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The Highly Unusual Behavior and Chemical Composition of Interstellar Comet 2I/Borisov

Year: 2026
Type: article
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.
DOI: 10.3847/psj/ae9f3d
Language: English
Open Access status: gold
Estimated APC paid: $1,800
Award: GOALS: INVESTIGATIONS OF THE CHEMICAL COMPOSITION AND PHYSICAL PROPERTIES OF COMETS ARE IMPORTANT BECAUSE IN ADDITION TO REVEALING THE CHARACTERISTICS OF COMETS THEMSELVES THE COMPOSITION AND STRUCTURE OF COMETS HOLD UNIQUE CLUES TO CONDITIONS IN THE EARLY SOLAR NEBULA THE SOLAR SYSTEM'S FORMATION PROCESSES AND EVEN THE EVOLUTION OF LIFE ON EARTH AS INFLUENCED BY COMET IMPACTS OVER THE PAST 4.6 BILLION YEARS. THE GOALS OF OUR PROPOSED RESEARCH ARE TO UNDERSTAND THE PHYSICAL AND CHEMICAL PROPERTIES OF COMETS BY APPLYING A VARIETY OF OBSERVATIONAL TECHNIQUES. WE PARTICULARLY EMPHASIZE SIMULTANEOUS OR COORDINATED OBSERVATIONS WITH DIFFERENT INSTRUMENTATION. WE AIM TO (1) MEASURE THE BASIC PROPERTIES OF COMETARY NUCLEI BY STUDYING COMETS WHOSE DUST COMAE ARE SO ANEMIC THAT THE SIGNAL FROM THE NUCLEUS CAN BE EXTRACTED OR ARE AT SUFFICIENT DISTANCES SUCH THAT THE COMA NO LONGER DOMINATES THE SIGNAL (2) MEASURE AND MODEL COMA JET MORPHOLOGY TO DETERMINE THE ORIENTATION OF THE ROTATIONAL AXIS THE LOCATION OF SOURCE REGIONS THE LIFETIME AND VELOCITIES OF THE OBSERVED SPECIES AND THEIR RESPECTIVE PARENTS AND HOW THE PRODUCTION OF MATERIAL FROM THE SURFACE OF EACH NUCLEUS VARIES WITH TIME AND SOLAR ILLUMINATION AND (3) MEASURE THE COMPOSITION AND PRODUCTION RATES OF COMETS AS A FUNCTION OF ORBITAL MOTION. DUE TO THE STRONGLY INTER-RELATED ASPECTS OF THESE MEASUREMENTS RESULTS FROM THE PROPOSED PROJECTS WILL ENABLE US TO SIGNIFICANTLY IMPROVE OUR UNDERSTANDING OF THE PHYSICAL NATURE LOCATION AND CHEMICAL COMPOSITION OF ACTIVE SOURCE REGIONS ON THE NUCLEUS AND HOW THESE PRODUCE THE OBSERVED COMA PHENOMENA. PREVIOUS RESULTS: WE HAVE HAD A NUMBER OF ACCOMPLISHMENTS UNDER CURRENT PAST FUNDING INCLUDING IMAGING PHOTOMETRY AND/OR SPECTROSCOPY OF 20+ SHORT-PERIOD AND 10+ LONG-PERIOD COMETS. OUR PRIMARY EFFORTS INCLUDED COMPLETING ANALYSES OF COMET ISON PERFORMING SUPPORT OBSERVATIONS FOR ROSETTA TARGET 67P/CHURYUMOV- GERASIMENKO AND STUDYING SEVERAL CLOSE-APPROACHING COMETS - 209P/LINEAR 252P/LINEAR 45P/HONDA-MRKOS-PAJDUSAKOVA AND 41P/TUTTLE-GIACOBINI-KRESAK - WITH MULTIPLE SCIENCE GOALS. WE'VE ALSO RECENTLY COMPLETED A ROTATIONAL STUDY OF 49P/AREND-RIGAUX AND OBTAINED ADDITIONAL NUCLEUS DATA OF 6P/D'ARREST 2P/ENCKE AND 103P/HARTLEY 2. PRODUCTION RATES WERE DETERMINED AND IMAGES OBTAINED OF SEVERAL OTHER COMETS. PROPOSED RESEARCH: IN THE PROPOSED INTERVAL WE PLAN DETAILED INVESTIGATIONS FOR A VARIETY OF COMETS INCLUDING SEVERAL THAT HAVE BEEN OR ARE POTENTIAL NASA SPACECRAFT MISSION TARGETS. NUCLEUS STUDIES WILL BE MADE OF COMETS 2P/ENCKE 6P/D'ARREST AND 103P/HARTLEY 2. ROTATIONAL PERIODS AND HOW THEY CHANGE WITH TIME WILL BE MEASURED AND WE WILL INVESTIGATE CLAIMS FOR COMPLEX ROTATION AND WHEN PRESENT ATTEMPT TO DETERMINE DETAILS OF THE SPECIFIC ROTATIONAL STATE INVOLVED. COMA IMAGING WILL BE OBTAINED FOR COMETS 21P/GIACOBINI-ZINNER 38P/STEPHAN-OTERMA 46P/WIRTANEN AND 88P/HOWELL TO STUDY AND MODEL JET MORPHOLOGY; ON-GOING OBSERVATIONS OF 41P/TUTTLE-GIACOBINI-KRESAK WILL ALSO BE MODELED UNDER THIS GRANT. EXPECTED RESULTS INCLUDE THE ORIENTATION OF THE ROTATION AXIS THE ROTATION PERIOD THE LOCATION AND SIZE OF EACH SOURCE REGION AND THE OUTFLOW VELOCITY OF THE GAS. PLANNED SPECTROSCOPY OF WIRTANEN WILL BE COMBINED WITH SIMILAR DATA RECENTLY OBTAINED FOR TWO OTHER VERY CLOSE APPROACHING COMETS -- 252P/LINEAR AND 45P/HONDA-MRKOS-PAJDUSAKOVA -- TO MEASURE THE ELUSIVE SPECIES DIATOMIC SULFUR. A NEW EXTREMELY HIGH RESOLUTION SPECTROGRAPH ON OUR 4.3-M TELESCOPE DCT WILL BE USED TO MEASURE ISOTOPIC RATIOS AND OTHER PROPERTIES IN THE BRIGHTEST COMETS. PRODUCTION RATES WILL BE OBTAINED TO INVESTIGATE TEMPORAL TRENDS AND/OR DETERMINE BASIC COMPOSITION. COORDINATED OBSERVATIONS WITH OTHER INVESTIGATORS WILL BE PURSUED WHEN PRACTICAL. WE WILL ALSO OBTAIN OBSERVATIONS OF NEWLY DISCOVERED COMETS AS CIRCUMSTANCES WARRANT.
OpenAlex ID: W7204246261
FWCI: 5.778
Cites: 57
Cited by: 2
Related to: 10