Quasi‐random narrow‐band model fits to near‐infrared low‐temperature laboratory methane spectra and derived exponential‐sum absorption coefficients

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Abstract
Near‐infrared 10‐cm−1 resolution spectra of methane obtained at various temperatures, pressures, and abundances are fit to a quasi‐random narrow‐band model. Exponential‐sum absorption coefficients for three temperatures (112 K, 188 K, and 295 K), and 20 pressures from 0.0001 to 5.6 bars, applicable to the cold environments of the major planets, are then derived from the band model for the 230 wavelengths measured from 1.6 to 2.5 μm. Root‐mean‐square deviations between the laboratory and the exponential‐sum synthetic transmissions are reported for the best fitting 50 wavelengths. Deviations relevant to broadband, 1% spectral resolution observations such as planned to be acquired by Galileo/Near Infrared Mapping Spectrometer (NIMS)and Cassini/Visual and Infrared Mapping Spectrometer are also presented. The validity of exponential‐sum coefficients derived from broadband (10 cm−1) transmission data is demonstrated via direct comparison with line‐by‐line calculations. The complete atlas of coefficients is available from the Planetary Data System‐Planetary Atmospheres Discipline Node.