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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This work makes use of ALMA dataset ADS/JAO.ALMA#2019.A.00002.S, which is available for download from the ALMA Science Archive (http://almascience.nrao.edu/aq/) following a 9-month proprietary period. The VLT dataset is available for download from the ESO Science Archive (http://archive.eso.org/eso/eso_archive_main.html), under ESO program ID 2103.C-5068 and 0105.C-0250, principal investigator O.R.H., following a 1-year proprietary period.
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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).
The authors declare no competing interests.
Peer review information Nature 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 5, 586–593 (2021). https://doi.org/10.1038/s41550-021-01336-w