The compositional taxonomy of parent volatiles in comets has been based primarily (though not entirely) on coma abundances of trace species relative to H2O (abundance ratio X/H2O, where X is any volatile). This choice of H2O as the compositional “baseline” includes important information; however, it is not exclusive, as water may exhibit either similar or distinct outgassing sources and/or time variability patterns in production rate compared to other species in a given comet. We therefore consider the value of extending the chemical taxonomy towards (a) additional compositional baselines and (b) a closer integration between global coma abundances and the underlying volatile associations as revealed by spatial distributions, which can test whether simultaneously measured species are associated with common or distinct outgassing sources (including the nucleus, extended coma sources, or combination of both). We provide one example comparing the chemistry of Jupiter-family comet 46P/Wirtanen and Oort Cloud comet C/2007 W1 (Boattini) using both H2O (a polar molecule) and C2H6 (a non-polar molecule) as complementary compositional baselines, and suggest that potential differences between X/H2O and X/C2H6 comet classifications might bring important insights. The choice of ethane is partly motivated by H2O and C2H6 showing distinctly different outgassing patterns in spacecraft targets 103P/Hartley 2 and 67P/Churyumov-Gerasimenko, and in some comets studied only from the ground, including 46P/Wirtanen. In addition, unlike water, ethane is a “hypervolatile”, with one of the lowest equilibrium sublimation temperatures among molecules commonly covered in ground-based studies of comets. However, because C2H6 is observable only in the infrared (IR), additional compositional baselines should also include species that can be more easily measured in a larger number of comets by both mm/sub-mm and near-IR techniques, as they represent equally important windows into parent volatile composition.
Various co-authors gratefully acknowledge support by NSF Astronomy and Astrophysics Research Grants, by NASA Earth and Space Science Fellowship, Emerging Worlds, Solar System Workings, and Solar System Observations Programs, and by Keck PI Data Award. This work was partly motivated by discussions at the International Space Science Institute in Bern during meetings of Team 361: “From Qualitative to Quantitative: Exploring the Early Solar System by Connecting Comet Composition and Protoplanetary Disk Models”.