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.