A Newly Developed Model for Fluorescence of C2H6 ν5 in Comets and Application to Infrared Spectra Acquired with NIRSPEC at Keck II

Exploring the compositional diversity of cometary nuclei is key to understanding the formation and evolution of the Solar System, and the origin of water and prebiotic organics on the Earth. For most polyatomic gases, a production rate can be obtained from measured ro-vibrational intensities if an accurate rotational temperature is available, and the comparison of temperatures displayed by individual molecular species can reveal additional insights into physics in the cometary inner coma. Until now, accurate temperatures could be derived for HCN, CO, and H₂O, but not for ethane (C₂H₆). As a symmetric hydrocarbon, C₂H₆ is uniquely sampled in the infrared, but the fine rotational structure of the ν₇ Q-branches at 3.3 μm cannot be resolved by current infrared spectrometers, ultimately limiting the retrievals of rotational temperatures from these lines. However, the complex ro-vibrational structure (P- and R-branches) of the ν₅ band at 3.5 μm is resolved by the Near Infrared Echelle Spectrograph on the Keck II telescope (NIRSPEC). We developed a fluorescence model for the infrared ν₅ band of C₂H₆, and applied it to interpret high-resolution infrared spectra acquired with NIRSPEC. We present rotational temperatures and production rates for C₂H₆ in multiple comets. We compare the extracted rotational temperatures for ethane with those obtained from simultaneous measurements of other species (H₂O and HCN). We also compare mixing ratios derived from the C₂H₆ ν₅ band with those derived from the previously analyzed (by NASA GSFC team members) ν₇ band. This work now establishes a robust method for quantifying additional physical parameters for ethane in comets.