The Volatile Carbon-to-oxygen Ratio as a Tracer for the Formation Locations of Interstellar Comets

Based on the occurrence rates implied by the discoveries of 1I/‘Oumuamua and 2I/Borisov, the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) should detect one interstellar object every year. We advocate for future measurements of the production rates of H₂O, CO₂, and CO in these objects to estimate their carbon-to-oxygen ratios, which trace formation locations within their original protoplanetary disks. We review similar measurements for solar system comets, which indicate formation interior to the CO snow line. By quantifying the relative processing in the interstellar medium and solar system, we estimate that production rates will not be representative of primordial compositions for the majority of interstellar comets. Preferential desorption of CO and CO₂ relative to H₂O in the interstellar medium implies that measured C/O ratios represent lower limits on the primordial ratios. Specifically, production rate ratios of Q(CO)/Q(H₂O) < 0.2 and Q(CO)/Q(H₂O) > 1 likely indicate formation interior and exterior to the CO snow line, respectively. The high C/O ratio of 2I/Borisov implies that it formed exterior to the CO snow line. We provide an overview of the currently operational facilities capable of obtaining these measurements that will constrain the fraction of ejected comets that formed exterior to the CO snow line. This fraction will provide key insights into the efficiency of and mechanisms for cometary ejection in exoplanetary systems.