Distribution, origin and retention of CO2 and CO in TNOs revealed by JWST

Introduction : Carbon dioxide (CO₂) and carbon monoxide (CO) are major components of interstellar ices, protoplanetary disks and comets [1,2,3]. Yet, they have long eluded detection on Trans-Neptunian Objects (TNOs) thought to be frozen remnants of the outer protoplanetary disk. Until now, CO₂ and CO had only been detected on the captured TNO Triton [4] while only CO was detected on the dwarf planet Pluto [5]. Instrumental and atmospheric limitations to the detection of CO₂ and CO on medium-sized TNOs (100-800 km) are now lifted by the capacities of the James Webb Space Telescope (JWST).

Methods : 54 TNOs and 5 Centaurs have been observed using the low spectral resolution PRISM grating on NIRSpec onboard JWST as part of the Cycle 1 Large Program “Discovering the Surface Composition of trans-Neptunian objects" (DiSCo-TNOs). Our analysis of the observations focused on the 4.26 and 2.70 µm CO₂ bands and on the 4.68 µm CO band. We extracted band parameters by gaussian fitting. We used available spectroscopic studies of various ices, pure and mixed, to compare their band characteristics to the ones of observational data. We also ran ion irradiation experiments to simulate space weathering on the surfaces of TNOs. We used in particular 30 keV H⁺ to irradiate CO₂ ices at 45K and methanol (CH₃OH) ices at 60K.

Results : We report here the detection of CO₂ across 95% of the sample. CO₂ abundance, mainly investigated through the depth of its bands, is found to greatly vary between objects and eventually to define different compositional groups. The 4.26 µm CO₂ fundamental band shows an unusual profile on objects where its abundance is high. Current modelling efforts have trouble reproducing these peculiar features but show that they are likely due to sub-wavelength ice grains and complex fine optical properties of the CO₂ ice. The 2.70 µm CO₂ combination band, present on 63 % of the sample, is highly sensitive to chemical environment. Its position reveals that CO₂ is likely pure or mixed with CO when most abundant while it is likely mixed with a polar component like H₂O and CH₃OH when less abundant [6]. We also report the detection of CO across at least 47% of the sample. CO is typically found on objects where CO₂ is most abundant. To study the abundance of CO relatively to that of CO₂ we use the band area ratio of the 4.68 µm and the 2.70 µm bands. We also retrieve this ratio from in-situ infrared spectroscopy of the irradiated CO₂ and CH₃OH ices. By comparing observations with laboratory experiments, we find that CO₂-rich objects are compatible with a formation of CO by CO₂ irradiation. Objects richer in complex organics have surfaces compatible with CO formed by CH₃OH irradiation.

Discussion : The detection of CO and CO₂ on TNOs give the unprecedented opportunity to map the distribution of volatiles across the different dynamical populations of the outer solar system. In fact, CO₂ was found to be one of the molecules that characterizes 3 distinct spectral types, defined by clustering techniques over the whole spectral range (0.7 – 5.1 µm), which are attributed to different formation regions and retention lines in the protoplanetary disk [7]. The chemical and physical state of CO₂ and CO, retrieved from the study of band characteristics (shape, position and area) allow to probe their potential origin and ongoing processes. Particularly, we argue that CO is an irradiation product of either CO₂ or CH₃OH, depending on the dominating molecule when the last planetary migration was triggered. CO would then be more or less efficiently retained depending on the matrix from which it formed. Finally, Centaurs’ rapid thermal evolution is evidenced by their loss of CO₂ and CO [8].

Acknowledgements : This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope under the GO-1 program #2418.  Support for this program was partially provided by NASA through a grant from the Space Telescope Science Institute. Irradiations were performed using the INGMAR setup, a joint IAS-IJCLab (Orsay, France) facility funded by the P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d’Avenir (ANR-11-IDEX-0003- 01). The work was supported by the CNES (JWST mission).

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