Synergy Between Astrochemical Models and Cometary Taxonomies of Parent Volatiles

The principal output in taxonomic studies of cometary primary (parent) volatiles is the suite of “mixing ratios” between observed species. These ratios relate the abundances of different molecules (CH4/C2H2/C2H6/H2CO/CH3OH/H2O, etc.) or isotopologues (HDO/H2O, CH3D/CH4, etc.). Infrared and radio observations have found strong evidence that mixing ratios vary substantially among comets. However, we still face serious uncertainties in decoding the cosmogonic significance of the measured abundances. The observed composition of comets may be an end product of a variety of processes, including chemical evolution in the protoplanetary disk, dynamical evolution in the young solar system, and (perhaps) thermal evolution during successive perihelion passages. Improved understanding of their relative importance requires additional sensitive measurements and a comprehensive synergy with astrochemical models. These models find that protoplanetary disks can be divided into three distinct regions: (1) a cold midplane, where ices freeze to dust grains; (2) a warm molecular layer, where ices sublimate and are processed via gas-phase reactions; and (3) a hot disk atmosphere containing predominantly atoms and atomic ions. Material from the different layers can be mixed by transport processes.We will show how this synergy is being realized via close collaboration between modeling and observing teams. The goal is a deeper insight into the processes preceding comet formation that may have influenced the composition - what chemical reaction pathways dominated the synthesis of cometary compounds? What processes in the protoplanetary disk have left strong signatures in cometary ices? Can models provide testable predictions for the chemical diversity observed among comets? Addressing these questions, we will show initial comparisons between relative abundances for several cometary volatiles and those predicted for the midplane of the protoplanetary disk where comets formed. We will also discuss how models link observations of volatiles in comets with studies of protoplanetary disks around solar type stars.This work is supported by NSF (AAG) and NASA (Astrobiology, PATM, PAST). Astrophysics at QUB is supported by a grant from STFC.