About unidentified features in the Voyager far infrared spectra of Jupiter and Saturn

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Abstract
Two faint spectral features are visible in Jupiter's spectra recorded during the infrared interferometer experiment aboard the two spacecraft of the Voyager mission. These features are located at the frequencies of the S(0) and S(1) pressure induced rotation lines of the free H2 molecule, and are superimposed over the absorption spectrum induced by collisions between H2–H2 and H2–He molecules, which are the main components of the atmospheres of the giant planets. We demonstrate conclusively that these features are real and not measurement artifacts. The feature located at the S(0) frequency, which also exists in the Voyager spectra of Saturn, exhibits a complex shape, characterized by a lack of absorption in the continuum at 354.4 cm−1 and a local excess of absorption over the continuum at 350.5 cm−1. The feature corresponding to the S(1) frequency seems to be due mainly to an absorption dip in the continuum at 587.1 cm−1. Physical conditions prevailing at the atmospheric levels of formation of the features rule out the possibility that they can be attributed to H2 quadrupolar lines or to a dip in absorption resulting from intercollisional effects in the H2–He component of the pressure induced absorption. We speculate that they could be due to an intercollisional effect in the H2–H2 component, but the lack of laboratory measurements precludes any firm interpretation.Laboratory studies of the S lines in the collision induced spectrum of H2, made at high spectral resolution and low temperatures, are absolutely necessary in order to be able to complete the interpretation of the observed phenomena and to derive from them information on the atmospheric thermal structure or composition of Jupiter and Saturn.

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