The Unusual Organic Volatile Composition of Comet 73P/Schwassmann-Wachmann 3: Is SW3 the Third Member of a Distinct Taxonomic Group?

Details The Unusual Organic Volatile Composition of Comet 73P/Schwassmann-Wachmann 3: Is SW3 the Third Member of a Distinct Taxonomic Group? The Unusual Organic Volatile Composition of Comet 73P/Schwassmann-Wachmann 3: Is SW3 the Third Member of a Distinct Taxonomic Group?

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.0308m&link_type=abstract

American Astronomical Society, DPS meeting #38, #03.08; Bulletin of the American Astronomical Society, Vol. 38, p.486

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We investigated the composition of comet 73P/Schwassmann-Wachmann-3 (SW3) using high-dispersion infrared echelle spectroscopy (NIRSPEC/Keck Observatory) (1). Fragments 'C' and 'B' were depleted in ethane, and 'C' was depleted in most other forms of volatile carbon. Of special note, CH3OH was severely depleted in 'C' but HCN (with similar volatility) was "normal". The ethane depletion seen in 'B' and 'C' could plausibly result from thermal processing of the pre-split nucleus, however, the depletion of CH3OH but normalcy of HCN cannot be explained in this way. Moreover, 'B' displayed frequent outbursts and ongoing fragmentation, suggesting it was composed of fresh material and that the observed ethane depletion is cosmogonic.
The composition of SW3-C is similar to that of the Oort Cloud (OC) comet C/1999 S4 (LINEAR) (3). Another OC comet (C/1996 Q1 (Tabur)) also showed unusual methanol depletion (4), suggesting that these three comets share a common chemistry distinct from the most dominat group of OC comets. All three comets disintegrated into multiple fragments, apparently spontaneously. The cause is not known, but the fact that they share two common factors (low methanol, and disintegration) is striking. The distinct chemistry of their volatile ices may be associated with sublimation of icy mantles from pre-cometary grains, followed by subsequent gas-phase chemistry and re-condensation of methanol-poor ices (3). This could result from accretion heating at the nebular skin during in-fall into a relatively warm and dense region of the protoplanetary disk. If so, these comets could represent a taxonomic group distinct from the dominant (chemical) group of comets.
1. Blake et al. IAUC 8704; Villanueva et al. Ap. J. Letters (submitted).
2. Kawakita et al. 2006. IAU CBET 532; this meeting.
3. Mumma et al. 2001. Science 292:1334-1339.
4. Biver et al. 2002. Earth, Moon, and Planets 90:323-333.

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