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Project Progress

Principal Objective:

Our long-range objective is to establish a taxonomy for comets based on chemistry, rather than orbital dynamics. The formation temperature of a given comet can be constrained by measures of the ortho-para ratios in H₂O and by the isotopic enhancements (e.g., in deuterium) in selected volatile species. We now measure the OPR routinely in even moderately bright comets, and often find relaxed spin ratios. A comparison of mid-plane disk temperatures (and chemical abundances) predicted by nebular models will assist in localizing the heliocentric distance at which the pre-cometary ices formed in the protoplanetary disk. Some volatile abundances test the presence of remnant ices from the interstellar natal cloud core. In this way, we will test cometary delivery of organics and water to the young terrestrial planets, and their role in enabling conditions favorable to the emergence of life.

Personnel:

Our core Team consists of co-I M. A. DiSanti (GSFC), Karen Magee-Sauer (Rowan University), Erika L. Gibb (Univ. Missouri-St. Louis), Geronimo Villanueva (NAS-NRC at Goddard), and graduate students Boncho P. Bonev (Univ. Toledo at GSFC) and William Anderson (Catholic Univ. at GSFC). We augment the Team with other scientists for specific projects, as appropriate. During year two of the NAI work, long-term collaborator Neil Dello Russo departed Goddard and began new responsibilities at Applied Physics Lab (Johns Hopkins Univ.). Post-doctoral associate Geronimo Villanueva arrived, and graduate student Avram Mandell (Penn State Univ., Astrobiology team) began his dissertation research at Goddard. Two undergraduate students conducted research with us at Goddard under the NAI Summer Undergraduate Internships in Astrobiology (SUIA) program: Yana Radeva (Connecticut College) returned for a second summer conducting research on Comet C/2001 WM1, and Constantinos Makrides (Iona College) worked on aspects of water fluorescence in comets.

Research:

Our group is recognized as a world-leader in studies of parent volatiles in comets through ground-based high-resolution spectroscopy at near-IR wavelengths (~ 1 — 5 µm):

Progress:

Year two has been very exciting. Using the high-resolution spectrometer (NIRSPEC) at the Keck-2 10-m telescope, we observed the long-period (Oort cloud) comet C/2004 Q2 (Machholz) in Fall/Winter 2004/2005 and the short-period (Jupiter Family) comet 9P/Tempel-1 in June and July 2005, and measured the volatile (ice) composition of both objects. Comet Machholz was observed in November 2004 by the core Team and in January 2005 by collaborators Richard Ellis and Dan Stark (Calif. Inst. Tech.). The spectral lines in January were the brightest ever observed with NIRSPEC, and multiple species were detected (H₂O, C₂H₆, HCN, CO, CH₃OH, H₂CO, C₂H₂, CH₄, NH₂, OH*, and more). A small portion of the spectrum is shown in Fig. 1. Boncho Bonev has incorporated these data into his dissertation study of OH prompt emission (OH*) as a tracer for water in comets. Other Team members quantified individual parent volatile species to characterize the overall chemistry.

The Jupiter-family 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. Eight investigators were added to six from our core team for this investigation. We quantified eight parent volatiles (H₂O, C₂H₆, HCN, CO, CH₃OH, H₂CO, C₂H₂, and CH₄) in Tempel-1 using high-dispersion infrared spectroscopy in the wavelength range 2.8-5.0 µm (Fig. 2). Our compositional measurements revealed that the impact ejecta displayed a different chemical signature compared with the nucleus surface material (determined from pre-impact measurements) (Mumma et al. 2005). The abundance ratio for ethane was significantly higher after impact (UT 4 July) whereas those for methanol and hydrogen cyanide were unchanged. The abundance ratios in the ejecta are similar to most Oort-cloud (OC) comets, but methanol and acetylene are lower in Tempel-1 by a factor of about two. These chemical similarities suggest that Tempel-1 and most OC comets originated in a common region of the protoplanetary disk; this is consistent with the view that comet nuclei in both the scattered Kuiper-Edgeworth disk (the proposed source reservoir for most ecliptic comets) and the Oort cloud originated in the outer giant-planets’ region of the protoplanetary disk. The depleted ethane abundance during quiescent release and its similarity to values found for 2P/Encke and 21P/ Giacobini-Zinner suggests that the surfaces of short period comets have been processed thermally.

Cryogenic laboratory measurements show that the efficiency of converting carbon monoxide to formaldehyde and methyl alcohol on the surfaces of icy grain mantles depends on temperature, and so their abundance ratios in a comet may reveal the temperature at which pre-cometary ices formed prior to their incorporation into the nucleus. Co-I DiSanti has now measured CO, H₂CO, and CH₃OH in Tempel-1 and six long-period comets in our database. The inferred conversion efficiencies among these comets range from ~ 30 percent in Tempel-1 to 80 percent in C/2002 T7 LINEAR (T7 displayed very strong signatures of both H₂CO and CH₃OH, yet relatively weak CO). Such measurements are important for establishing whether comets seeded Earth with pre-biotic organic molecules after Earth-Moon formation. Along with HCN and NH₃ (both of which we also study, lead by Magee-Sauer), H₂CO is thought to play a particularly significant role in the latter process. (see Report by DiSanti)

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