nai

Project Progress

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, both observationally and in terms of interpretation of molecular spectra through continually improving analysis software and through comparison with fluorescence models.

Observations

The current progress period featured observational campaigns on two comets: (1) the world-wide effort on Jupiter Family Comet (JFC) 103P/Hartley 2 in support of the EPOXI/DIXI space craft encounter with closest approach of 700 km on 2010 November 4. Our efforts at the Goddard Center for Astrobioloby (GCA) led to a detailed study of the short (hourly/daily, associated with the complex rotation of the nucleus) and long term (seasonal, 2010 Sept. 18 – Dec. 17) behavior of the comet (Mumma et al. 2011). This paper, one of eight featured in a special issue of Astrophysical Journal Letters, represents a tremendous effort (> 1 person-year) by people under the GCA umbrella, including several months by Co-I DiSanti (additional details given below). An additional paper, led by GCA scientific collaborator B. Bonev, will map the rotational and spin temperatures of H₂O in the coma of comet Hartley 2, with targeted submission to a special issue of Icarus in winter/spring 2012. (2) Since August 2011, our Team is conducting serial observations of long-period comet (LPC) C/2009 P1 (Garradd), which comes to perihelion on 2011 December 23. Observations using three ground-based telescope/instrument combinations (VLT/CRIRES, Keck/NIRSPEC, IRTF/CSHELL) are already in hand that encompass the period 2011 August 7 – October 13, and proposals for post-perihelion observations in observing semester 2012A are pending. A number of papers (2-3) reporting these pre-perihelion observations are in progress, including one led by Co-I DiSanti on the NIRSPEC observations, in which all key primary volatiles were measured in less than two hours of clock time (Fig. 2).

In addition to these extended campaigns, we have papers either nearing submission reporting the compositions of comets we have observed, some within the current performance period:

• 10P/Tempel 2, JFC measured with NIRSPEC in July and September 2010. A paper is currently in revision (Paganini et al.).
• C/2010 G2 (Hill), LPC measured with NIRSPEC. This was a back-up target to C/2010 X1 (Elenin), which disrupted and subsequently disintegrated beginning in August 2011, approximately one month prior to its perihelion passage inside 0.5 AU from the Sun. Although 5-6 magnitudes fainter than predictions for Elenin, hyper-volatiles (CH₄, C₂H₆) were clearly seen in Comet Hill (Fig. 3), which had a heliocentric distance R = 2.06 AU. These results demonstrate the capability of NIRSPEC in studying hyper-volatiles in relatively faint comets at larger R.
• C/2007 N3 (Lulin), LPC measured in 2009 with NIRSPEC, are coming to fruition in a paper led by scientific collaborator Erika Gibb. Co-I DiSanti was primary in analysis of echelle orders targeting CO (Fig. 4A) and H₂CO (Fig. 4B).
• C/2007 W1 (Boattini), LPC measured in 2008 with CRIRES and NIRSPEC. The NIRSPEC results are recently published (Villanueva et al. 2011a), and these suggest at least two phases of ice, one polar and one apolar. The CRIRES results were obtained over a series of R-values and a paper (led by Co-I DiSanti) is in preparation.

Fluorescence Modeling

To interpret our spectroscopic observations of comets we rely heavily on accurate models of predicted line-by-line intensities. The current progress period has featured significant advances in this capacity, and these feature both quantum mechanical and empirical approaches. Rigorous quantum mechanical models for C₂H₆ (ν7 band), H₂O (multiple bands), and CH₃OH (ν3 band) have been led by scientific collaborator Geronimo Villanueva.

Co-I DiSanti has developed an empirically-based model for the ν2 band of CH₃OH centered near 2999 cm-1 (3.33 µm), incorporating approximately 100 of the most prominent lines based on laboratory measurements and assignments. Empirical g-factors were established using NIRSPEC spectra of 8P/Tuttle (Figure 5, top; 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 lines were based on laboratory jet-cooled spectra of CH3OH near 20 K (Xu et al. 1997, J. Mol. Sp. 185, 158-172), and lower (ground) state energies were taken from Mekhtiev et al. 1999 (J. Mol. Sp. 194, 171-178). These empirical g-factors were then applied to the short-period (Jupiter Family) comet 21P/Giacobini-Zinner (Figure 5, middle).

Objectives of 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. Application of up-to-date fluorescence models (particularly for CH3OH) will modify conversion efficiencies for the comets shown in Fig. 1, and are being applied to the additional comets in our database. Also, accounting for reduced pumping solar photon intensities is expected to increase production rates (by up to ~ 20 percent) for CO in comets at small heliocentric velocities.

E/PO activities:

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 2011: Co-I DiSanti began serving as mentor to graduate student Adam McKay (New Mexico State U), who was awarded a NASA Graduate Student Researchers Program fellowship. His thesis will use optical and infrared spectra of comets to constrain the relative contributions from H₂O and other molecules (principally CO, CO₂) to atomic oxygen, as observed through its forbidden lines at 5577 and 6300, 6364 A. Mr. McKay’s stipend is for 3 years at no cost to the NAI.