The volatile composition of comets as inferred from gas production

Comets are small (1-10 km in radius) icy objects that orbit the Sun on highly eccentric orbits. The composition of comets has been relatively unalterred since their formation 4.5 billion years ago due to their small size and their cold storage in the Kuiper Belt and Oort Cloud. This makes comets "fossils" that can be studied in order to understand the physical conditions and composition of our Solar System during its infancy. Specifically, studying the volatile (ice) composition of comets can place constraints on molecule formation during the planetary formation stage and volatile transport to the inner Solar System. However, for most comets we must infer the volatile composition of the nucleus from gas present in the coma. The composition of the coma is alterred by physical and chemical processes, so the composition of the coma does not exactly reflect that of the nucleus. In this thesis we present analysis of observations of comets 103P/Hartley and C/2009 P1 Garradd in an effort to understand the physical and chemical processes operating in cometary comae. We obtained optical and NIR spectra in an effort to understand the gas production of comets Hartley and Garradd. We employed the ARCES instrument mounted on the ARC 3.5-meter telescope at Apache Point Observatory in Sunspot, NM to acquire optical spectra, while we used the CSHELL instrument mounted on NASA IRTF on Mauna Kea in Hawaii to acquire NIR spectra. We started our analysis with studies of atomic oxygen using the optical spectra and of CO and H2O using the NIR spectra. Specifically, the 5577 A, 6300, and 6300 A lines can potentially used as a proxy for CO2 in comets, which is very imporant because CO2 cannot be observed from the ground directly. Our analysis of the oxygen lines in several comets confirms that analysis of the oxygen line intensities can be employed to obtain quantitative measurements of CO2 in comets, though the accuracy of this method still needs to be firmly established. We also confirmed from observations of CO, H2O, and atomic oxygen in Garradd that CO photodissociation is not an important source of atomic oxygen in cometary comae. Our analysis of comets C/2006 W3 Christensen and C/2009 P1 Garradd at large heliocentric distance showed that the CO2 abundance in comets at heliocentric distances of > 2.5 AU is systematically higher than that of comets that are observed when they are closer to the Sun. Applying our analysis to other comets at heliocentric distances of < 2.5 AU demonstrates that comets have much higher CO2/H20 ratios than previously thought. This may suggest that comets formed in an oxidizing environment. We extended our analysis to the simple molecules CN, C2, CH, and NH2. These molecules are all products of coma photochemistry, and are not inherently present in the nucleus of the comet in ice form. Therefore understanding the progeny of these molecules is important for understanding coma photochemistry. We found that the CN and NH2 abundances in both Hartley and Garradd can be accounted for by HCN and NH 3 photodissociation, respectively. However, the C2 abundance in both comets cannot be accounted for by invoking only C2H 2 photodissociation. Therefore another source is needed. From studies of the rotational variation of C2 production in Hartley and heliocentric distance variation in Garradd, we present the hypothesis that a large fraction of the observed C2 in these comets originates from the sublimation of carbonaceous dust grains. We provide evidence that CH4 photodissociation cannot be the sole source of CH, and that another source, possibly carbonaceous dust grains or PAH's, is required. From analysis of the rotational variation of mixing ratios in Hartley and heliocentric distance variation of mixing ratios in Garradd, we found evidence that the parent of CN (HCN) is spatially correlated with CO 2 in the nucleus and is distinct from the H2O ice. This suggests that two or more phases of ice exist in cometary nuclei, thereby exhibiting small scale compositional heterogeneity. These results have profound consequences for cometary science, and pave the way for future work in the field. These results will prove beneficial to the in-tepretation of cosmogonie parameters, such as isotope and ortho-para ratios, in photodissociation products such as CN and NH2. This,.along with the possibility of atomix oxygen and C2 serving as tracers for CO2 and carbonaceous dust grains, respectively, will provide new avenues for cometary science that have previously been unexplored.