Spectroscopic evidence of molecular hydrogen in the atmospheres of Uranus and Neptune

The unidentified diffuse feature at 8270 A observed by Kuiper in the spectra of Uranus and Neptune is reproduced in the laboratory in an 80-meter path of hydrogen at 100-atm pressure and a temperature of 78 K. It is the S(0) line of the 3A) pressure-induced rotation-vibration band of the H2 molecule. Two otber lines of this band, as well as three lines of a double transition (2A), 14)), are observed in the laboratory spectrum. Most of these features are hidden in the planetary spectra by strong CH4 bands, but a line, 8166 A, of the double transition is only partially overlapped by an adjacent CH4 band. Its low intensity in the planetary spectra can be accounted for by assuming that there are even larger amounts of helium than of hydrogen in the atmospheres of Uranus and Neptune. Helium does not induce the double transition. The observed spectra are very roughly compatible with a 3:1 (or higher) helium-hydrogen ratio, a temperature of 78 K (or lower), and a partial pressure of hydrogen at the bottom of the visible atmosphere of about 2 atm. The thickness of an equivalent atmosphere of uniform density would be about 18 km for Uranus.