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Project Progress

Co-I DiSanti’s research emphasizes the chemistry of volatile oxidized carbon, 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 seven long-period comets, plus comet Tempel-1. Our inferred conversion efficiencies among comets in our database range from near 90 percent (in C/2006 M4), to a relatively low efficiency (maximum of ~ 30 percent) in Tempel-1. The C/2006 M4 results will be presented in a poster headed by Co-I DiSanti at the 2007 BioAstronomy conference in San Juan, Puerto Rico. 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.

In spring 2006, we used the high-resolution spectrometers NIRSPEC at the Keck2 10-m telescope and CSHELL at the NASA-IRTF 3-m telescope to characterize the short-period (Jupiter Family) comet 73P/Schwassmann-Wachmann 3, which split in 1995 and so is exposing material from the interior of its pre-split nucleus. We also used CSHELL to observe the long-period (Oort cloud), dynamically new comet C/2006 M4 (SWAN) in November 2006. Comets 2P/Encke, 9P/Tempel-1, and 73P are the only JFCs to be completely characterized thus far in several parent volatiles – results for 9P (Mumma et al. 2005 Science 310:270-274, DiSanti et al. 2007a) and 73P (Villanueva et al. 2006, DiSanti et al. 2007b) have been published, while analysis of 2P should be finalized in the near future.

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), in that the ejecta composition was consistent with that seen in the majority of Oort cloud comets (Mumma et al. 2005 Science). A second paper, on time-resolved evolution of parent volatiles and dust, has now been published in a special issue of Icarus (DiSanti et al. 2007a). This revealed a delay of about 20 minutes after impact until emergence of the volatile component of the ejecta. Once seen, the ejected parent volatile composition remained constant throughout our observing period (until about 90 minutes post-impact; Figure 1). In contrast, 73P was depleted in most parent volatiles (Villanueva et al. 2006, DiSanti et al. 2007b).
Along with HCN and NH3 (both of which we also study), H2CO is thought to play a particularly significant role in the origin of life, through Strecker synthesis of simple amino acids. Co-I DiSanti has applied a fluorescence model for two vibrational bands of H2CO to existing spectral observations of comets within our database. Originally developed for interpretation of H2CO in comet Halley, this is the first application of the model to high-resolution spectra that permit a line-by-line comparison between predicted and observed line intensities. We have developed a methodology for measuring molecular excitation (rotational temperature), essential for retrieval of robust production rates for non-linear species. Figure 2 shows the application of the H2CO model to observations of C/2002 T7, and of our H2O fluorescence model to C/2004 Q2 (Machholz), an Oort cloud comet we observed in late 2004/early 2005. Our methodology is generally applicable to any molecular species for which fluorescent g-factors exist over a range of rotational temperatures.

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Goals of this Research:

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. (See reports of the GCA Cosmic Ice Laboratory, and the GCA Analytical Astrobiology Laboratory.)

E/PO activities:

Co-I DiSanti is currently mentoring an undergraduate student, Ms. Cara Rahon (Iona College), through the GCA Summer Undergraduate Internships in Astrobiology (SUIA) program. Ms. Rahon is analyzing CO, H2O, and other parent volatiles in C/2004 Q2 (Machholz) from NIRSPEC observations in November 2004, and applying the methodology for measuring excitation and production rates in a manner analogous to that shown in Figure 2.

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 using the 1.3-m telescope on Kitt Peak. This effort is led by Dr. Donald Walter (South Carolina State University), who is here at the Goddard Space Flight Center for ten weeks in summer 2007, to prepare a proposal for submission to NSF to request funding for the purchase of filters. 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 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.

Refereed Conference Review Papers

Oxygen in Comets and Interplanetary Dust Particles, S. A. Sandford, S. Messenger, M. A. DiSanti, L. Keller, and K. Altwegg 2007, Chapter in Oxygen in the Earliest Solar System, at which DiSanti gave an Invited talk.
Temporal Evolution of DI Ejecta Based on NIRSPEC Observations at Keck 2: Parent Volatiles and Dust, M. A. DiSanti, G. L. Villanueva, B. P. Bonev, K. Magee-Sauer, J. Lyke, M. J. Mumma 2007. Contribution to the Conference Proceedings for the Workshop on Deep Impact as a World Observatory Event, held August 2006, Brussels, Belgium.
Reservoirs for Comets: Compositional Differences, M. A. DiSanti and M. J. Mumma 2007. Chapter in volume for the Workshop on Comet Nuclei, held October 2006, Bern, Switzerland, at which DiSanti gave an Invited talk.

Meeting Abstracts/Talks

“Infrared Observations of Oxidized Carbon in Comet C/2002 T7 (LINEAR).” Anderson, W. M., M. A. DiSanti, M. J. Mumma, N. Dello Russo, B. P. Bonev, K. Magee-Sauer, and E. L. Gibb, BAAS 38(3), p.545, October 2006, Pasadena, CA. (Oral presentation at the 38th annual meeting of the Division for Planetary Sciences of the American Astronomical Society)