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Observations of CO, CO₂, and H₂O in Comets and Centaurs with a Variety of Instruments and their Implications for Solar System Formation and Evolution

Presentation #218.04D in the session Comets: Coma, Nucleus, Dynamics (Oral Presentation)

Published onOct 23, 2023
Observations of CO, CO₂, and H₂O in Comets and Centaurs with a Variety of Instruments and their Implications for Solar System Formation and Evolution

Comets and Centaurs are remnants from the formation of the solar system. These small bodies are usually divided into subgroups based on their orbital dynamical properties, such as main belt, Centaurs, Jupiter-family (JF), Oort cloud (OC), and interstellar. Analyzing the relative amounts of the CO, CO2, and H2O in their comae can provide strong observational constraints to models of solar system formation and evolution, and comet nuclei behavior.

First, this talk will focus on our analysis of published measurements of simultaneous CO and CO2 production rates for 25 comets obtained pre-James Webb Space Telescope [1]. About half of the comae produce more CO2 than CO, about a third produce more CO, and about a tenth produce comparable amounts of both gases. Most (89%) of JFCs have CO2-dominated comae compared to CO, while OCCs show no preference. Among 14 OCCs, dynamically new OCCs produce more CO2 than CO, while dynamically older OCCs release more CO than CO2. A trend may be present with increasing CO/CO2 ratio that is associated with more thermal processing. These observations may also be explained by a model of cosmic-ray processing of outer layers of very distant OCCs leading to preferential depletion of CO over CO2 in the outer layers. Within 2.5 au, we find median values of the relative production rates (QA/QB), QCO/QH2O = 3±1% and QCO2 /QH2O = 12±2%. From 0.7 to 4.6 au, CO2 outgassing appears more intimately tied to the H2O production in a way that the CO is not. We estimate a median value of C/O = 13% for the comets within 2.5 au.

The second part of the talk focuses on JWST NIRSpec observations of active Centaur 39P/Oterma from UTC 2022 July 27 at Rhelio = 5.82 au [2]. Centaurs are minor solar system bodies transitioning from Scattered Disk orbits in the Trans-Neptunian region to those of JFCs. 39P’s orbit has changed substantially over decades due to gravitational perturbations from Jupiter. For the first time, CO2 gas emission was detected in a Centaur, with a production rate of QCO2 = (6.69 ± 0.49) x 1023 molecules s-1. CO and H2O were not detected with respective 1-σ upper limits of QCO < 8.4 x 1023 molecules s-1 and QH2O < 2.0 x 1023 molecules s-1, and abundance ratios of QCO/ QCO2 ≤ 0.98, and QCO2 /QH2O ≥ 0.38. Measurements are consistent with CO2 and/or CO outgassing driving the activity of this Centaur and shows significant differences with another Centaur at a similar heliocentric distance, 29P/SW1.

[1] Harrington Pinto O., et. al. 2022,

[2] Harrington Pinto O., et. al. 2023, “First detection of CO2 emission in a Centaur: JWST NIRSpec observations of 39P/Oterma”, submitted

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