Cometary Science Newsletter

May 2020
Michael S. P. Kelley (

PSJ Focus Issue for the A'Hearn Symposium

We would like to announce that the AAS Planetary Science Journal will be hosting a Focus Issue for publishing papers relating to the meeting, "New Cometary Insights from the Close Approach of 46P/Wirtanen: A Symposium in Celebration of Mike A'Hearn", which was held August 6-8, 2019 at the University of Maryland.

The PSJ is a gold open access journal with Faith Vilas as Editor. The A'Hearn Focus Issue will be included in the first issue of 2021. The issue is now open and will accept submissions until August 31. Papers will be released online as they are accepted. You can find more information about the PSJ at

Content of the articles could include 1) Results that were presented at the meeting, 2) Related studies of Wirtanen; 3) Studies of other recent comets; 4) Synthesis articles that bring together several studies that address bigger questions about comets.

This Focus Issue is open to all who knew Mike (personally or by reputation), so that members of the comet community who were unable to attend the meeting can also contribute.

If you are interested in submitting an article please let us know, or if you would like more information, please contact us at

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

Unusually High CO Abundance of the First Active Interstellar Comet

  • Cordiner, M. A.1,2
  • Milam, S. N.1
  • Biver, N.3
  • Bockelée-Morvan, D.3
  • Roth, N. X.1,4
  • Bergin, E. A.5
  • Jehin, E.6
  • Remijan, A. J.7
  • Charnley, S. B.1
  • Mumma, M. J.1
  • Boissier, J.8
  • Crovisier, J.3
  • Paganini, L.9
  • Kuan, Y.-J.10,11
  • Lis, D. C12
  1. Solar System Exploration Division, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA.
  2. Department of Physics, Catholic University of America, Washington, DC 20064, USA.
  3. LESIA, Observatoire de Paris, Universite PSL, CNRS, Sorbonne Universite, Universite de Paris, 5 place Jules Janssen, 92195 Meudon, France.
  4. Universities Space Research Association, Columbia, MD 21046, USA.
  5. Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109, USA.
  6. STAR Institute, Universite de Liege, Allee du 6 Aout, 19C, 4000 Liege, Belgium.
  7. National Radio Astronomy Observatory, Charlottesville, VA 22903, USA.
  8. IRAM, 300 Rue de la Piscine, 38406 Saint Martin d'Heres, France.
  9. NASA Headquarters, Washington, DC, United States of America.
  10. National Taiwan Normal University, Taipei 116, Taiwan, ROC.
  11. Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 106, Taiwan, ROC.
  12. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.

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 15th-16th 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.

Nature Astronomy (Published)

DOI: 10.1038/s41550-020-1087-2 arXiv: 2004.09586

The carbon monoxide-rich interstellar comet 2I/Borisov

  • Bodewits, D. 1
  • Noonan, J. W. 2
  • Feldman, P. D. 3
  • Bannister, M. T. 4
  • Farnocchia, D. 5
  • Harris, W. M. 2
  • Li, J.-Y. 6
  • Mandt, K. E. 7
  • Parker, J. Wm. 8
  • Xing, Z.-X. 1,9
  1. Auburn University
  2. Lunar and Planetary Laboratory, U. Arizona
  3. Johns Hopkins U
  4. U. Canterbury
  5. JPL
  6. Planetary Science Inst.
  7. Johns Hopkins Applied Physics Laboratory
  8. SWRI
  9. U. Hong Kong

Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our Solar System[1]. Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles[2], organic material and prebiotic chemicals around their host system[3]. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions[4]. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide[5]. Here we report that the coma of 2I/Borisov contains significantly more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System[4]. Our ultraviolet Hubble Space Telescope observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is significantly different from our own.

Nature Astronomy (Published)

DOI: 10.1038/s41550-020-1095-2 NASA ADS: 2020arXiv200408972B arXiv: 2004.08972

Water Production Rates and Activity of Interstellar Comet 2I/Borisov

  • Xing, Zexi 1,2
  • Bodewits, Dennis 2
  • Noonan, John 3
  • Bannister, Michele T. 4,5
  1. Department of Physics and Laboratory for Space Research, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People's Republic of China
  2. Physics Department, Leach Science Center, Auburn University, Auburn, AL 36849, USA
  3. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
  4. Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, UK
  5. School of Physical and Chemical Sciences - Te Kura Matū, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

We observed the interstellar comet 2I/Borisov using the Neil Gehrels-Swift Observatory's Ultraviolet/Optical Telescope. We obtained images of the OH gas and dust surrounding the nucleus at six epochs spaced before and after perihelion (-2.56 AU to 2.54 AU). Water production rates increased steadily before perihelion from (7.0±1.5)×10^26 molecules s−1 on Nov. 1, 2019 to (10.7±1.2)×10^26 molecules s−1 on Dec. 1. This rate of increase in water production rate is quicker than that of most dynamically new comets and at the slower end of the wide range of Jupiter-family comets. After perihelion, the water production rate decreased to (4.9±0.9)×10^26 molecules s−1 on Dec. 21, which is much more rapidly than that of all previously observed comets. Our sublimation model constrains the minimum radius of the nucleus to 0.37 km, and indicates an active fraction of at least 55% of the surface. A(0)fρ calculations show a variation between 57.5 and 105.6 cm with a slight trend peaking before the perihelion, lower than previous and concurrent published values. The observations confirm that 2I/Borisov is carbon-chain depleted and enriched in NH2 relative to water.

The Astrophysical Journal Letters (Published)

DOI: 10.3847/2041-8213/ab86be NASA ADS: 2020arXiv200104865X arXiv: 2001.04865