The Interstellar Heritage of Comets

Abstract Comets have similar compositions to interstellar medium ices, suggesting at least some of their molecules may be inherited from an earlier stage of evolution. To investigate the degree to which this might have occurred, we compare the composition of individual comets to that of the well-studied protostellar region IRAS 16293–2422B. We show that the observed molecular abundance ratios in several comets correlate well with those observed in the protostellar source. However, this does not necessarily mean that the cometary abundances are identical to protostellar. We find the abundance ratios of many molecules present in comets are enhanced compared to their protostellar counterparts. For COH molecules, the data suggest higher abundances relative to methanol of more complex species, e.g., HCOOH, CH 3 CHO, and HCOOCH 3 , are found in comets. For N-bearing molecules, the ratios of nitriles relative to CH 3 CN—HC 3 N/CH 3 CN and HCN/CH 3 CN—tend to be enhanced. The abundances of cometary SO and SO 2 relative to H 2 S are enhanced, whereas OCS/H 2 S is reduced. Using a subset of comets with a common set of observed molecules, we suggest a possible means of determining the relative degree to which they retain interstellar ices. This analysis suggests that over 84% of COH-bearing molecules can be explained by the protostellar composition. The possible fraction inherited from the protostellar region is lower for nitrogen molecules, at only 26%–74%. While this is still speculative, especially since few comets have large numbers of observed molecules, it provides a possible route for determining the relative degree to which comets contain disk-processed material.