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M. J. Mumma (NASA/GSFC), N. Dello Russo, M. A. DiSanti (Cath. Univ. Amer. at GSFC), K. Magee-Sauer (Rowan Univ.), R. Novak (Iona Coll.)
We investigated organic volatiles in six Oort-cloud comets at infrared wavelengths, using CSHELL at NASA's IRTF and NIRSPEC at the Keck observatory. For reasonably bright comets, the production rates of seven parent volatile species are measured in a space of two hours, with NIRSPEC. These include H2O, CO, CH3OH, CH4, C2H2, C2H6, and HCN. We extract intensities for multiple lines of select species, permitting determination of rotational temperatures, apparent production rates, and spatial profiles for these at various points along the slit. (Spatial profiles are used to eliminate systematic errors that would otherwise be introduced by factors such as atmospheric seeing, and to identify and compensate for optical depth effects. They also permit measurement of asymmetric release from the nucleus, and separate identification of native and distributed sources for a given species.) Systematic errors are reduced further when all species are measured with a single instrument on a single telescope, and over a time interval short compared with nucleus rotation. The resulting parent volatile production rates are highly robust, permitting a search for compositional diversity among these comets. Four have similar chemistry (excepting CO) and independent evidence reveals low formation temperatures (~ 30K), so they probably originated beyond 30 AU. The fifth is depleted in hypervolatiles and also in methanol, and likely formed near 5 - 10 AU. The sixth is enriched in ethane - its formation zone will be discussed. These results will be discussed in the context of current models for interstellar and nebular processing, and for delivery of volatiles to Earth.
This work was supported by the NASA Planetary Astronomy program (RTOP 344-32-30-07; NAG5-7905), the NASA Planetary Atmospheres Program (NAG5-7753), and the NSF (AST-9619461).
The author(s) of this abstract have provided an email address for comments about the abstract: mmumma@kuiper.gsfc.nasa.gov