The Unusual Chemical Composition of Interstellar Comet 2I/Borisov

Comet 2I/Borisov, discovered 2019 August 30, is the second interstellar object detected in our solar system, and the first inter-stellar object exhibiting cometary activity. Initial observations of the comet’s composition proved most interesting, with clear detections of CN but comparatively extremely low abundances of C₂ and low upper limits for C₃. These early observations placed 2I/Borisov firmly in the carbon-chain depleted class of comets as defined by A’Hearn et al. (1995). Our updated database, which builds on the dataset from A’Hearn et al. and now includes 40 years of observations, indicates these early observations place Borisov more specifically into the strongly carbon-chain depleted (i.e. strongly depleted in both C₂ and C₃) compositional class. When we froze our database in 2016 (Schleicher and Bair 2016), eight of our 114 well-determined comets were grouped into this compositional class, including the prototype depleted comet 21P/Giacobini-Zinner, 73P/Schwassmann-Wachmann 3 and 43P/Wolf-Harrington, among others. Since 2016, we have determined several additional comets are carbon-chain depleted, with 48P/Johnson, 123P/West-Hartley, and 260P/McNaught falling in the strongly carbon-chain depleted class and 114P/Wiseman-Skiff borderline between the strongly and moderately carbon-chain depleted classes. Unlike the other carbon-chain depleted comets we have observed, which show no variation in composition with heliocentric distance, 2I/Borisov became far less depleted as it moved closer to the Sun, and by December exhibited only moderate carbon-chain depletion. A further indication of its unusual composition, 2I/Borisov has a higher NH/OH ratio than any of the 114 objects in our restricted database. We will compare our compositional observations of 2I/Borisov with those obtained by other researchers, and place it into the context of our database, showing that while 2I/Borisov contains the same chemical components as comets originating in our solar system, its composition and behavior with changing temperature appears to be unique. This work is supported by NASA’s Solar System Observations Program grant 80NSSC18K0856.