Cometary Science Newsletter

June 2023
Michael S. P. Kelley (

Refereed Articles

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

Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE

  • Kwon, Y. G. 1
  • Opitom, C. 2
  • Lippi, M. 1,3
  1. Technical University of Braunschweig, Germany.
  2. University of Edinburgh, UK.
  3. INAF-Firenze, Italy.

We report a new imaging spectroscopic observation of Oort-cloud comet C/2017 K2 (hereafter K2) on its way to perihelion at 2.53 au, around a heliocentric distance where H2O ice begins to play a key role in comet activation. Normalized reflectances over 6 500–8 500 AA for its inner (cometocentric distance ρ ≈ 10^3 km) and outer (ρ ≈ 2 x 10^4 km) comae are 9.7±0.5 and 7.2±0.3 % (10^3 AA)^-1, respectively, the latter being consistent with the slope observed when the comet was beyond the orbit of Saturn. The dust coma of K2 at the time of observation appears to contain three distinct populations: mm-sized chunks prevailing at ρ ≲ 10^3 km; a 10^5-km steady-state dust envelope; and fresh anti-sunward jet particles. The dust chunks dominate the continuum signal and are distributed over a similar radial distance scale as the coma region with redder dust than nearby. They also appear to be co-spatial with OI1D, suggesting that the chunks may accommodate H2O ice with a fraction (≳1 %) of refractory materials. The jet particles do not colocate with any gas species detected. The outer coma spectrum contains three significant emissions from C2(0,0) Swan band, OI1D, and CN(1,0) red band, with an overall deficiency in NH2. Assuming that all OI1D flux results from H2O dissociation, we compute an upper limit on the water production rate Q_H2O of ~7 × 10^28 molec s^−1 (with an uncertainty of a factor of two). The production ratio log[Q_C2/Q_CN] of K2 suggests that the comet has typical carbon-chain composition, with the value potentially changing with increasing distance from the Sun. Our observations suggest that water ice-containing dust chunks (>0.1 mm) near K2’s nucleus emitted beyond 4 au may be responsible for its very low gas rotational temperature and the discrepancy between its optical and infrared lights reported at similar heliocentric distances.

Astronomy & Astrophysics (In press)

arXiv: 2305.01385

Spectroscopic identification of water from a main-belt comet

  • Kelley, M. S. P. 1
  • Hsieh, H. H. 2,3
  • Bodewits, D. 4
  • Saki, M. 4
  • Villanueva, G. L. 5
  • Milam, S. N. 5
  • Hammel, H. B. 6
  1. University of Maryland, College Park
  2. Planetary Science Institute
  3. Institute of Astronomy and Astrophysics, Academia Sinica, Taipei
  4. Auburn University
  5. Solar System Exploration Division, NASA Goddard Space Flight Center
  6. Association of Universities for Research in Astronomy

Main-belt comets are small solar system bodies located in the asteroid belt that repeatedly exhibit comet-like activity (i.e., dust comae or tails) during their perihelion passages, strongly suggesting ice sublimation. Although the existence of main-belt comets implies the presence of extant water ice in the asteroid belt, no gas has been detected around these objects despite intense scrutiny with the world’s largest telescopes. Here, we present JWST observations which clearly show that main-belt comet 238P/Read has a coma of water vapour, but lacks a significant CO2 gas coma. Our findings demonstrate that the activity of comet Read is driven by water-ice sublimation, and implies that main-belt comets are fundamentally different from the general cometary population. Whether comet Read experienced different formation circumstances or evolutionary history, it is unlikely to be a recent asteroid belt interloper from the outer solar system. Based on these results, main-belt comets appear to represent a sample of volatile material that is currently unrepresented in observations of classical comets and the meteoritic record, making them important for understanding the early solar system’s volatile inventory and its subsequent evolution.

The full text is freely available for viewing.

Nature (In press)

DOI: 10.1038/s41586-023-06152-y