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M. F. A'Hearn (Univ. Md.), C. Arpigny (Univ. Liege), P. D. Feldman (Johns Hopkins Univ.), W.M. Jackson (UC-Davis), R. Meier (Gretag Imaging), H. A. Weaver (Johns Hopkins Univ.), D. D. Wellnitz (Univ. Md.), L. M. Woodney (Lowell Obs.)
Recent observations have shown that S2 is probably ubiquitous in comets, rather than rare as previously thought. Thus it can become a very important constraint on models of solar system formation if it is incorporated into comets via icy grain mantles as proposed after its discovery. Chemical reactions have previously been invoked involving methylene, C2H4, but this species is unlikely to be abundant and there is a significant problem with getting reactions to proceed rapidly enough to reproduce the very short spatial scale (< 100 km) needed to explain the S2 observed in comets observed at the highest spatial resolution (IRAS-Araki-Alcock and Hyakutake).
We present here a new mechanism for production in the innermost coma based on the extremely fast reaction between OCS and atomic sulphur in the metastable 1D state. The metastable S is produced efficiently in the innermost coma during the dissociation of CS2, which itself has a very short lifetime. Other sources of metastable S have too long a lifetime against photodissociation to produce enough S. Reactions of metastable S with other sulfur-bearing species such as H2S do not have sufficiently large cross sections to dominate, even though they are more abundant than OCS. The key question in producing S2 is the rate of collisional quenching of the metastable level by other, abundant species such as H2O, but the reaction can still produce sufficient S2 if only 1 metastable level.