Letter to the Editor

Unveiling the ice and gas nature of active centaur (2060) Chiron using the James Webb Space Telescope

¹ Institute for Space Sciences and Technologies in Asturias (ICTEA), University of Oviedo, Oviedo, Spain
² Florida Space Institute, University of Central Florida, Orlando, FL, USA
³ Instituto Astrofísico de Canarias, La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, Tenerife, Spain
⁵ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, Orsay, France
⁶ University of Central Florida, Physics Department, Orlando, FL, USA
⁷ Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, UMR 5276 CNRS, UCBL, ENSL, Villeurbanne, France
⁸ Space Telescope Science Institute, Baltimore, MD, USA
⁹ Northern Arizona University, Flagstaff, AZ, USA
¹⁰ Lowell Observatory, Flagstaff, AZ, USA
¹¹ NASA Goddard Space Flight Center, Greenbelt, MD, USA
¹² American University, Washington, DC, USA
¹³ Cornell University, Department of Astronomy, Ithaca, NY, USA
¹⁴ Southwest Research Institute, Boulder, CO, USA
¹⁵ Association of Universities for Research in Astronomy, Washington, DC, USA

^⋆ Corresponding author; npinilla@ucf.edu

Received: 25 March 2024
Accepted: 25 June 2024

Abstract

Context. (2060) Chiron is a large centaur that has been reported active on multiple occasions at relatively large heliocentric distances, including during aphelion passage. Studies of Chiron’s coma during active periods have resulted in the detection of C≡N and CO outgassing. Additionally, Chiron is surrounded by a disk of debris that varies with time. Significant work remains to be undertaken to comprehend the activation mechanisms on Chiron and the parent molecules of the gas phases detected.

Aims. This work reports the study of the ices on Chiron’s surface and coma and seeks spectral indicators of volatiles associated with the activity. Additionally, we discuss how these detections could be related to the activation mechanism for Chiron and, potentially, other centaurs.

Methods. In July 2023, the James Webb Space Telescope (JWST) observed Chiron when it was active near its aphelion. We present JWST/NIRSpec spectra from 0.97 to 5.27 μm with a resolving power of ∼1000, and compare them with laboratory data for identification of the spectral bands.

Results We report the first detections on Chiron of absorption bands of several volatile ices, including CO₂, CO, C₂H₆, C₃H₈, and C₂H₂. We also confirm the presence of water ice in its amorphous state. A key discovery arising from these data is the detection of fluorescence emissions of CH₄, revealing the presence of a gas coma rich in this hyper-volatile molecule, which we also identify to be in non-local thermal equilibrium (non-LTE). CO₂ gas emission is also detected in the fundamental stretching band at 4.27 μm. We argue that the presence of CH₄ emission is the first proof of the desorption of CH₄ due to a density phase transition of amorphous water ice at low temperature in agreement with the estimated temperature of Chiron during the JWST observations (61 K). Detection of photolytic and proton irradiation products of CH₄ and CO₂ on the surface, in the coma ice grains, or in the ring material is also detected via a forest of absorption features from 3.5 to 5.3 μm.