The Highly Unusual Behavior and Chemical Composition of Interstellar Comet 2I/Borisov
4 affiliations
- 1Lowell Observatory
- 2Villanova University
- 3United States Naval Academy
- 4University of Maryland
Abstract
We report on imaging, spectroscopy, and narrowband photometry of interstellar comet 2I/Borisov obtained at Lowell Observatory from 2019 September 13 to December 16. We measured chemical composition on 4 nights before perihelion, from October 4 to November 25 (2.466–2.028 au), with CN detections on all nights. Our measurements from October 27 represent the earliest reported detections of C ₂ and C ₃ , where C ₂ -to-CN and C ₃ -to-CN log production rate ratios were −0.7 and −1.3, respectively. Both ratios increased by nearly 4× to −0.3 and −0.9 by November 25, an extremely large increase with heliocentric distance that is unprecedented in the Lowell Observatory comet database. We additionally observed OH and NH on November 25, with a derived H ₂ O production rate of 4.5 × 10 ²⁶ mol s ⁻¹ and very high log production rate ratios for CN-to-OH (−2.0) and NH-to-OH (−1.7). Monitoring from broadband R images from September 13 to December 16 suggests dust ( A (0°) fρ ) production peaked around October 19, 50 days before its December 8 perihelion passage. Narrowband images from October 4 and 27 reveal a nearly symmetric CN distribution in Borisov’s coma, suggesting activity over its entire sunward hemisphere and possibly its entire nucleus. Our results, combined with those from others, reveal a chemical composition and behavior with heliocentric distance unlike any other comet we have observed.
Related
- High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS
- The Central Molecular Zone of our Galaxy, a chemical complexity factory: first interstellar detections of molecules towards G+0.693
- Measuring the Age and Initial Composition of Interstellar Objects
- Postperihelion Coma Composition of the Interstellar Comet 3I/ATLAS from Optical Spectroscopy
- High-resolution optical spectroscopy reveals the distinctive volatile composition of the interstellar comet 3I/ATLAS
BibTeX
@article{bair2026highly,
title = {{The Highly Unusual Behavior and Chemical Composition of Interstellar Comet 2I/Borisov}},
author = {Allison N. Bair and David G. Schleicher and Theodore Kareta and Matthew M. Knight and Michael S. P. Kelley},
journal = {The Planetary Science Journal},
year = {2026},
month = oct,
doi = {10.3847/psj/ae9f3d},
url = {https://iopscience.iop.org/article/10.3847/PSJ/ae9f3d},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The Highly Unusual Behavior and Chemical Composition of Interstellar Comet 2I/Borisov
AU - Allison N. Bair
AU - David G. Schleicher
AU - Theodore Kareta
AU - Matthew M. Knight
AU - Michael S. P. Kelley
PY - 2026
DA - 2026/10/09
JO - The Planetary Science Journal
DO - 10.3847/psj/ae9f3d
UR - https://iopscience.iop.org/article/10.3847/PSJ/ae9f3d
AB - We report on imaging, spectroscopy, and narrowband photometry of interstellar comet 2I/Borisov obtained at Lowell Observatory from 2019 September 13 to December 16. We measured chemical composition on 4 nights before perihelion, from October 4 to November 25 (2.466–2.028 au), with CN detections on all nights. Our measurements from October 27 represent the earliest reported detections of C ₂ and C ₃ , where C ₂ -to-CN and C ₃ -to-CN log production rate ratios were −0.7 and −1.3, respectively. Both ratios increased by nearly 4× to −0.3 and −0.9 by November 25, an extremely large increase with heliocentric distance that is unprecedented in the Lowell Observatory comet database. We additionally observed OH and NH on November 25, with a derived H ₂ O production rate of 4.5 × 10 ²⁶ mol s ⁻¹ and very high log production rate ratios for CN-to-OH (−2.0) and NH-to-OH (−1.7). Monitoring from broadband R images from September 13 to December 16 suggests dust ( A (0°) fρ ) production peaked around October 19, 50 days before its December 8 perihelion passage. Narrowband images from October 4 and 27 reveal a nearly symmetric CN distribution in Borisov’s coma, suggesting activity over its entire sunward hemisphere and possibly its entire nucleus. Our results, combined with those from others, reveal a chemical composition and behavior with heliocentric distance unlike any other comet we have observed.
SN - 2632-3338
ER -