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Project Progress

Along with HCN and NH₃ (both of which we also study), H₂CO is thought to play a particularly significant role in the origin of life, through Strecker synthesis of simple amino acids. Co-I DiSanti’s research emphasizes the chemistry of volatile oxidized carbon, in particular the efficiency of converting carbon monoxide to formaldehyde and methyl alcohol on the surfaces of icy grain mantles prior to their incorporation into the nucleus. This process has been shown experimentally to be temperature-dependent, and we have now measured CO, H₂CO, and CH₃OH in eight long-period comets, plus comet Tempel-1. Our inferred conversion efficiencies among comets in our database range from near 90 percent (in comets C/2006 M4 Swan and, more recently, in 8P/Tuttle), to a relatively low efficiency (~ 30 percent) in Tempel-1. Such measurements are important for establishing the role of comets in delivering water to Earth along with the seed organic molecules from which life emerged.

The past year has been an extremely busy and productive time for our comet research, beginning with the surprisingly high activity of the short period (Jupiter Family) comet 17P/Holmes in late October 2006. With NIRSPEC at Keck-2, we measured six parent volatiles (including H₂O; Salyk et al. IAUC 8890, 2007 November 6). Compared with the majority of comets in our database, 17P displayed an enrichment of organic ices.

We used the ultra high-resolution CRIRES (which achieves up to λ/Δλ ~ 10⁵) at the VLT to observe the Halley-family comet 8P/Tuttle (Boehnhardt et al. 2008) in January/February 2008, and the dynamically new long-period (Oort cloud) C/2007 W1 (Boattini) in May and July. These observations, combined with NIRSPEC observations (e.g., Bonev et al. 2008), permitted the volatile compositions of these two objects to be studied in detail. Their inter-comparison revealed distinct chemical signatures: 8P had severely depleted CO and H₂CO, somewhat depleted C₂H₆, and somewhat enriched CH₃OH, while W1 had highly enriched C₂H₆, somewhat enriched CO, depleted H₂CO, and fairly normal CH₃OH (Figs. 1 and 2).

In addition, we used CSHELL to observe the long-period, dynamically new comet C/2006 M4 (SWAN) in November 2006. In July 2007, the SWAN results were presented in a poster headed by Co-I DiSanti at the 2007 BioAstronomy conference in San Juan, Puerto Rico, and a manuscript is nearing submission (DiSanti et al., in preparation). Our recent observations continue to support compositional diversity among comets (DiSanti and Mumma 2008; Fig. 3).

Co-I DiSanti has applied a fluorescence model for two vibrational bands of H₂CO to existing spectral observations of comets within our database. Originally developed for interpretation of H₂CO in comet Halley, this is the first application of the model to high-resolution spectra that permit a line-by-line comparison between predicted and observed line intensities. We have developed a methodology for measuring molecular excitation (rotational temperature), essential for retrieval of robust production rates for non-linear species. Figure 4 shows the application of the H₂CO model to observations of C/2002 T7, and of our H₂O fluorescence model to C/2004 Q2 (Machholz), an Oort cloud comet we observed in late 2004/early 2005. Our methodology is generally applicable to any molecular species for which fluorescent g-factors exist over a range of rotational temperatures. We are currently extending our H₂CO model to higher rotational quantum number, based on new fits to laboratory spectra.

Goals of this Research:

1. Compare modeled and observed H₂CO line intensities, to accurately measure its abundance in comets, as well as to reveal potential discrepancies between model and data. This involves utilizing comets as test beds for the H₂CO fluorescence model.

2. Measure relative abundances of CO, H₂CO, and CH₃OH in observed comets, for comparison with the overall volatile chemistry.

3. From these measured abundances, determine the efficiency of CO conversion in an attempt to establish the conditions (e.g., H-atom density, temperature) to which the pre-cometary ices were exposed. This requires comparison with yields from irradiation experiments on cometary ice analogues as a function of temperature, and also with observational data on these ices in interstellar and proto-stellar sources. (See reports of the GCA Cosmic Ice Laboratory, and the GCA Analytical Astrobiology Laboratory.)

E/PO activities:

Co-I DiSanti is mentoring graduate student William M. Anderson (Catholic U. of America). Mr. Anderson will defend his PhD thesis in fall 2008. His thesis topic is the chemistry of oxidized carbon in comet C/2002 T7 (LINEAR), and how its chemistry compares with other comets belonging to two principal dynamical reservoirs: Long-period comets, coming from the Oort cloud (such as T7 LINEAR), and short-period comets that are in dynamical resonance with the Sun and Jupiter (the so-called “Jupiter Family” comets, JFCs). The JFCs are thought to come primarily from the scattered Kuiper disk population of comets.

Co-I DiSanti is also the GCA contact for the research effort, within the Minority Institution Astrobiology Cooperative (MIAC), to systematically observe comets through emission-line filters at optical wavelengths using the 1.3-m telescope on Kitt Peak. Dr. Donald Walter (South Carolina State University) leads this effort. Molecules giving rise to the IR emissions are photo-dissociated in the coma, producing the “daughter” fragments (radicals) to be targeted by the MIAC filter imaging program. The intent is to provide early molecular detections to assist in evaluating newly discovered comets as Targets-of-Opportunity and to provide production rates and 2-D images for selected daughter volatiles, in support of our principal comet program. GCA provided scientific/technical guidance for the cometary filter set along with funding for their acquisition.

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