Within a Cometary Nucleus High-Resolution Infrared Spectroscopy of Comet 73P/Schwassmann-Wachmann 3: A Test for Chemical Heterogeneity Within a Cometary Nucleus N. Dello Russo1, R. J. Vervack Jr.1, H. A. Weaver1, N. Biver2, D. Bockelée-Morvan2, J. Crovisier2 and C. M. Lisse1 1 JHU/APL, 2Observatoire de Paris, France 38th Annual DPS Meeting, Pasadena, California Monday, October 9, 2006 Comet Schwassmann-Wachmann 3 - I: 03:05

May 3.5 & 9.6 observations were obtained with CSHELL at the NASA IRTF ( ~ 25,000). May 14.6 & 15.6 observations were obtained with NIRSPEC at the Keck Observatory ( ~ 25,000).

On this date 73P-B was significantly more productive than 73P-C. Species detected in 73P-B on this date include: H2O, HCN, C2H6, and CH3OH. Species detected in 73P-C: H2O, HCN, and C2H6 (tentative).

Multiple strong volatile emission lines detected in both fragments. A spectral comparison of the fragments suggests a very similar composition for 73P-B & 73P-C.

Relative line intensities are sensitive to rotational temperature. Line-by-line analysis constrains Trot. Aperture size: (9 x 3 pixels) (~ 80-100 km) x (~ 21-26 km) centered on the nucleus.

These data are consistent with “normal” HCN and depleted C2H2 in both 73P-B and 73P-C.

Significant variation in the H2O production rates with date, especially in 73P-B. The relative abundances within a fragment were relatively stable from day-to-day despite significant changes in the overall activity. Relative abundances between 73P-B and 73P-C are also very similar suggesting a homogeneous volatile composition in the nucleus of 73P.

Compositionally, with the exception of HCN, 73P more closely resembles the depleted volatile comet C/1999 S4 than a “typical” comet.

Conclusions 73P has been fragmenting and shedding material for at least the past 11 years making it likely that relatively pristine comet material was sampled. The measured rotational temperature was significantly higher for H2O than HCN in the inner coma (within ~ 50 km of the nucleus). 73P-B & 73P-C are similar in composition and depleted (with respect to H2O) in all measured volatile species except for HCN. Low C2 abundances measured in 73P (Fink and Hicks 1996; Schleicher 2006, IAUC 8681) are consistent with depleted C2H2 in 73P. C2H6 >> C2 indicates that C2H6 is not an efficient source of C2 in the coma (consistent with predictions e.g. Helbert et al. 2005). These data suggest the nucleus of 73P is homogeneous in volatile composition. 73P provided an excellent opportunity to measure the chemical composition of a Jupiter-family comet. The number of comets sampled to date is probably still too small to define a compositional taxonomy, but recent progress in measurement capabilities promises to change that over the next decade.