DPS 34th Meeting, October 2002
Session 16. Comets
Poster, Chair(s): , Tuesday, October 8, 2002, 3:30-6:00pm, Exhibit Hall

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[16.13] The organic volatile composition of Oort-cloud comets: Evidence for chemical diversity in the giant-planets' nebular region*

M. J. Mumma (NASA GSFC), M. A. DiSanti, N. Dello Russo (CUA at NASA GSFC), K. Magee-Sauer (Rowan University), E. Gibb (NAS-NRC at NASA GSFC), R. Novak (Iona College)

Parent volatiles (H2O, CO, CH3OH, CH4, C2H2, C2H6, and HCN) in comets were characterized using CSHELL at NASA’s IRTF and NIRSPEC at Keck. Long-slit infrared spectra are taken at high spectral dispersion and high spatial resolution, eliminating many sources of systematic error and permitting a sensitive search for compositional diversity.

Eight Oort-cloud comets were investigated in this way. Five have compositions similar to that of comet Halley (excepting CO), and three also reveal low formation temperatures (~30K); this group probably formed beyond 30 AU from the young sun. C/1999 S4 LINEAR is depleted in hypervolatiles and (surprisingly) also in methanol; it likely formed near 5-10 AU. Reductions for the remaining comets are in progress.

An emerging hypothesis is that most comets now in the Oort-cloud were formed beyond 30 AU of the young sun, with a small fraction formed between 5 and 30 AU. Although under-represented in the Oort-cloud for dynamical reasons, the latter group dominated the population of icy planetesimals in the 5-40 AU region and thus they likely delivered water and pre-biotic organics to early Earth. Comets Halley, Hyakutake, and Hale-Bopp are not representative of this group. Measurements of HDO/H2O in future comets are planned, to test this idea.

*This work was supported by NASA (Planetary Astronomy RTOP 344-32-30-07 to MJM and NAG 5-7905 to MAD; Planetary Atmospheres NAG 5-10795 and NAG 5-7793 to NDR), NSF (Research at Undergraduate Institutions Grant 0098411 to KM-S), and the National Research Council (NAS-NRC Award to EG).

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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.