with alternating layers of transparent gas and scattering cloud, provides the most satisfactory description of the observed behavior of the methane lines. The results with the IHSL model are encouraging; however, since the latter still involves certain simplifying assumptions, more elaborate analyses using somewhat more powerful radiative transfer techniques may be useful.