Infrared Spectroscopy of Cometary Parent Molecules

Abstract

Most cometary parent molecules do not strongly fluoresce at ultraviolet and visible wavelengths, and some do not possess permanent electric dipole moments, preventing their study in the radio region as well. However, many of these molecules have strong ro-vibrational transitions in the near infrared (λ ~ 2 – 5 µm). Since the solar flux at these wavelengths is quite strong, parent molecules in cometary comae can be probed directly via fluorescence in these infrared transitions. The feasibility of this approach was convincingly demonstrated by the detection of H2O in comet Halley (1986 III) from the Kuiper Airborne Observatory and by the detection of H2O, CO2, and H2CO using an infrared spectrometer (IKS) on VEGA. Tentative detections of near infrared lines of CH4 were also reported during ground-based and airborne observations of comets Halley and Wilson (1987 VII). High resolution spectroscopy of the infrared water transitions has yielded a wealth of new information on cometary physics: the absolute line intensities and spatial brightness profiles are used to determine water production rates and lifetimes, the relative line intensities probe the kinetic temperature profile in the coma, the line widths and line positions shed light on coma outflow dynamics, and the temporal variability in the lines provides information on the structure of the nucleus. These observations also allow the determination of the water ortho-to-para ratio, which may provide fundamental insight into the origin and/or evolutionary history of cometary nuclei. Similar observations of other molecules (those mentioned above plus others) will provide important complementary data and will also allow us to compile a volatile inventory for cometary nuclei, but such observations are extremely difficult due to the low abundances of these molecules (≤10% relative to water) and the limitations of present infrared facilities. Recent advances in infrared instrumentation promise to extend sensitivities for parent molecule searches to relative abundances well below 1%, especially if cooled, Earth-orbiting facilities are available.