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M. A. DiSanti (Catholic U. / NASA-GSFC), M. J. Mumma (NASA-GSFC), N. Dello Russo (Catholic U. / NASA-GSFC), K. Magee-Sauer (Rowan U.), R. Novak (Iona Coll.), T. W. Rettig (U. Notre Dame / NSF)
The high volatility of carbon monoxide makes it an excellent probe of conditions in the early-solar system environment in which comets are thought to have formed. A fundamental question in cometary science is: Did the constituent ices of the nucleus originate in the interstellar natal cloud, or were they formed later by condensation from gas in the pre-planetary disk? Interstellar CO-rich ice could not have survived within about 40 AU of the proto-sun, but interstellar CO trapped as a minor component in a polar (i.e., water-rich) ice could have survived in the giant-planets nebular region. Thus the amount of CO retained in the cometary nucleus (native CO), and its abundance relative to water, provide important clues to the origin of this material and to conditions in the pre-cometary environment.
A summary of CO abundances in several recent Oort-cloud comets will be presented, based on high-resolution (RP ~ 15,000 - 25,000) observations with CSHELL at the NASA-IRTF and NIRSPEC at Keck 2, and comparisons to water and other volatile (e.g., methyl alcohol, CH3OH) abundances will be made. Recently observed CO abundances range from ``high CO" in comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), for which Q(CO) / Q(H2O) > 10 %, to CO-poor comets C/1999 H1 (Lee; CO/H2O ~ 2 %) and D/1999 S4 (Linear; CO/H2O ~ 1 % or less). Implications of these diverse chemistries will be discussed.
This work was supported through the NASA Planetary Astronomy Program (Grant NAG5-7905).