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Project Progress

Project Progress (1 September 2009 – 31 August 2010):
Our studies of native ices in comets (i.e., those contained in the nucleus) have important implications for Astrobiology. These volatiles, which we measure routinely in comets, include several important pre-biotic molecules that are thought to play significant roles in the origin of life on Earth. For example, HCN and NH₃ lead to amino acids, H₂CO leads to sugars, and CH₄ and C₂H₆ are suggested as respective precursors to the methylamine and ethylamine that were unambiguously identified in Stardust samples returned to Earth from comet 81P/Wild 2.

Co-I DiSanti’s research emphasizes the chemistry of volatile oxidized carbon in comets, in particular the efficiency of converting CO to H₂CO and CH₃OH on the surfaces of icy interstellar grains, through H-atom addition reactions prior to their incorporation into the nucleus. Laboratory experiments have shown this process to be dependent both on temperatures and H-atom densities in the pre-cometary (interstellar cloud) environment. We see considerable variation among comets, both in abundances of CO, H₂CO, and CH₃OH (Fig. 1A) and in CO conversion efficiency (Fig. 1B). 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. Our recent observations continue to support compositional diversity among comets (DiSanti and Mumma 2008).

We have demonstrated continued productivity in our comet research over the past year. Coordinated observations with Dr. Stefanie Milam, a recent civil service hire at Goddard, were used to investigate the volatile composition of the long-period comet C/2009 R1 (McNaught) in both the infrared (using CSHELL at the IRTF; Figure 2) and millimeter (using the KPNO 12-m)/sub-millimeter (using SMT) (Milam et al., in preparation). At least seven molecular species were detected. This extended our previous effort that led to a paper combining IR and millimeter observations of C/2006 M4 (SWAN), which represented our first effort to measure parent molecules in these two frequency regimes (DiSanti et al. 2009a).

During the current period the fluorescence modeling component of Co-I DiSanti’s work has shifted emphasis from H₂CO to CH₃OH. This modeling effort has two aspects: (1) A rigorous quantum mechanical application to the ν₃ band centered near 2844 cm-1 (3.52 µm), using measured line strengths (near 200 K) together with rotational quantum numbers and energies. (2) An empirical approach to lines in the ν₂ band centered near 2999 cm^-1 (3.33 µm). Empirical g-factors were established for approximately 50 lines using NIRSPEC spectra of 8P/Tuttle (Figure 3A; see also Figs. 1c and 1d in Bonev et al. 2008 ApJ 680:L61-L64). A high abundance ratio CH₃OH/C₂H₆ (approaching 10) was reported previously from observations of 8P/Tuttle with NIRSPEC (Bonev et al. 2008) and CRIRES (Bohnhardt et al. 2008 ApJ 683:L100-L104). Line assignments and rest frequencies for these ν2 lines were based on laboratory jet-cooled spectra of CH₃OH near 20 K (Xu et al. 1997, J. Mol. Sp. 185, 158-172). These empirical g-factors were then applied to the short-period (Jupiter Family) comet 21P/Giacobini-Zinner (Figure 3B). The resulting production rate of CH₃OH was consistent with that found from the ν3 Q-branch (Figure 3C), and a similarly high CH₃OH/C₂H₆ was also revealed for 21P (Figures 3D & 3E).

Goals of this Research:

1. Compare modeled and observed H₂CO and CH₃OH 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 fluorescence models, both quantum mechanical (for both H₂CO and CH₃OH) and empirical (for CH₃OH).
2. Measure relative abundances of CO, H₂CO, and CH3OH in observed comets, for comparison with the overall volatile chemistry.
3. From these measured abundances, extend the measurement of CO conversion (see Figure 1) to additional comets, 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.

E/PO activities:

Co-I DiSanti mentored graduate student William Anderson (Catholic U. of America). His thesis topic is the chemistry of oxidized carbon in comet C/2002 T7 (LINEAR), and this work led to the first light curve for a comet based on measurements of multiple parent volatiles. Mr. Anderson currently holds a position in the U.S. Patent Office and his written dissertation is nearing completion.
Co-I DiSanti is 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 of radicals (e.g., C₂, CN, OH, NH₂, etc.) 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. For example, simultaneous (or nearly simultaneous) measurements at optical and IR wavelengths can provide information regarding their parentage. GCA provided scientific/technical guidance for the cometary filter set along with funding for their acquisition.
Summer 2010: Co-I DiSanti co-mentored (with GCA Collaborator Dr. Boncho Bonev as primary mentor) undergraduate student Emily Sudholdt (U. Missouri/St. Louis.) through the NAI Summer Undergraduate Intern Associateship (SUIA) program. Ms. Sudholdt’s project during her 10-week internship at the NASA-Goddard Space Flight Center was to compare lines of OH prompt emission (OH*) in comet C/2007 N₃ (Lulin) in two separate yet overlapping NIRSPEC settings. Although NIRSPEC has broad spectral coverage, H₂O and HDO cannot be sampled simultaneously – these settings were chosen such that one encompasses H₂O and OH* and the other encompasses HDO and OH*, with multiple lines of OH* being common to both settings. The science driver behind this project is to use OH*, which is a proxy for H₂O, to better constrain the abundance ratio HDO/H₂O in comets.

Talks/Presentations

Co-I DiSanti gave an oral talk at the NAI Organics Workshop in March 2010, and also at the Astrobiology Science Conference in April 2010. Both talks concentrated on oxidized carbon in comets.

Refereed Journal Articles during this Reporting Period:
DiSanti, M.A., G.L. Villanueva, S.N. Milam, L.N. Zack, B.P. Bonev, M.J. Mumma, L.M. Ziurys, W.M. Anderson 2009. A multi-wavelength study of parent volatile abundances in comet C/2006 M4 (SWAN). Icarus 203, 589-598.
DiSanti, M.A., G.L. Villanueva, B.P. Bonev, K. Magee-Sauer, J.E. Lyke, M.J. Mumma 2009. Temporal evolution of DI ejecta based on NIRSPEC observations at Keck 2: Parent volatiles and dust. In “Deep Impact as a World Observatory Event: Synergies in Space, Time, and Wavelength,” ESO Astrophysics Symposia, pp. 251-263.
Radeva, Y.L., M.J. Mumma, B.P. Bonev, M.A. DiSanti, G.L. Villanueva, K. Magee-Sauer, E.L. Gibb, H.A. Weaver 2010. The organic composition of Comet C/2000 WM1 (LINEAR) revealed through infrared spectroscopy. Icarus 206, 764-777.