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Project Progress

Overview
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 (Mumma and Charnley 2011, DiSanti and Mumma 2008).

Observations during the past year
C/2009 P1 (Garradd): Our observations of comets over the past year were centered primarily on long-period comet Garradd, which reached perihelion on 2011 December 23. Using three ground-based telescope/instrument combinations (VLT/CRIRES, Keck/NIRSPEC, IRTF/CSHELL) produced two published papers featuring pre-perihelion results from August and September 2011 (Paganini et al. 2012, Villanueva et al. 2012). An additional paper (led by Co-I DiSanti) combines pre- and post-perihelion observations with NIRSPEC (Fig. 2), and is nearing submission for publication.

Recent results have suggested the importance of oxidation reactions in addition to reduction (H-atom addition) in explaining observed abundances of ices in comets. Finding abundant CO₂ (much higher than CO) in a substantial number of comets surveyed by the Japanese Akari space craft (Ootsubo et al. 2012, ApJ 752:15) argues for oxidation, on interstellar grains and/or following vaporization in the proto-planetary nebula. Comet Garradd had abundant CO (~ 10% relative to H₂O), as first observed by us near R = 2.1 AU using CSHELL (together with NIRSPEC observations on the following day; Villanueva et al. 2012). Together with its strongly depleted C₂H₂ (~ 0.06%) yet nearly “normal”, this also suggests a potential role for oxidation in additional to reduction on its pre-cometary grains (neither C₂H₆ nor CH₃OH are produced efficiently by gas-phase chemistry). Furthermore, spectra of Garradd from the DIXI space craft suggest a non-negligible abundance of CO₂ in Garradd, perhaps rivaling that of CO (Feaga et al. 2012, DPS poster 313.08).

Fig. 3 provides a comparison of ratios of measured abundance ratios (relative to H₂O) for “reactants” (CO and C₂H₂, both condensed onto interstellar grains) versus “products” (C₂H₆ and CH₃OH) among a number of comets in our database. Comets with high CO/C₂H₂ lie well above the horizontal dotted line; this is the case with Garradd, which may represent the best case to date supporting the importance of oxidation on its pre-cometary grains.

Additional comets within the past year. We observed the relatively weak long-period Comet Hill with NIRSPEC (Fig. 4), supporting our program of extending observations to comets at larger Rh (> 2 AU). These observations demonstrate the power of NIRSPEC in studying hyper-volatiles in relatively faint comets at larger R. Together with C/2009 P1 (Garradd), C/2006 W3 (Christensen), and 29P/Schwassmann-Wachmann 1, Comet Hill brings our current total of such comets to four; papers on Garradd (DiSanti et al.), Christensen (Bonev et al.), and S-W 1 (Paganini et al.) are currently in preparation, with submission targeted for fall 2012.

Recent Progress on 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, including both quantum mechanical and empirical approaches. Rigorous quantum mechanical models for C₂H₆ (ν₇ band), H₂O (multiple bands), and CH₃OH (ν₃ band) were led by scientific collaborator Geronimo Villanueva. Co-I DiSanti led development of an empirical fluorescence model for the ν₂ band of CH₃OH (see below).

Progress on studies prior to September 2011, in which Co-I DiSanti played major roles
➢ Comet 21P/Giacobini-Zinner. In June 2005, we obtained spectra in one NIRSPEC setting that simultaneously measured H₂O, C₂H₆ and CH₃OH. As of September 1, a paper is in revision that reports abundances for these primary volatiles (DiSanti et al. 2012). The principal elements of this paper are:

* An extremely precise and accurate production rate for H₂O, incorporating analyses of rotational and spin temperatures, which were highly correlated in 21P/G-Z; this is required for obtaining robust abundances of C₂H₆ and CH₃OH. * Confirmation of highly depleted C₂H₆ in 21P/G-Z, as suggested from observations during its 1998 using IRTF/CSHELL. * Development and application of an empirical model for CH₃OH, including 157 prominent lines of the ν₂ band. Fluorescence g-factors were established using NIRSPEC spectra of 8P/Tuttle (Fig. 5A; see also Figs. 1c and 1d in Bonev et al. 2008 ApJ 680:L61-L64). These empirical g-factors were then applied to the short-period (Jupiter Family) comet 21P/Giacobini-Zinner (Fig. 5B). This yields a production rate for CH₃OH consistent with that we measured by applying our recently published fluorescence model (Villanueva et al. 2012) to ν3 band emissions in an adjacent order (Fig. 5C).

➢ C/2007 N3 (Lulin). We observed this long-period comet in January/February 2009 with NIRSPEC, has come to fruition in the form of a paper led by scientific collaborator Erika Gibb (Gibb et al. 2012). Co-I DiSanti led analyses of echelle orders targeting CO and H₂CO.
➢ C/2009 R1 (McNaught). We observed this long-period comet with IRTF/CSHELL in May and June 2010, when it was near R_h = 1.0 (Fig. 6) and 0.5 AU (Fig. 7), respectively. These data formed the basis for the research project of Undergraduate Research Associate Maria Sofia Stenborg (U. Maryland), who was mentored by Co-I DiSanti (see and text below, under “E/PO activities”).

➢ C/2007 W1 (Boattini). We measured this long-period comet in 2008 with CRIRES and NIRSPEC. The NIRSPEC results are 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 heliocentric distances, and a paper (led by Co-I DiSanti) is in preparation. A new fluorescence model for HCN was applied to Boattini, 8P/Tuttle, and C/2008 Q3 (Garradd), forms a manuscript that is nearing acceptance for publication (M. Lippi et. al).

E/PO activities:
Summer 2012: Co-I DiSanti served as mentor to GCA-NAI Undergraduate Research Associate Maria Sofia Stenborg (U. Maryland). Ms Stenborg processed CSHELL infrared spectra of long-period comet C/2009 R1 (McNaught). Six primary volatiles (H₂O, CO, H₂CO, CH₃OH, C₂H₆, CH₄) were measured near heliocentric distance 0.5 AU, and H₂O and CO were measured near 1.0 AU. These observations, together with sub-millimeter-wavelength measurements processed by fellow Intern Lillian Haynes (mentored by GSFC civil servant Dr. Stefanie Milam), comprise a manuscript led by Dr. Milam, with submission to The Astrophysical Journal planned for Fall 2012.

Summer 2012: Co-I DiSanti is serving as mentor to graduate student Adam McKay (New Mexico State U.), who spent ten weeks in residence at GSFC processing pre- and post-perihelion spectra acquired on long-period Comet C/2009 P1 (Garradd), using CSHELL. Mr. McKay was awarded a NASA Graduate Student Researchers Program fellowship, with his second of three years beginning 2012 August 1. His thesis uses 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 support is for 3 years at no cost to the NAI. His work supports our effort of building a compositional taxonomy of comets.