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2005 Annual Science Report

NASA Goddard Space Flight Center Reporting  |  JUL 2004 – JUN 2005

The Organic Volatile Composition of Comets: A Window on the Early Solar System

Project Summary

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.

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

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.


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.


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):

  • Since 1996, we characterized ten Oort cloud comets and four Jupiter Family comets.
  • We routinely measure abundances for seven parent volatiles (H2O, CO, C2H6, CH4, C2H2, CH3OH, HCN) and prompt OH emission (OH* – a proxy for H2O; many multiplets observed ~2.9 —3.7 μm, spanning rotational states J = 2.5 through 17.5).
  • We definitively detected H2CO in six comets (see Report by DiSanti), NH3 in three comets, and OCS in three comets. We developed a search strategy for HDO, and have applied it to four comets.
  • We use CSHELL/IRTF and NIRSPEC/Keck. A principal advantage of these instruments is the small beam size. This allows the study of the spatial distribution of emitters along the slit, and is ideal for native (parent) ices. We typically nod along slit between A- and B-beam positions. This mode of observation (coupled with our small beam sizes) further emphasizes native sources.
  • 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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    Invited Talks:

    2005 March 28 "Methane on Mars: Geochemistry or Biology?" Director’s Seminar Series, NASA Astrobiology Institute.
      March 17 "Methane on Mars: Geochemistry or Biology?" University College of New Jersey ( Trenton ).
      Feb. 25 "Methane on Mars: Geophysics or Biology?" Scientific Colloquium Series, Goddard Space Flight Center ( Greenbelt , MD ).
      Jan. 20 "Detection and mapping of Methane on Mars: Evidence for intense local sources." Space Telescope Science Institute ( Baltimore , MD ).
    2004 Nov. 29 "Comets! What are they? Why are they important? Did they deliver water and pre-biotic organics to early Earth? What mysteries are being solved? What new Puzzles are emerging?". Keck Observatory Public Lecture Series ( Waimea , HI ).
      July 23 "An Emerging Chemical Classification of Comets: Implications for the early Solar System." Invited review in Symposium B1.2: "Interpretation of the remote and in-situ observations of small bodies", 35th COSPAR 2004 ( Paris , France , 18-25 July).
      July 16 "Chemical Diversity among Comets: Implications for delivery of Water and pre-biotic organics to early Earth." Invited Review. BioAstronomy 2004 – Habitable Worlds ( Rejkyavik , Iceland , 12-16 July).
      July 15 "Discovery and Mapping of Methane on Mars." (Inserted Invited). BioAstronomy 2004 – Habitable Worlds ( Rejkyavik , Iceland , 12-16 July).