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Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov


The interstellar traveller, 2I/Borisov, is the first clearly active extrasolar comet ever detected in our Solar System. We obtained high-resolution interferometric observations of 2I/Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA) and multi-colour optical observations with the Very Large Telescope (VLT) to gain a comprehensive understanding of the dust properties of this comet. We found that the dust coma of 2I/Borisov consists of compact ‘pebbles’ of radii exceeding ~1 mm, suggesting that the dust particles have experienced compaction through mutual impacts during the bouncing collision phase in the protoplanetary disk. We derived a dust mass-loss rate of 200 kg s−1 and a dust-to-gas ratio 3. Our long-term monitoring of 2I/Borisov with the VLT indicates a steady dust mass-loss with no significant dust fragmentation and/or sublimation occurring in the coma. We also detected emissions from carbon monoxide (CO) gas with ALMA and derived the gas production rate of Q(CO) = (3.3 ± 0.8) × 1026 s−1. We found that the CO/H2O mixing ratio of 2I/Borisov changed drastically before and after perihelion, indicating the heterogeneity of the cometary nucleus, with components formed at different locations beyond the volatile snow-line with different chemical abundances. Our observations suggest that 2I/Borisov’s home system, much like our own system, experienced efficient radial mixing from the innermost parts of its protoplanetary disk to beyond the frost line of CO.

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Fig. 1: ALMA dust thermal continuum maps of 2I/Borisov.
Fig. 2: Dust thermal emission models and ALMA photometry of 2I/Borisov.
Fig. 3: Optical reflectivity gradient of 2I/Borisov.
Fig. 4: CO flux maps and spectra of 2I/Borisov obtained with ALMA.

Data availability

This work makes use of ALMA dataset ADS/JAO.ALMA#2019.A.00002.S, which is available for download from the ALMA Science Archive ( following a 9-month proprietary period. The VLT dataset is available for download from the ESO Science Archive (, under ESO program ID 2103.C-5068 and 0105.C-0250, principal investigator O.R.H., following a 1-year proprietary period.


  1. 1.

    Charnoz, S. & Morbidelli, A. Coupling dynamical and collisional evolution of small bodies: an application to the early ejection of planetesimals from the Jupiter-Saturn region. Icarus 166, 141–156 (2003).

    ADS  Article  Google Scholar 

  2. 2.

    Meech, K. J. et al. A brief visit from a red and extremely elongated interstellar asteroid. Nature 552, 378–381 (2017).

    ADS  Article  Google Scholar 

  3. 3.

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

    ADS  Article  Google Scholar 

  4. 4.

    Guzik, P. et al. Initial characterization of interstellar comet 2I/Borisov. Nat. Astron. 4, 53–57 (2020).

    ADS  Article  Google Scholar 

  5. 5.

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

    ADS  Article  Google Scholar 

  6. 6.

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

    ADS  Article  Google Scholar 

  7. 7.

    Bodewits, D. et al. The carbon monoxide-rich interstellar comet 2I/Borisov. Nat. Astron. 4, 867–871 (2020).

    ADS  Article  Google Scholar 

  8. 8.

    Cordiner, M. A. et al. Unusually high CO abundance of the first active interstellar comet. Nat. Astron. 4, 861–866 (2020).

    ADS  Article  Google Scholar 

  9. 9.

    Jewitt, D. et al. The nucleus of interstellar comet 2I/Borisov. Astrophys. J. Lett. 888, L23 (2020).

    ADS  Article  Google Scholar 

  10. 10.

    Hui, M.-T., Ye, Q.-Z., Föhring, D., Hung, D. & Tholen, D. J. Physical characterization of interstellar comet 2I/2019 Q4 (Borisov). Astron. J. 160, 92 (2020).

    ADS  Article  Google Scholar 

  11. 11.

    Cochran, A. L. et al. The composition of comets. Space Sci. Rev. 197, 9–46 (2015).

    ADS  Article  Google Scholar 

  12. 12.

    Fray, N. et al. Nitrogen-to-carbon atomic ratio measured by COSIMA in the particles of comet 67P/Churyumov-Gerasimenko. Mon. Not. R. Astron. Soc. 469, S506–S516 (2017).

    Article  Google Scholar 

  13. 13.

    Levasseur-Regourd, A.-C. et al. Cometary dust. Space Sci. Rev. 214, 64 (2018).

    ADS  Article  Google Scholar 

  14. 14.

    A’Hearn, M. F. Comets: looking ahead. Philos. Trans. R. Soc. Lond. Ser. A 375, 20160261 (2017).

    ADS  Google Scholar 

  15. 15.

    Davidsson, B. J. R. et al. The primordial nucleus of comet 67P/Churyumov-Gerasimenko. Astron. Astrophys. 592, A63 (2016).

    Article  Google Scholar 

  16. 16.

    Fulle, M. The ice content of Kuiper belt objects. Nat. Astron. 1, 0018 (2017).

    ADS  Article  Google Scholar 

  17. 17.

    Güttler, C. et al. Synthesis of the morphological description of cometary dust at comet 67P/Churyumov-Gerasimenko. Astron. Astrophys. 630, A24 (2019).

    Article  Google Scholar 

  18. 18.

    Li, A. & Greenberg, J. M. From interstellar dust to comets: infrared emission from comet Hale-Bopp (C/1995 O1). Astrophys. J. Lett. 498, L83–L87 (1998).

    ADS  Article  Google Scholar 

  19. 19.

    Wentworth, C. K. A scale of grade and class terms for clastic sediments. J. Geol. 30, 377–392 (1922).

    ADS  Article  Google Scholar 

  20. 20.

    Johansen, A. & Lambrechts, M. Forming planets via pebble accretion. Annu. Rev. Earth Planet. Sci. 45, 359–387 (2017).

    ADS  Article  Google Scholar 

  21. 21.

    Jewitt, D. & Matthews, H. Particulate mass loss from comet Hale-Bopp. Astron. J. 117, 1056–1062 (1999).

    ADS  Article  Google Scholar 

  22. 22.

    Bohren, C. F. & Huffman, D. R. Absorption and Scattering of Light by Small Particles (Wiley, 1983).

  23. 23.

    Divine, N. et al. The comet Halley dust and gas environment. Space Sci. Rev. 43, 1–104 (1986).

    ADS  Article  Google Scholar 

  24. 24.

    Ye, Q. et al. Pre-discovery activity of new interstellar comet 2I/Borisov beyond 5 au. Astron. J. 159, 77 (2020).

    ADS  Article  Google Scholar 

  25. 25.

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

    ADS  Article  Google Scholar 

  26. 26.

    Cremonese, G. et al. Dust environment model of the interstellar comet 2I/Borisov. Astrophys. J. Lett. 893, L12 (2020).

    ADS  Article  Google Scholar 

  27. 27.

    Kim, Y. et al. Coma anisotropy and the rotation pole of interstellar comet 2I/Borisov. Astrophys. J. Lett. 895, L34 (2020).

    ADS  Article  Google Scholar 

  28. 28.

    Pätzold, M. et al. The nucleus of comet 67P/Churyumov-Gerasimenko—Part I: the global view—nucleus mass, mass-loss, porosity, and implications. Mon. Not. R. Astron. Soc. 483, 2337–2346 (2019).

    ADS  Article  Google Scholar 

  29. 29.

    Bolin, B. T. et al. Characterization of the nucleus, morphology, and activity of interstellar comet 2I/Borisov by optical and near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck observations. Astron. J. 160, 26 (2020).

    ADS  Article  Google Scholar 

  30. 30.

    Kolokolova, L., Jockers, K., Chernova, G. & Kiselev, N. Properties of cometary dust from color and polarization. Icarus 126, 351–361 (1997).

    ADS  Article  Google Scholar 

  31. 31.

    Yang, B. et al. Searching for water ice in the coma of interstellar object 2I/Borisov. Astron. Astrophys. 634, L6 (2020).

    ADS  Article  Google Scholar 

  32. 32.

    Xing, Z., Bodewits, D., Noonan, J. & Bannister, M. T. Water production rates and activity of interstellar comet 2I/Borisov. Astrophys. J. Lett. 893, L48 (2020).

    ADS  Article  Google Scholar 

  33. 33.

    Läuter, M., Kramer, T., Rubin, M. & Altwegg, K. Surface localization of gas sources on comet 67P/Churyumov-Gerasimenko based on DFMS/COPS data. Mon. Not. R. Astron. Soc. 483, 852–861 (2019).

    ADS  Google Scholar 

  34. 34.

    Rotundi, A. et al. Dust measurements in the coma of comet 67P/Churyumov-Gerasimenko inbound to the Sun. Science 347, aaa3905 (2015).

    Article  Google Scholar 

  35. 35.

    Choukroun, M. et al. Dust-to-gas and refractory-to-ice mass ratios of comet 67P/Churyumov-Gerasimenko from Rosetta observations. Space Sci. Rev. 216, 44 (2020).

    ADS  Article  Google Scholar 

  36. 36.

    Singh, P. D., de Almeida, A. A. & Huebner, W. F. Dust release rates and dust-to-gas mass ratios of eight comets. Astron. J. 104, 848 (1992).

    ADS  Article  Google Scholar 

  37. 37.

    Lamy, P. L., Toth, I., Weaver, H. A., A’Hearn, M. F. & Jorda, L. Properties of the nuclei and comae of 13 ecliptic comets from Hubble Space Telescope snapshot observations. Astron. Astrophys. 508, 1045–1056 (2009).

    ADS  Article  Google Scholar 

  38. 38.

    Lorek, S., Gundlach, B., Lacerda, P. & Blum, J. Comet formation in collapsing pebble clouds. What cometary bulk density implies for the cloud mass and dust-to-ice ratio. Astron. Astrophys. 587, A128 (2016).

    ADS  Article  Google Scholar 

  39. 39.

    Mannel, T. et al. Dust of comet 67P/Churyumov-Gerasimenko collected by Rosetta/MIDAS: classification and extension to the nanometer scale. Astron. Astrophys. 630, A26 (2019).

    Article  Google Scholar 

  40. 40.

    Weidenschilling, S. J. Aerodynamics of solid bodies in the solar nebula. Mon. Not. R. Astron. Soc. 180, 57–70 (1977).

    ADS  Article  Google Scholar 

  41. 41.

    Zsom, A., Ormel, C. W., Güttler, C., Blum, J. & Dullemond, C. P. The outcome of protoplanetary dust growth: pebbles, boulders, or planetesimals? II. Introducing the bouncing barrier. Astron. Astrophys. 513, A57 (2010).

    ADS  Article  Google Scholar 

  42. 42.

    Mukai, T. & Fechtig, H. Packing effect of fluffy particles. Planet. Space Sci. 31, 655–658 (1983).

    ADS  Article  Google Scholar 

  43. 43.

    Feaga, L. M. et al. Uncorrelated volatile behavior during the 2011 apparition of comet C/2009 P1 Garradd. Astron. J. 147, 24 (2014).

    ADS  Article  Google Scholar 

  44. 44.

    Cooper, J. F., Christian, E. R., Richardson, J. D. & Wang, C. Proton irradiation of Centaur, Kuiper Belt, and Oort Cloud objects at plasma to cosmic ray energy. Earth Moon Planets 92, 261–277 (2003).

    ADS  Article  Google Scholar 

  45. 45.

    Garrod, R. T. Simulations of ice chemistry in cometary nuclei. Astrophys. J. 884, 69 (2019).

    ADS  Article  Google Scholar 

  46. 46.

    Jehin, E. et al. Monitoring of the optical spectrum of comet 2I/Borisov at the VLT. In European Planetary Science Congress EPSC2020-653 (Europlanet Science Conference, 2020).

  47. 47.

    Bolin, B. T. & Lisse, C. M. Constraints on the spin-pole orientation, jet morphology, and rotation of interstellar comet 2I/Borisov with deep HST imaging. Mon. Not. R. Astron. Soc. 497, 4031–4041 (2020).

    ADS  Article  Google Scholar 

  48. 48.

    Walsh, K. J., Morbidelli, A., Raymond, S. N., O’Brien, D. P. & Mandell, A. M. A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475, 206–209 (2011).

    ADS  Article  Google Scholar 

  49. 49.

    Batalha, N. M. Exploring exoplanet populations with NASA’s Kepler Mission. Proc. Natl Acad. Sci. USA 111, 12647–12654 (2014).

    ADS  Article  Google Scholar 

  50. 50.

    Kolokolova, L., Hanner, M. S., Levasseur-Regourd, A. C. & Gustafson, B. Å. S. in Comets II 577–604 (Univ. Arizona Press, 2004).

  51. 51.

    Kimura, H., Kolokolova, L., Li, A. & Lebreton, J. in Light Scattering Reviews 363–418 (Springer, 2016).

  52. 52.

    Draine, B. T. & Lee, H. M. Optical properties of interstellar graphite and silicate grains. Astrophys. J. 285, 89–108 (1984).

    ADS  Article  Google Scholar 

  53. 53.

    Li, A. & Greenberg, J. M. A unified model of interstellar dust. Astron. Astrophys. 323, 566–584 (1997).

    ADS  Google Scholar 

  54. 54.

    Bardyn, A. et al. Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta. Mon. Not. R. Astron. Soc. 469, S712–S722 (2017).

    Article  Google Scholar 

  55. 55.

    Jessberger, E. K., Christoforidis, A. & Kissel, J. Aspects of the major element composition of Halley’s dust. Nature 332, 691–695 (1988).

    ADS  Article  Google Scholar 

  56. 56.

    Li, A. & Lunine, J. I. Modeling the infrared emission from the HD 141569A disk. Astrophys. J. 594, 987–1010 (2003).

    ADS  Article  Google Scholar 

  57. 57.

    Lasue, J., Levasseur-Regourd, A. C., Hadamcik, E. & Alcouffe, G. Cometary dust properties retrieved from polarization observations: application to C/1995 O1 Hale Bopp and 1P/Halley. Icarus 199, 129–144 (2009).

    ADS  Article  Google Scholar 

  58. 58.

    Draine, B. T. & Flatau, P. J. Discrete-dipole approximation for scattering calculations. J. Opt. Soc. Am. A 11, 1491–1499 (1994).

    ADS  Article  Google Scholar 

  59. 59.

    Hainaut, O. R. et al. Disintegration of active asteroid P/2016 G1 (PANSTARRS). Astron. Astrophys. 628, A48 (2019).

    Article  Google Scholar 

  60. 60.

    Tonry, J. L. et al. The Pan-STARRS1 photometric system. Astrophys. J. 750, 99 (2012).

    ADS  Article  Google Scholar 

  61. 61.

    A’Hearn, M. F., Schleicher, D. G., Millis, R. L., Feldman, P. D. & Thompson, D. T. Comet Bowell 1980b. Astron. J. 89, 579–591 (1984).

    ADS  Article  Google Scholar 

  62. 62.

    Hainaut, O. R., Boehnhardt, H. & Protopapa, S. Colours of minor bodies in the outer solar system. II. A statistical analysis revisited. Astron. Astrophys. 546, A115 (2012).

    ADS  Article  Google Scholar 

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We thank P.-Y. Hsieh and C.-T. Yang for help with the ALMA data, and Z. Wahhaj, D. Jewitt and X.-J. Yang for their constructive comments. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2019.A.00002.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program 105.205Q.001. A.L. was supported in part by the National Science Foundation (under grant no. AST-1816411) and by NASA (under grants HST-AR-15037.001-A and Chandra TM9-20009X). M.A.C. was supported by the National Science Foundation (under grant no. AST-1614471) and the NASA Planetary Science Division Internal Scientist Funding Program through the Fundamental Laboratory Research work package (FLaRe). K.J.M. and J.V.K. were supported by NASA (grant no. 80NSSC18K0853).

Author information




B.Y. led the application and organization of the ALMA observations and led the writing of this paper. A.L. performed dust modelling of the ALMA data and participated in the writing of this paper and the ALMA proposal. O.R.H. analysed the FORS data and M.A.C. analysed the CO data. C.-S.C. assisted in writing the ALMA proposal and reduced the ALMA data. J.P.W. contributed to ALMA observation design and data interpretation. K.J.M., J.V.K. and E.V. were co-investigators on the telescope proposals and commented on the manuscript.

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Correspondence to Bin Yang.

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The authors declare no competing interests.

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Peer review informationNature Astronomy thanks Bryce Bolin, Anny-Chantal Levasseur-Regourd and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Yang, B., Li, A., Cordiner, M.A. et al. Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Borisov. Nat Astron (2021).

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