A Caltech Library Service

Unusually high CO abundance of the first active interstellar comet

Cordiner, M. A. and Milam, S. N. and Biver, N. and Bockelée-Morvan, D. and Roth, N. X. and Bergin, E. A. and Jehin, E. and Remijan, A. J. and Charnley, S. B. and Mumma, M. J. and Boissier, J. and Crovisier, J. and Paganini, L. and Kuan, Y.-J. and Lis, D. C. (2020) Unusually high CO abundance of the first active interstellar comet. Nature Astronomy . ISSN 2397-3366. (In Press)

[img] PDF - Accepted Version
See Usage Policy.

[img] PDF (Supplementary Fig. 1, Tables 1–2 and references) - Supplemental Material
See Usage Policy.


Use this Persistent URL to link to this item:


Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation. To date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here, we present high-resolution interferometric observations of 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 15–16 December 2019. Our observations reveal emission from hydrogen cyanide (HCN) and carbon monoxide (CO) coincident with the expected position of 2I/Borisov’s nucleus, with production rates Q(HCN) = (7.0 ± 1.1) × 10²³ s⁻¹ and Q(CO) = (4.4 ± 0.7) × 10²⁶ s⁻¹. While the HCN abundance relative to water (0.06–0.16%) appears similar to that of typical, previously observed comets in our Solar System, the abundance of CO (35–105%) is among the highest observed in any comet within 2 au of the Sun. This shows that 2I/Borisov must have formed in a relatively CO-rich environment—probably beyond the CO ice-line in the very cold, outer regions of a distant protoplanetary accretion disk, as part of a population of small icy bodies analogous to our Solar System’s own proto-Kuiper belt.

Item Type:Article
Related URLs:
URLURL TypeDescription ReadCube access Paper
Cordiner, M. A.0000-0001-8233-2436
Milam, S. N.0000-0001-7694-4129
Roth, N. X.0000-0002-6006-9574
Bergin, E. A.0000-0003-4179-6394
Remijan, A. J.0000-0001-9479-9287
Mumma, M. J.0000-0003-4627-750X
Paganini, L.0000-0002-4639-5488
Lis, D. C.0000-0002-0500-4700
Additional Information:© 2020 Springer Nature Limited. Received 12 February 2020; Accepted 24 March 2020; Published 20 April 2020. We thank R. Simon for setting up the ALMA scheduling blocks, and D. Cruikshank for discussions on the composition of Kuiper belt objects. This work was supported by the National Science Foundation (under grant no. AST-1614471), and by the Planetary Science Division Internal Scientist Funding Program through the Fundamental Laboratory Research (FLaRe) work package, as well as the NASA Astrobiology Institute through the Goddard Center for Astrobiology (proposal 13-13NAI7-0032). Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. ALMA is a partnership of ESO, NSF (USA), NINS (Japan), NRC (Canada), NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The JAO is operated by ESO, AUI/NRAO and NAOJ. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. TRAPPIST is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (Fonds de la Recherche Scientifique–FNRS) under grant FRFC 2.5.594.09.F. E.J. is a FNRS Senior Research Associate. N.X.R. was supported by the NASA Postdoctoral Program, administered by the Universities Space Research Association. Data availability: This work makes use of ALMA dataset ADS/JAO.ALMA#2019.01008.T, which is available for download from the ALMA Science Archive ( following a 1-year proprietary period. All data that support the findings of this study are available on resonable request from the corresponding author. Code availability: The radiative transfer model required to reproduce the results of this study is available on reasonable request from the corresponding author. Author Contributions: M.A.C. performed the data reduction and radiative transfer modelling, and generated most of the text and figures. S.N.M. obtained the ALMA observations and helped write the manuscript. N.B. performed independent radiative transfer calculations and statistical comparisons. D.B.-M. made Fig. 4. E.A.B. wrote part of the interpretation. N.X.R. worked on Supplementary Table 1, and generated upper limits. A.J.R. helped obtain the observations and identify spectral lines. E.J. provided ancillary optical data from TRAPPIST. S.B.C., J.C., D.C.L., L.P., Y.-J.K., J.B. and M.J.M. contributed to the interpretation and helped write the manuscript. The authors declare no competing interests.
Funding AgencyGrant Number
Fonds de la Recherche Scientifique (FNRS)UNSPECIFIED
Fond National de la Recherche Scientifique2.5.594.09.F
NASA Postdoctoral ProgramUNSPECIFIED
Subject Keywords:Asteroids, comets and Kuiper belt; Atmospheric chemistry
Record Number:CaltechAUTHORS:20200507-082038890
Persistent URL:
Official Citation:Cordiner, M.A., Milam, S.N., Biver, N. et al. Unusually high CO abundance of the first active interstellar comet. Nat Astron (2020).
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103060
Deposited By: Tony Diaz
Deposited On:07 May 2020 19:02
Last Modified:07 May 2020 19:02

Repository Staff Only: item control page