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Project Progress

The second year accomplishments of Dr. Michael DiSanti (Co-I, Goddard Center for Astrobiology, NAI) fall into two distinct although related categories: (1) Ongoing research on the organic volatile composition of comets, and (2) E/PO-related activities.

Research:

Our group is recognized as the world-leader in studies of parent volatiles in comets through ground-based high-resolution spectroscopy at near-IR wavelengths (~ 1 — 5 µm). Year two has been very exciting. Using the high-resolution spectrometer (NIRSPEC) at the Keck2 10-m telescope, we observed the long-period (Oort cloud) comet C/2004 Q2 (Machholz) in Fall/Winter 2004/2005 and the short-period (Jupiter Family) comet 9P/Tempel-1 in June and July 2005, and measured the volatile (ice) composition of both objects.

Comet Tempel-1 was the target of the NASA Deep Impact Mission, in which a projectile impacted the nucleus on July 4, 2005 at a speed of about 10 km/s. Our compositional measurements revealed a different chemical signature between the nucleus surface material (determined from pre-impact measurements) and the impact ejecta (from post-impact spectra; Mumma et al. 2005).

Co-I DiSanti’s research emphasizes the volatile carbon-oxygen chemistry, 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 six long-period comets, plus comet Tempel-1. Our inferred conversion efficiencies among comets in our database range from as high as 80 percent (in C/2002 T7 (LINEAR), which displayed very strong signatures of both H₂CO and CH₃OH, yet relatively weak CO), to a relatively low efficiency (maximum of ~ 30 percent) in Tempel-1.

Such measurements are important for establishing the role of comets in replenishing Earth’s oceans and for delivery of the seed organic molecules from which life emerged. Along with HCN and NH₃ (both of which we also study), H₂CO is thought to play a particularly significant role in the latter process. Co-I DiSanti has applied a fluorescence model of H₂CO to existing spectral observations of comets within our database. This is the first application of the model to high-resolution spectra, allowing a line-by-line comparison between predicted and observed line intensities. We have developed a methodology for accurately measuring molecular excitation (rotational temperature); this is essential for retrieval of robust production rates. Fig. 1 shows the application to observations of C/2002 T7.

Research Goals:

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.
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.

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E/PO activities:

Co-I DiSanti mentored an undergraduate student, Constantinos Makrides, through the NAI Summer Undergraduate Internships in Astrobiology (SUIA) program. Mr. Makrides streamlined the calculation of line-by-line fluorescence efficiency factors for water, at one-degree intervals over a wide range of temperatures, for comparison with emissions seen in our comet spectra. This effort found immediate application in establishing the rotational temperature of H₂O in, and amount of H₂O released from comet Tempel-1. The methodology for measuring the excitation and production rate of H₂O is completely analogous to that shown for H₂CO in Fig. 1.

Co-I DiSanti is also the node contact for the research effort, within the Minority Institution Astrobiology Cooperative (MIAC), to systematically observe comets through emission-line filters at optical wavelengths. This effort is led by Dr. Donald Walter (South Carolina State University), and will utilize telescopes in Arizona. Imaging studies on the Kitt Peak 1.3-m telescope are in the science-testing phase. The 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.