Ortho-Para Equilibrium of Hydrogen in Neptune's Atmosphere from HST Spectroscopic Observations

Details Ortho-Para Equilibrium of Hydrogen in Neptune's Atmosphere from HST Spectroscopic Observations Ortho-Para Equilibrium of Hydrogen in Neptune's Atmosphere from HST Spectroscopic Observations

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0505c&link_type=abstract

American Astronomical Society, DPS meeting #28, #05.05; Bulletin of the American Astronomical Society, Vol. 28, p.1077

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In Neptune's atmosphere, Raman scattering (besides nearly conservative Rayleigh scattering) of solar UV photons by H_2 is significant because of the relative transparence of the stratospheric haze and because of the deep location of the methane condensation cloud. Since the frequencies of the ro-vibrational transitions of H_2 --- mainly the S_0 and S_1 rotational as well as the Q_1 vibrational --- are different in the ortho and para states, the spectral shift of a Raman-scattered solar photon is also different whether it is produced by one or the other H_2 modification. Hence, the contrast of several prominent Raman ``ghost" features is influenced by the ortho/para partition of H_2 in the upper atmosphere, and it may be used as a diagnostic of that fundamental thermodynamical parameter. Archived HST spectra of Neptune, obtained with the Faint Object Spectrograph between 2000 and 3100 Angstroms, were used to contrain the relative abundances of ortho- and para-hydrogen using the above scheme. Unfortunately, at the resolution of a few Angstroms/ that is presently attained on Neptune, the effect is significant for only a single Raman ghost feature near 2850 Angstroms. The amplitude of that feature in the HST/FOS data is consistent with an equilibrium partition between the ortho and para states, in agreement with a previous determination from ground-based observations of H_2 quadrupole lines (Baines et al./ 1995).

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