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Project Progress

This research is targeted on building a new taxonomy for comets based on their parent volatile compositions, especially organics. This taxonomy is crucial for understanding the role of each taxonomic group in delivering prebiotic organics and water to early Earth. During this reporting period, we identified the first comet of Kuiper belt origin belonging to the “organics-depleted” class (joining two Oort cloud comets of similar composition). This demonstration that icy planetesimals of both “organics-normal” and “organics-depleted” groups are found in both reservoirs is consistent with a new model for disruption of the proto-planetary disk. We also identified a comet that is enriched in organics. This could be the first example of a third taxonomic class. Our long-range objective is to link each organic group to a temperature and location of formation through measurements of the D/H ratio in individual chemicals and the nuclear spin temperatures of water and other compounds, and to assess the role of each taxonomic group in exogenous delivery of organics and water to early Earth through dynamical models. Our Team made significant progress during this reporting period by observing additional comets while also working off a large backlog of earlier investigations. This was enabled by major new software tools that streamlined our pipeline data processing and science extraction very significantly.

Detailed Description of Principal Results:

We investigated the organic composition of native ices in comet 73P/Schwassman-Wachmann 3 (SW3), a Jupiter Family Comet (JFC) that likely originated in the scattered Kuiper Disk. This comet split into several pieces in 1995, several of which survived its next perihelion passage in 2001. Its apparition in 2006 was widely anticipated because it provided an unprecedented opportunity to characterize freshly exposed material from the (presumably) pristine interior of the pre-split nucleus. We investigated the comet using high-dispersion infrared spectroscopy at 3 major telescopes on Mauna Kea (HI): the 8-m Japanese National Telescope (Subaru, with IRCS), the 10-m Keck-2 telescope (with NIRSPEC), and the 3-m NASA Infrared Telescope (IRTF, with CSHELL). We characterized the organic composition of two major fragments (SW3-B and SW3-C), showing that they displayed similar organic compositions and that both were severely depleted in organic volatiles, compared with the “organics-normal” comets. The composition of this comet was indistinguishable from another “organics-depleted” comet that came from the Oort cloud reservoir, demonstrating that comets of this unusual composition reside in both reservoirs. This is consistent with emerging models for migration and ejection of icy bodies from the outer region of the proto-planetary disk. Three papers were published.

In a prior year, we investigated the nature of material excavated from the sub-surface material from the JFC 9P/Tempel 1 by a controlled impact (Deep Impact Mission), using NASA IRTF/CSHELL and Keck-2/NIRSPEC. We showed that the ejecta displayed a chemical composition consistent with the “organics-normal” class of Oort cloud comets (Mumma et al. Science 310: 270-274 (2005)). During this reporting period, we published a sequel in which we explored the detailed temporal evolution of parent volatiles (H2O, C2H6, and HCN) and dust in the ejecta cloud during the first hour after impact. NPP Fellow Geronimo Villanueva developed a Monte-Carlo model for the expanding dust cloud, and showed that it reproduced the measured temporal development (both spatial development and intensity) of the thermal continuum (see Figure 1, below). GCA co-Investigator Michael DiSanti showed that ejected ethane and water in the plume increased suddenly 20 minutes after impact, suggesting that icy grains vaporized after being released from the nucleus. This could be the signature of decreasing optical depth in the expanding dust coma, modeled by Villanueva. Both gaseous species peaked 30 minutes after impact and then declined steadily (as did HCN). DiSanti provides further details in his contribution to this Annual Report. Mumma, DiSanti, and Villanueva gave reviews at a major International Workshop on this event (see below).

“Temporal Evolution of Parent Volatiles and Dust in Comet 9P/Tempel 1 Resulting from the Deep Impact Experiment.” M. A. DiSanti, G. L. Villanueva, B. P. Bonev, K. Magee-Sauer, J. E. Lyke, and M. J. Mumma, Icarus 187:240-252 (2006).

Also in this reporting period, we published the first comprehensive test of chemical heterogeneity within the nucleus of a split Oort cloud comet (C/2001 A2 (LINEAR)). GCA collaborator Erika Gibb reported quantitative measurements of methane, formaldehyde, and water in this comet, acquired with Keck-2/NIRSPEC spanning a period of five weeks. Gibb et al. (2007) showed that the evidence supported internal heterogeneous chemistry on macroscopic scale, possibly the signature of radial migration of chemically diverse cometesimals in the proto-planetary disk. GCA collaborator Karen Magee-Sauer extended this work to the overall organic composition of this unusual comet, showing that A2 is the first example of an “organics-enriched” comet. Based on seven organic parent volatiles (C₂H₆, C₂H₂, HCN, CH₄, CO, CH₃OH, H₂CO) measured simultaneously with H₂O, she reports this comet is enriched in several of these (HCN, C₂H₂, C₂H₆, and CH₃OH) by a factor of ~ 2 – 3, compared with comets of the “organics-normal” group. A paper is undergoing review at this time.

“The Organic Composition of Comet C/2001 A2 (LINEAR): Search for Heterogeneity within a Comet Nucleus.” E. L. Gibb, M. A. DiSanti, K. Magee-Sauer, N. Dello Russo, B. P. Bonev, and M. J. Mumma, Icarus 188:224-232 (2007).

“The Organic Composition of Comet C/2001 A2 (LINEAR): Evidence for an Unusual Organic Chemistry.” K. Magee-Sauer, M. J. Mumma, M. A. DiSanti, N. Dello Russo. E. L. Gibb, B. P. Bonev, and G. L. Villanueva, Icarus (submitted).

Also in this reporting period, we published the first comprehensive spectral survey of the key organics wavelength region (2.9 -3.6 μm) that encompasses vibrational fundamental bands of aliphatic and aromatic organic volatiles (Dello Russo et al. 2006). This survey was acquired in 1999 with Keck-2 NIRSPEC during its commissioning. The organic composition of Oort cloud comet C/1999 H1 (Lee) derived from these data was published earlier (Mumma et al., Ap. J. 546:1183-1193 (2001)), but the full survey appeared only now. (Comet Lee now is recognized as an “organics-normal” comet.) The full survey contains 525 spectral lines, of which 427 are identified with specific molecules.

“A High-Resolution Spectral Survey of Comet C/1999 H1 (Lee)”. N. Dello Russo, M. J. Mumma, M. A. DiSanti, K. Magee-Sauer, E. L. Gibb, B. P. Bonev, I. S. McLean, and Li-Hong Xu , Icarus 184:255-276 (2006).

Several basic advances were made through studies of individual molecular species. Boncho P. Bonev published two basic papers on OH prompt emission in comets, augmenting his initial paper on this topic (the combined work constituted his Ph. D. dissertation, performed in residence at Goddard and reported in 2006). Photolysis of H₂O produces excited fragments, in this case OH in highly excited rotational levels in v=1. Bonev et al. (2007) obtained quantitative emission efficiencies for OH doublets in the P-branch region of the 1-0 vibrational band, and showed that they track the spatial profile of water, and not that of quiescent OH in the cometary coma. This work permits these lines to be used as a direct proxy for water itself, enabling simultaneous measurements of the dominant H₂O and an individual trace gas (parent volatile). Their imultaneous measurement eliminates many sources of systematic error when they are measured separately. Bonev and Mumma (2007) related the OH prompt emission to specific characteristics of the internal dissociative channels of the H₂O parent.

“A Comprehensive Study of Infrared OH prompt Emission in Two Comets. I. Observations and Effective g-factors.” B. P. Bonev, M. J. Mumma, M. A. DiSanti, N. Dello Russo, K. Magee-Sauer, R. S. Ellis, and D. P. Stark, Ap. J. 653:774-787 (2006).

“A Comprehensive Study of Infrared OH Prompt Emission in Two Comets. II. Implications for Unimolecular Dissociation of H₂O.” B. P. Bonev and M. J. Mumma, Ap. J. 653:788-791 (2006).

We measured spectrally resolved vibrational spectra of formaldehyde (H₂CO) for the first time, capping a 20-year development of this capability that began with comet Halley (Mumma and Reuter 1989, Ap. J. 344:940-948). This permitted extraction of rotational temperatures for H₂CO, and thus of production rates for release of the monomer from the cometary nucleus in C/2002 T7 (LINEAR). DiSanti provides further details in his contribution to this Annual Report.

“Detection of Formaldehyde Emission in Comet C/2002 T7 (LINEAR) at Infrared Wavelengths: Line-by-Line Validation of Modeled Fluorescent Intensities.” M. A. DiSanti, B. P. Bonev, K. Magee-Sauer, N. Dello Russo, M. J. Mumma, D. C. Reuter, and G. L. Villanueva, Ap. J. 650: 470-483 (2006).

We also broke new ground in testing the significance of the ortho-para ratio in cometary water. First measured in comet 1P/Halley, the OPR (and the inferred nuclear spin temperature) has been sought in every comet since then (see Figure 2). In many comets, the OPR is less than the statistical equilibrium value (3.0) leading to the suggestion that the OPR is a cosmogonic invariant that is set when the H₂O molecule is formed. If so, it would be a key test of the temperature at which the water in a specific comet formed and thus of its formation region. We measured, for the first time, the OPR vs. distance from the nucleus in an active comet, testing whether it was invariant. Bonev et al. (2007) report that the OPR was invariant in C/2004 Q2 Machhholz), even though the rotational temperature changed greatly.

“A Search for variation in the H₂O Ortho-Para ratio and Rotational Temperature in the inner coma of Comet C/2004 Q2 (Machholz).” B. P. Bonev, M. J. Mumma, G. L. Villanueva, Michael A. DiSanti, Richard S. Ellis, K. Magee-Sauer, and Neil Dello Russo, Ap. J. 661:L97-L100 (2007).

In the coming year, graduate student Yana Radeva (Univ. of Maryland) will conduct similar tests for a comet in which the nucleus-centered OPR was relaxed (2.4). This will test whether the spin temperature (30K) is reset to the H₂O rotational temperature (ca. 70K) by collisions in the coma.

Invited Talks and Reviews by M. J. Mumma

“Parent Volatiles in 9P/Tempel 1 as seen with NIRSPEC at Keck-2: Before and After Impact.” ESO Workshop: Deep Impact as a World Observatory Event – Synergies in Space, Time, and Wavelength (Vrije Universitat, Brussels, Belgium, MD 7-10 August 2006).


“The Icy Planetesimals: A record of the early Planetary System.” Workshop on Astrophysics Enabled by a Return to the Moon (STScI, Baltimore, MD 27-30 Nov. 2006).


“Comets as Messengers from the Early Solar System: A Guide to the Properties of Other Planetary Systems.” ESO Workshop on Planetary Systems (Santiago, Chile, 5-8 March 2007)


“Comets, and implications for delivery of water and pre-biotic organics to Young Planets (and a bit of Atomic-Molecular Physics, too!).” Inst. Phys. Sci. & Tech., Univ. of Maryland (9 April 2007, College Park).


“Comets and Astrobiology: implications for delivery of water and pre-biotic organics to early Earth.” Centro de Astrobiologia (20 April 2007, Madrid, Spain)

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