Modeling Optically Thick Molecular Emission Spectra of Comets Using Asymmetric Spherical Coupled Escape Probability

Abstract

Comets are frozen remnants from the formation of the Solar System. As such, their chemical composition is of great significance to understanding the origin of the planets and the distribution of important molecules, including water and other volatiles, throughout the Solar System. Recent observations, in particular those of the Deep Impact and EPOXI Missions, have provided better spectra of a cometary coma than were previously available. These observations include spectra with high spatial resolution very near to the nucleus. The purpose of this research is to better understand the abundances, distributions and creation mechanisms of various volatiles observed in cometary comae, in particular those of comet 9P/Tempel 1, the target of the Deep Impact Mission, and 103P/Hartley 2, the subject of the EPOXI mission. In order to do so, I have built a computer model of the spectrum of the comet's coma which includes the difficult and often ignored problem of accurately including radiative transfer to account for the potentially optically thick coma (or regions of the coma) near the nucleus. I have adapted Coupled Escape Probability, a new exact method of solving radiative transfer problems, from its original plane-parallel formulation for use in asymmetrical spherical situations. My model is designed specifically for use in modeling optically thick cometary comae, although not limited to such use. By providing for asymmetric geometry in the coma, the model is able to include the morphology of the near nucleus coma, as observed by the Deep Impact spacecraft for Tempel 1 and Hartley 2, and include this in the modeling of radiative transfer. This method enables the accurate modeling of comets' spectra even in the potentially optically thick regions nearest the nucleus, such as those seen in Deep Impact observations of 9P/Tempel 1 and EPOXI observations of 103P/Hartley 2. This model will facilitate analyzing the actual spectral data from the Deep Impact and EPOXI missions to better determine abundances of key volatile species, including CO, CO2 and H2O, as well as remote sensing data on active comets.