Cometary Science Newsletter

Comet C/2023 P1 (Nishimura) can cast light on the physical mechanism of low Pmax in comets

Observations of Comet C/2023 P1 (Nishimura) at the Ussuriysk Astrophysical Observatory conducted on September 6 and 7 reveal an extremely low positive polarization. The modeling suggests Mg-rich silicate dust is the dominant species in the coma; whereas, an alternative explanation is that such low positive polarization in comets is attributed to the depolarizing effect of gaseous emission. These two alternative explanations predict dramatically different polarimetric responses at small phase angle that can be discriminated in further polarimetric observations of Comet C/2023 P1 (Nishimura). Due to the orientation of orbit of Comet Nishimura we no longer can measure its polarization. We are searching for collaborators having access to a polarimeter located in the southern hemisphere and willing to explore polarization in Comet Nishimura and, thus, to cast light on the physical mechanism of low Pmax in comets. For further details, please, contact Evgenij Zubko: evgenij.s.zubko@gmail.com

First Detection of CO₂ Emission in a Centaur: JWST NIRSpec Observations of 39P/Oterma

• O. Harrington Pinto ^1,2
• M. S. P. Kelley ³
• G. L. Villanueva ⁴
• M. Womack ^1,5
• S. Faggi ^4,6
• A. McKay ⁷
• M. A. DiSanti ^8,9
• C. Schambeau ^1,10
• Y. Fernandez ^1,10
• J. Bauer ³
• L. M. Feaga ³
• K. Wierzchos ¹¹

1. Department of Physics, University of Central Florida, USA; harrington.olga@gmail.com, oharrington@knights.ucf.edu
2. Department of Physics, Auburn University, USA
3. Department of Astronomy, University of Maryland, College Park, MD 20742, USA
4. NASA Goddard Space Flight Center, USA
5. National Science Foundation, USA
6. Department of Physics, American University, USA
7. Department of Physics and Astronomy, Appalachian State University, Boone, NC 28608, USA
8. Solar System Exploration Division, Planetary Science Laboratory Code 693, NASA, USA
9. Goddard Center for Astrobiology, NASA, USA
10. Florida Space Institute, University of Central Florida, USA
11. Lunar and Planetary Laboratory, University of Arizona, USA

Centaurs are minor solar system bodies with orbits transitioning between those of trans-Neptunian scattered disk objects and Jupiter-family comets (JFCs). 39P/Oterma (39P) is a frequently active centaur that has recently held both centaur and JFC classifications and was observed with the JWST NIRSpec instrument on 2022 July 27 UTC while it was 5.82 au from the Sun. For the first time, CO₂ gas emission was detected in a centaur, with a production rate of Q_CO2 = (5.96 ± 0.80) × 10²³ molecules s⁻¹. This is the lowest detection of CO₂ of any centaur or comet. CO and H₂O were not detected down to constraining upper limits. Derived mixing ratios of Q_CO/Q_CO2 ≲ 2.03 and Q_CO2/Q _H2O ≳ 0.60 are consistent with CO₂ and/or CO outgassing playing large roles in driving the activity, but not water, and show a significant difference between the coma abundances of 29P/Schwassmann–Wachmann 1, another centaur at a similar heliocentric distance, which may be explained by thermal processing of 39P’s surface during its previous JFC orbit. To help contextualize the JWST data we also acquired visible CCD imaging data on two dates in 2022 July (Gemini-North) and September (Lowell Discovery Telescope). Image analysis and photometry based on these data are consistent with a point-source detection and an estimated effective nucleus radius of 39P in the range of R_nuc = 2.21–2.49 km.

The Planetary Science Journal (Published)

DOI: 10.3847/PSJ/acf928 NASA ADS: 2023arXiv230911486H arXiv: 2309.11486