October 2013


Far and away the most common molecule in the universe, molecular hydrogen (H2) is a difficult compound to observe directly because it has no dipole moment and thus no dipole-allowed rotational transitions. H2 was identified in space for the first in an unusual way, via a spectrograph lifted into the upper atmosphere aboard an Aerobee-150 rocket, as described by Carruthers in 1970. The spectrum taken toward the star ξ Persei exhibited Lyman ultraviolet bands. Smith subsequently observed 10 bands of the Lyman system as well as one band of the Werner series with another rocket-launched spectrograph aimed toward δ Scorpii.

© GK Burnett

Later observations with the Copernicus Satellite reported by Spitzer et al. confirmed the earlier detection of H2 and also reported the first detection of HD, deuterated molecular hydrogen. Launched in 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) allowed nearly a decade of high resolution observations of hydrogen (see, for example, Snow et al.). FUSE detected H2 in comet C/2001 A2 (LINEAR) as reported by Feldman et al. in 2002. The gas giants are predominately composed of H2: Jupiter, ~90%, Saturn, ~96%, Uranus, ~83%, and Neptune, ~80%.

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