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J.J. O'Brien, E.A. Amin, B.L. Kalmar (Department of Chemistry, University of Missouri-St. Louis)
Methane spectral features are prominent in the reflected sunlight spectra from the outer planets and some of their major satellites and can provide detailed information on the atmospheres of those bodies. Methane bands in the visible to near-IR region are particularly important because for many of these planetary bodies, methane IR bands are found to be saturated. Laboratory data acquired at appropriate low temperature conditions are required to interpret the observational data. Methane bands in the visible to near-IR region are intrinsically weak so sensitive techniques such as intracavity laser spectroscopy (ILS) are required to perform the measurements.
The ILS technique with a Ti:sapphire laser has been used to record spectra for the 790 nm and the major portion of the 889 nm bands of methane. Spectra were obtained at methane temperatures of 77 and 296 K for the 790 nm band and at temperatures of 77, 123, 173 and 296 K for the 889 nm band. Spectra for the more strongly absorbing sections of these bands are presented. The spectra are acquired at a resolution of ~500,000. From the spectra, absorption coefficients are determined and these are presented as averages over 1 cm-1 intervals. The results are compared with low resolution absorption spectral measurements on methane at room temperature and with absorption coefficients derived from methane features observed in spectra of the outer planets and Titan. To obtain our results, spectra are deconvolved for the instrument function using a Fourier transform technique. The validity of the approach is verified from studies of isolated water absorption lines occurring in the same spectral region.
Supported by NASA's Planetary Atmospheres Program under grant numbers NAGW-2474 and NAG5-6091.