Unusually high CO abundance of the first active interstellar comet


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 formation1. 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.

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Fig. 1: ALMA molecular flux maps.
Fig. 2: ALMA spectra of 2I/Borisov.
Fig. 3: ALMA CO autocorrelation spectrum.
Fig. 4: Cometary CO/HCN mixing ratios.

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 (http://almascience.nrao.edu/aq/) 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.


  1. 1.

    Mumma, M. J. & Charnley, S. B. The chemical composition of comets—emerging taxonomies and natal heritage. Annu. Rev. Astron. Astrophys. 49, 471–524 (2011).

    ADS  Google Scholar 

  2. 2.

    Cleeves, L. I. et al. Constraining gas-phase carbon, oxygen, and nitrogen in the IM Lup protoplanetary disk. Astrophys. J. 865, 155–166 (2018).

    ADS  Google Scholar 

  3. 3.

    Podio, L. et al. Organic molecules in the protoplanetary disk of DG Tauri revealed by ALMA. Astron. Astrophys. 623, L6 (2019).

    ADS  Google Scholar 

  4. 4.

    Bergner, J. B. et al. A survey of C2H, HCN, and C18O in protoplanetary disks. Astrophys. J. 876, 25 (2019).

    ADS  Google Scholar 

  5. 5.

    Walsh, C. et al. Complex organic molecules in protoplanetary disks. Astron. Astrophys. 563, A33 (2014).

    Google Scholar 

  6. 6.

    Drozdovskaya, M. N. et al. Cometary ices in forming protoplanetary disc midplanes. Mon. Not. R. Astron. Soc. 462, 977–993 (2016).

    ADS  Google Scholar 

  7. 7.

    Lodders, K. Jupiter formed with more tar than ice. Astrophys. J. 611, 587–597 (2004).

    ADS  Google Scholar 

  8. 8.

    Öberg, K. I., Murray-Clay, R. & Bergin, E. A. The effects of snowlines on C/O in planetary atmospheres. Astrophys. J. Lett. 743, L16 (2011).

    ADS  Google Scholar 

  9. 9.

    Dodson-Robinson, S. E., Willacy, K., Bodenheimer, P., Turner, N. J. & Beichman, C. A. Ice lines, planetesimal composition and solid surface density in the solar nebula. Icarus 200, 672–693 (2009).

    ADS  Google Scholar 

  10. 10.

    Bailer-Jones, C. A. L., Farnocchia, D., Ye, Q., Meech, K. J. & Micheli, M. A search for the origin of the interstellar comet 2I/Borisov. Astron. Astrophys. 634, A14 (2020).

    ADS  Google Scholar 

  11. 11.

    Torbett, M. V. Capture of V = 20 km s−1 interstellar comets by three-body interactions in the planetary system. Astron. J. 92, 171–175 (1986).

    ADS  Google Scholar 

  12. 12.

    The ’Oumuamua ISSI Team. The natural history of ’Oumuamua. Nat. Astron. 3, 594–602 (2019).

    ADS  Google Scholar 

  13. 13.

    Micheli, M. et al. Non-gravitational acceleration in the trajectory of 1I/2017 U1 (’Oumuamua). Nature 559, 223–226 (2018).

    ADS  Google Scholar 

  14. 14.

    Fitzsimmons, A. et al. Detection of CN gas in interstellar object 2I/Borisov. Astron. Astrophys. 885, L9 (2019).

    Google Scholar 

  15. 15.

    Opitom, C. et al. 2I/Borisov: a C2-depleted interstellar comet. Astron. Astrophys. 631, L8 (2019).

    ADS  Google Scholar 

  16. 16.

    Xing, Z., Bodewits, D., Noonan, J. & Bannister, M. Water production rates and activity of interstellar comet 2I/Borisov. Astrophys. J. Lett. (in the press).

  17. 17.

    McKay, A., Cochran, A., Dello Russo, N. & DiSanti, M. Detection of a water tracer in interstellar comet 2I/Borisov. Astrophys. J. Lett. 889, L10 (2020).

    ADS  Google Scholar 

  18. 18.

    Jewitt, D. & Luu, J. Initial characterization of interstellar comet 2I/2019 Q4 (Borisov). Astrophys. J. Lett. 886, L29 (2019).

    ADS  Google Scholar 

  19. 19.

    Fray, N., Bénilan, Y., Cottin, H., Gazeau, M.-C. & Crovisier, J. The origin of the CN radical in comets: a review from observations. Planet. Space Sci. 53, 1243–1262 (2005).

    ADS  Google Scholar 

  20. 20.

    Bockelée-Morvan, D. & Biver, N. The composition of cometary ices. Phil. Trans. R. Soc. A 375, 20160252 (2017).

    ADS  Google Scholar 

  21. 21.

    Cordiner, M. A. et al. ALMA autocorrelation spectroscopy of comets: the HCN/H13CN ratio in C/2012 S1 (ISON). Astrophys. J. Lett. 870, L26 (2019).

    ADS  Google Scholar 

  22. 22.

    Paganini, L. et al. C/2013 R1 (Lovejoy) at IR wavelengths and the variability of CO abundances among Oort Cloud comets. Astrophys. J. 791, 122 (2014).

    ADS  Google Scholar 

  23. 23.

    McKay, A. J. et al. The peculiar volatile composition of CO-dominated comet C/2016 R2 (PanSTARRS). Astron. J. 158, 128–152 (2019).

    ADS  Google Scholar 

  24. 24.

    Biver, N. et al. The extraordinary composition of the blue comet C/2016 R2 (PanSTARRS). Astron. Astrophys. 619, A127 (2018).

    Google Scholar 

  25. 25.

    Bockelée-Morvan, D. et al. A study of the distant activity of comet C/2006 W3 (Christensen) with Herschel and ground-based radio telescopes. Astron. Astrophys. 518, L149 (2010).

    ADS  Google Scholar 

  26. 26.

    Wierzchos, K. & Womakck, M. C/2016 R2 (PANSTARRS): a comet rich in CO and depleted in HCN. Astron. J. 156, 34–40 (2018).

    ADS  Google Scholar 

  27. 27.

    Womack, M., Sarid, G. & Wierzchos, K. CO in distantly active comets. Publ. Astron. Soc. Pac. 129, 031001 (2017).

    ADS  Google Scholar 

  28. 28.

    Ootsubo, T. et al. AKARI near-infrared spectroscopic survey for CO2 in 18 comets. Astrophys. J. 752, 15 (2012).

    ADS  Google Scholar 

  29. 29.

    Fitzsimmons, A. et al. Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘Oumuamua. Nat. Astron. 2, 133–137 (2018).

    ADS  Google Scholar 

  30. 30.

    Stern, S. A. ISM-induced erosion and gas-dynamical drag in the Oort Cloud. Icarus 84, 447–466 (1990).

    ADS  Google Scholar 

  31. 31.

    Bergin, E. & Cleeves, L. I. in Handbook of Exoplanets (eds Deeg, H. J. & Belmonte, J. A.) 2221–2250 (Springer, 2018).

  32. 32.

    Eistrup, C., Walsh, C. & van Dishoeck, E. F. Molecular abundances and C/O ratios in chemically evolving planet-forming disk midplanes. Astron. Astrophys. 613, A14 (2018).

    Google Scholar 

  33. 33.

    Qi, C. et al. Imaging of the CO snow line in a solar nebula analog. Science 341, 630–632 (2013).

    ADS  Google Scholar 

  34. 34.

    Morbidelli, A. & Nesvorný, D. in The Trans-Neptunian Solar System (eds Prialnik, D. et al.) 25–29 (Elsevier, 2019).

  35. 35.

    Gladman, B., Marsden, B. G. & VanLaerhoven, C. in The Solar System Beyond Neptune (eds Barucci, M. A. et al.) 43–57 (Springer, 2008).

  36. 36.

    Zhang, K. E., Bergin, E. A., Schwarz, K., Krijt, S. & Ciesla, F. Systematic variations of CO gas abundance with radius in gas-rich protoplanetary disks. Astrophys. J. 883, 98 (2019).

    ADS  Google Scholar 

  37. 37.

    Qi, C. et al. Probing CO and N2 snow surfaces in protoplanetary disks with N2H+ emission. Astrophys. J. 882, 160 (2019).

    ADS  Google Scholar 

  38. 38.

    Andrews, S. Observations of protoplanetary disk structures. Annu. Rev. Astron. Astrophys. (in the press).

  39. 39.

    Zhang, S. et al. The Disk Substructures at High Angular Resolution Project (DSHARP). VII. The planet–disk interactions interpretation. Astrophys. J. Lett. 869, L47 (2018).

    ADS  Google Scholar 

  40. 40.

    Stevenson, D. J. & Lunine, J. I. Rapid formation of Jupiter by diffusive redistribution of water vapor in the solar nebula. Icarus 75, 146–155 (1988).

    ADS  Google Scholar 

  41. 41.

    De Sanctis, M. C., Capria, M. T. & Coradini, A. Thermal evolution and differentiation of Edgeworth-Kuiper Belt objects. Astron. J. 121, 2792–2799 (2001).

    ADS  Google Scholar 

  42. 42.

    Dello Russo, N., Kawakita, H., Vervack, R. J. & Weaver, H. Emerging trends and a comet taxonomy based on the volatile chemistry measured in thirty comets with high-resolution infrared spectroscopy between 1997 and 2013. Icarus 278, 301–332 (2016).

    ADS  Google Scholar 

  43. 43.

    Jaeger, S. in Astronomical Data Analysis Software and Systems XVII (Astronomical Society of the Pacific Conference Series) Vol. 394 (eds Argyle, R. W. et al.) 623–626 (Astronomical Society of the Pacific, 2008).

  44. 44.

    Remijan, A. et al. ALMA Technical Handbook Doc 7.3, ver. 1.1 (ALMA, 2019).

  45. 45.

    Downes, D. in Diffraction-Limited Imaging with Very Large Telescopes (eds Alloin, D. M. & Mariotti, J.-M.) 53–84 (Springer, 1989).

  46. 46.

    Brinch, C. & Hogerheijde, M. R. LIME—a flexible, non-LTE line excitation and radiation transfer method for millimeter and far-infrared wavelengths. Astron. Astrophys. 523, A25 (2010).

    ADS  Google Scholar 

  47. 47.

    Schöier, F. L., van der Tak, F. F. S., van Dishoeck, E. F. & Black, J. H. An atomic and molecular database for analysis of submillimetre line observations. Astron. Astrophys. 432, 369–379 (2005).

    ADS  Google Scholar 

  48. 48.

    Huebner, W. F. & Mukherjee, J. Photoionization and photodissociation rates in solar and blackbody radiation fields. Planet. Space Sci. 106, 11–45 (2015).

    ADS  Google Scholar 

  49. 49.

    Biver, N. et al. Long-term monitoring of the outgassing and composition of comet 67P/Churyumov-Gerasimenko with the Rosetta/MIRO instrument. Astron. Astrophys. 630, A19 (2019).

    Google Scholar 

  50. 50.

    Cochran, A. & Schleicher, D. G. Observational constraints on the lifetime of cometary H2O. Icarus 105, 235–253 (1993).

    ADS  Google Scholar 

  51. 51.

    Biver, N. et al. Spectroscopic monitoring of comet C/1996 B2 (Hyakutake) with the JCMT and IRAM Radio Telescopes. Astrophys. J. 118, 1850–1872 (1999).

    Google Scholar 

  52. 52.

    Biver, N. et al. Submillimetre observations of comets with Odin: 2001-2005. Planet. Space Sci. 55, 1058–1068 (2007).

    ADS  Google Scholar 

  53. 53.

    Hartogh, P. et al. HIFI observations of water in the atmosphere of comet C/2008 Q3 (Garradd). Astron. Astrophys. 518, L150 (2010).

    ADS  Google Scholar 

  54. 54.

    Jehin, E. et al. TRAPPIST: TRAnsiting Planets and PlanetesImals Small Telescope. Messenger 145, 2–6 (2011).

    ADS  Google Scholar 

  55. 55.

    Farnham, T. L., Schleicher, D. G. & A’Hearn, M. F. The HB narrowband comet filters: standard stars and calibrations. Icarus 147, 180–204 (2000).

    ADS  Google Scholar 

  56. 56.

    Opitom, C. Monitoring of the chemical composition of comets in the framework of the TRAPPIST survey. PhD Thesis, Université de Liège, Belgium (2016).

  57. 57.

    Moulane, Y. et al. Monitoring of the activity and composition of comets 41P/Tuttle-Giacobini-Kresak and 45P/Honda-Mrkos-Pajdusakova. Astron. Astrophys. 619, A156 (2018).

    Google Scholar 

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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.

Author information




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.

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Correspondence to M. A. Cordiner.

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Cordiner, M.A., Milam, S.N., Biver, N. et al. Unusually high CO abundance of the first active interstellar comet. Nat Astron 4, 861–866 (2020). https://doi.org/10.1038/s41550-020-1087-2

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