Infrared Spectroscopy of Parent Volatiles in Comets: Chemical Diversity and a New Fluorescence Model for the Ethane nu5 Band
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This work investigates the chemical and dynamical diversities of comets, and explores the clues they hold to understanding the formation and evolution of the Solar System. This research is based on analysis of high-resolution infrared spectroscopic data obtained with the Near Infrared Echelle Spectrograph on the Keck II telescope. Gas production rates of parent volatile species released from cometary nuclei are measured, and the relative enrichment of organics in comets, with respect to the dominant volatile - H2O - is determined. These measurements require fluorescence models for each species, as well as derivation of an accurate rotational temperature.
A major contribution of this work is the development of a theoretical model of the fluorescence of the infrared C2H6 nu5 band in comets (at 2896 cm-1), which can be used to derive an accurate rotational temperature for this parent volatile (unlike the C2H6 nu7 band at 2985 cm-1). As a symmetric hydrocarbon C2H6 is uniquely observed in the infrared, and now brings the number of molecules for which we can derive a rotational temperature to four (along with H2O, HCN and CO). Also, C2H6 nu5 is observed simultaneously with H2CO, OH, CH4, HCN, C2H2 and H2O, which eliminates many systematic effects.
The C2H6 nu5 model is applied to cometary spectra, and it used to extract ethane rotational temperatures, production rates and mixing ratios. The rotational temperatures derived from C2H6 nu5 agree with those measured for H2O (and other species). Mixing ratios from the C2H6 nu7 band are also confirmed by the nu5 band - agreement is within 1-sigma (2-sigma in one case).
Analysis of the depleted organic composition of the Oort cloud comet C/2000 WM1 (LINEAR) is presented, along with the ecliptic comet 2P/Encke, and their compositions are compared with those of other comets. The results from this dissertation contribute to understanding physics in the inner cometary coma, and on a grander scale - to the exploration of cometary origins in terms of Solar System formation and evolution.