Ethane in Planetary and Cometary Atmospheres: Transmittance and Fluorescence Models of the nu7 Band at 3.3 Micrometers
Ethane and other hydrocarbon gases have strong rovibrational transitions in the 3.3 micron spectral region owing to C-H, CH2, and CH3 vibrational modes, making this spectral region prime for searching possible biomarker gases in extraterrestrial atmospheres (e.g., Mars, exoplanets) and organic molecules in comets. However, removing ethane spectral signatures from high-resolution terrestrial transmittance spectra has been imperfect because existing quantum mechanical models have been unable to reproduce the observed spectra with sufficient accuracy. To redress this problem, we constructed a line-by-line model for the n7 band of ethane (C2H6) and applied it to compute telluric transmittances and cometary fluorescence efficiencies. Our model considers accurate spectral parameters, vibration-rotation interactions, and a functional characterization of the torsional hot band. We integrated the new band model into an advanced radiative transfer code for synthesizing the terrestrial atmosphere (LBLRTM), achieving excellent agreement with transmittance data recorded against Mars using three different instruments located in the Northern and Southern hemispheres. The retrieved ethane abundances demonstrate the strong hemispheric asymmetry noted in prior surveys of volatile hydrocarbons. We also retrieved sensitive limits for the abundance of ethane on Mars. The most critical validation of the model was obtained by comparing simulations of C2H6 fluorescent emission with spectra of three hydrocarbon-rich comets: C/2004 Q2 (Machholz), 8P/Tuttle, and C/2007 W1 (Boattini). The new model accurately describes the complex emission morphology of the nu7 band at low rotational temperatures and greatly increases the confidence of the retrieved production rates (and rotational temperatures) with respect to previously available fluorescence models.