Atmospheric Composition of the Jovian Planets

Abstract

Models for line formation in the atmospheres of the outer planets are discussed critically. It is shown that Jupiter's cloud deck cannot he regarded as a simple reflecting surface and that scattering processes must be involved to some extent in the detailed mechanism of line formation. The theory of line formation in a homogeneous, semi-infinite, isotropically scattering atmosphere is reviewed from an intuitive viewpoint, and it is shown that qualitatively this theory can account for much of the observed morphological behavior of absorption lines in the atmospheres of Jupiter and Saturn. Uranus and Neptune possess very deep hydrogen atmospheres and Rayleigh scattering plays a role in line formation for these planets. Molecular hydrogen is the most abundant constituent in Jupiter's atmosphere. The mixing ratio of helium is less than 0.5, as deduced from analysis of line widths of a weak methane band by Owen. The mixing ratio of methane is of the order 5?1?4 and, in the region of the atmosphere probed at infrared frequencies, ammonia is apparently saturated. The only gases positively identified in the atmospheres of Saturn, Uranus and Neptune are molecular hydrogen and methane, and the observed abundance of these gases increases with increasing distance from the sun. This behavior apparently results from a corresponding increase in the depth of the effective scattering level in these atmospheres. An early inference of substantial amounts of helium in the atmospheres of Uranus and Neptune is based on an erroneous conclusion as to the efficiency with which helium can induce overtone vibrational transitions in hydrogen. Consequently, the abundance of helium in the atmospheres of Saturn, Uranus and Neptune is unknown.