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).

References : [1] McClure et al. (2023). Nat Astron 7, 431–443. [2] Sturm et al. (2023). A&A, 679, A138. [3] Harrington-Pinto et al. (2022). PsJ, 3:247 (25pp). [4] Cruikshank et al. (1993). Science, 261(5122), 742-745. [5] Owen et al. (1993). Science, 261(5122), 745-748. [6] De Prá et al. (2024). Nature Astronomy, in press. [7] Pinilla-Alonso et al. (2024). Nature Astronomy, under revision. [8] Licandro et al. (2024). Nature Astronomy, in press.

How to cite: Henault, E., N. De Prá, M., Brunetto, R., Pinilla-Alonso, N., Holler, B. J., Baklouti, D., and Djouadi, Z. and the DiSCo-TNO team: Distribution, origin and retention of CO2 and CO in TNOs revealed by JWST, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-64, https://doi.org/10.5194/epsc2024-64, 2024.

We report the first detection of carbon dioxide (CO₂) and hydrogen peroxide (H₂O₂) on Charon’s frozen surface as revealed by JWST NIRSpec instrument. With the extended spectral range of NIRSpec, we have expanded Charon’s compositional inventory to include these two new species. Previously, the inventory primarily consisted of water ice (mostly in crystalline form), ammoniated species, and a tholin-like darkening constituent. The synergy of laboratory measurements and modeling analysis reveals a stratified surface rich in crystalline water ice with ammonia diluted in water ice at penetration depths of approximately ~100 micron. Additionally, a layer of pure crystalline CO₂ is evident at shallower penetration depths of about ~1 micron. This feature is likely attributable to an endogenous source, unearthed by external impacts. This layering configuration is believed to cause a scattering effect, which may account for the peculiarly strong CO₂ absorption band at longer wavelengths. Moreover, the surface is undergoing continuous alteration by photolysis and radiolysis, which are responsible for the presence of H₂O₂ and amorphous water ice.

How to cite: Protopapa, S., Raut, U., Wong, I., Stansberry, J., Villanueva, G., Cook, J., Holler, B., Grundy, W. M., Brunetto, R., Cartwright, R. J., Mamo, B., Emery, J., Parker, A. H., Guilbert-Lepoutre, A., Pinilla-Alonso, N., Milam, S., and Hammel, H. B.: Detection of Carbon Dioxide and Hydrogen Peroxide on the Layered Surface of Charon Using the James Webb Space Telescope, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1277, https://doi.org/10.5194/epsc2024-1277, 2024.

Introduction: Bodies orbiting the trans-Neptunian regions (TNOs) are windows into Solar System formation, history and evolution. They are remnants from the ancient time of planetary formation in the protoplanetary disk, but they have all been affected to some extent by subsequent dynamical evolution of the Solar System. Many are probably very primitive, in the sense that they incorporated solids from the protoplanetary disk and never melted, and can provide precious information about conditions in the solar nebula. Others remelted and differentiated, and we can use them to learn about the processes of planet growth and evolution.

Methods: Before the launch of the James Webb Space Telescope (JWST), the detection of molecules on TNOs has long been limited by the terrestrial atmosphere and the sensitivity of the available instrumentation. We used the low spectral resolution PRISM grating on the Near-Infrared Spectrograph (NIRSpec) of JWST as part of the Cycle 1 Large Program “Discovering the Surface Composition of trans-Neptunian objects" (DiSCo-TNOs) to observe 54 medium-sized TNOs and 5 Centaurs. The sample contains objects spanning the diversity of the TNO population in terms of size, visible colors, geometric albedo, and dynamical properties, except for the volatile rich dwarf planets.

Results: JWST is providing an unprecedented view of the molecular diversity on the surfaces of TNOs [1]. We report the detection of several molecular ices throughout the TNO population, including H₂O, CO₂, ¹³CO₂, CO, CH₃OH, and complex molecules and refractory materials containing aliphatic C–H, C≡N, O–H, and N–H bonds. In particular, CO₂ is widespread in all TNO populations [2]. As a result of the imprint that these molecules leave on the spectra, three main compositional groups consistently emerge from multiple independent cluster analysis efforts: Bowl-type surfaces are water- and dust-rich and CH-poor, while Cliff and Double-dip surfaces are C-rich and water-poor, with Double-dip being rich in CO₂ and Cliff surfaces being rich in organics. The C/O and (CH+NH)/(C+O) ratios on the surface of TNOs are the primary indicators of the spectral differences among the three TNO groups that we observe today.

Discussion: Our results unlock the long-standing question of the interpretation of TNO colour diversity providing compositional information. The marked separation of the three spectral clusters reveals sharp variations in the surface molecular constituents. CO₂ and CH₃OH play a major role, probably because these ices were abundant in the protoplanetary disk. We propose that medium-sized TNOs are fossil remnants of icy planetesimals, and that the three compositional groups provide a picture of the ice retention lines in the Solar System that likely occurred in the outer protoplanetary disk, possibly just before a major planetary migration. Finally, Centaurs observed by DiSCo provide direct evidence of how the three TNO spectral groups evolve when surface activity is triggered by entering the region between the orbits of Jupiter and Neptune [3].

References: [1] Pinilla-Alonso N. et al. (2024). Nature Astronomy, under revision. [2] DePrà M. et al. (2024). Nature Astronomy, in press. [3] Licandro J. et al. (2024). Nature Astronomy, in press.

How to cite: Brunetto, R., Pinilla-Alonso, N., N. De Prá, M., Hénault, E., Holler, B. J., and Licandro, J. and the DiSCo-TNO team: The molecular diversity of TNOs revealed by JWST and its links to protoplanetary composition and processes, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-65, https://doi.org/10.5194/epsc2024-65, 2024.

This research explores the surface properties of ten Centaurs through their reflective properties in the 0.6-5.3 μm range obtained using the JWST/NIRSpec spectrograph in the framework of two different programs: the JWST GO-1 large program “Discovering the Surface Composition of the trans-Neptunian Objects, Icy Embryos for Planet Formation” (DiSCo-TNOs; PID 2418) and the first Cycle One programs of Guaranteed Time “Kuiper Belt Science with JWST” (PIDs 1272 and 1273).

Our study includes 52872 Okyrhoe, 3253226 Thereus, 136204, 250112, 310071 (Licandro et al. 2024), 10199 Chariklo, 55576 Amycus, 281371, and 459865 (Licandro et al, in preparation), and 2060 Chiron (Pinilla-Alonso et al., in preparation). We observe considerable diversity in their surface composition. Our analysis reveals two main categories among these bodies that mirrors similar findings in the Trans-Neptunian Objects (TNOs): those with surfaces composed of refractory materials plus some degree of water ice, and those with a higher content of carbon-based materials. Centaurs also include objects with a surface largely composed of refractory materials and little or no volatiles. As Centaurs travel closer to the Sun their surfaces tend to become less icy and more dominated by non-volatile materials due to the sublimation of volatile substances such as ice. Our compositional analysis suggests these Centaurs have a high concentration of amorphous silicates, indicating surfaces composed of primitive, comet-like dust. These findings suggest that similar groups in the solar system – including comets, Jupiter Trojans, Main Belt Comets, and D-type asteroids – which are initially thought to have had icy compositions, may undergo comparable surface changes due to thermal processing. Finally, our data strongly suggest that the color bimodal visible color distribution of the Centaur population is due to their original composition (’nature’) and not to surface evolution (’nurture’) as in the case of TNOs (Pinilla et al., submitted).

How to cite: Licandro, J., Pinilla-Alonso, N., Holler, B., Wong, I., de Pra, M., Melita, M., Souza Feliciano, A. C., Brunetto, R., Guilbert-Lepoutre, A., Hénault, E., Lorenzi, V., Stansberry, J., Schambeau, C., Harvison, B., Pendleton, Y., Cruikshank, D., Mueller, T., Emery, J., McClure, L., and Peixinho, N.: Deciphering TNOs thermal evolution through Centaur surface studies using JWST, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-984, https://doi.org/10.5194/epsc2024-984, 2024.

The spin of dwarf planets and their satellites in the outer solar system is governed by their formation history and the tidal forces acting between them. By studying the rotation of these objects, we can track their evolution over time. In the case of the dwarf planet (136199) Eris the previously obtained rotation periods were not consistent in the literature. The values ranged from a few hours to ~15 days.

We used new data from ground based ~1-2m telescopes and light curves from Gaia and TESS space telescopes to determine the rotation period of Eris. The data from TESS wasn’t conclusive, but the combined ground based and Gaia data showed a period almost identical to the orbital period of Eris’s moon, Dysnomia with a Δm ≈ 0.03 mag amplitude. We concluded that the rotation of Eris should be tidally locked.

Figure 1.: C(P, ∆m) (period cost function) contour map; The most prominent minima is identified at a period of ∼16 d, very close to the orbital period of Dysnomia, 15.78 d.

We used a simple tidal evolution model, assuming that Dysnomia has a collisional origin, and we found that Dysnomia must be relatively massive (mass ratio of q = 0.01-0.03) and large (radius of R_s ≥ 300 km) or Eris must have a low Q tidal parameter similar to that of terrestrial planets to have the potential to slow Eris down to a synchronised rotation. In the first scenario assuming usual tidal parameters for trans-Neptunian objects the calculations indicate that the density of Dysnomia should be 1.8-2.4 g cm⁻³. This density considered very high among similarly sized objects and could set important constraints on their formation conditions.

The size (R≈1100 km) suggests that Eris should be spherical and the observed light curve probably comes from some surface variegations. This is not unusual among large trans-Neptunian objects because they can be characterized by bright surfaces and variable volatile compositions.

We compared the visible range and J - H colors of Eris from the literature and while the visible range colors showed consistent values the J - H colors varied in a large range. To clarify whether the J - H color changes we used the GuideDog camera of the Infrared Telescope Facility to observe Eris for a two week period to cover a full rotation. We found that the J - H color of Eris indeed changes with the rotational phase. This suggests notable surface heterogenity in chemical composition and/or other material properties. With a simple calculation we tested that the grain size of the dominant CH₄ may in general be responsible for notable changes in the J - H color, but in the current observing geometry of the system it can only partially explain the observed J - H variation.

Figure 2.:  Top: (J − H) colors of Eris converted to the 2MASS system, as a function of rotational phase (phase zero epoch is 2457357.8929 JD, as obtained from
Bernstein et al. 2023). The dark blue markers represent the J − H colors from this work, labeled as I1...I4. The colored markers represent the J − H
values from the literature. Bottom: An approximate representation of the Eris visible range light curve using a properly phased sinusoidal curve. The phases
of IRTF, XShooter (X1 and X2) and JWST/NIRSpec (JWST) measurements are marked by vertical dashed lines.

How to cite: Szakáts, R., Kiss, C., Ortiz, J. L., Morales, N., Pál, A., Müller, T., Greiner, J., Santos-Sanz, P., Marton, G., Duffard, R., Sági, P., and Forgács-Dajka, E.: Tidally locked rotation and spin phase dependent J - H color of the dwarf planet (136199) Eris, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1046, https://doi.org/10.5194/epsc2024-1046, 2024.

Recent stellar occultations have allowed accurate instantaneous size and apparent shape determinations of the large Kuiper belt object (50000) Quaoar and the detection of two rings with spatially variable optical depths. Here we present new visible range light curve data of Quaoar from the Kepler/K2 mission, and thermal light curves at 100 and 160 µm obtained with Herschel/PACS. The K2 data provide a single-peaked period of 8.88 h, very close to the previously determined 8.84 h, and it favours an asymmetric double-peaked light curve with a 17.76 h period. We clearly detected a thermal light curve with relative amplitudes of ~10% at 100 and at 160 µm. A detailed thermophysical modelling of the system shows that the measurements can be best fit with a triaxial ellipsoid shape, a volume-equivalent diameter of 1090 km, and axis ratios of a/b = 1.19 and b/c = 1.16. This shape matches the published occultation shape, as well as visual and thermal light curve data. The radiometric size uncertainty remains relatively large (±40 km) as the ring and satellite contributions to the system-integrated flux densities are unknown. In the less likely case of negligible ring or satellite contributions, Quaoar would have a size above 1100 km and a thermal inertia ≤ 10 J m⁻²K⁻¹s^−1/2. A large and dark Weywot in combination with a possible ring contribution would lead to a size below 1080 km in combination with a thermal inertia ≳10 J m⁻²K⁻¹s^−1/2, notably higher than that of smaller Kuiper belt objects with similar albedo and colours. We find that Quaoar's density is in the range 1.67-1.77 g cm⁻³, significantly lower than previous estimates. This density value closely matches the relationship observed between the size and density of the largest Kuiper belt objects. 

Figure 1: Thermophysical model results for Quaoar. Left: All available IR observations divided by the corresponding TPM predictions, using a triaxial shape with an equivalent diameter of 1090 km, and a thermal inertia of 12 J m⁻²K⁻¹s^−1/2 . The calculations take small flux contributions from the ring and Weywot into account. These absolute ratios are very sensitive to the size and thermal inertia assumptions in the model, the 24 μm points is strongly influenced by surface roughness effects. Right: Far-infrared light curves. The small ring and Weywot flux contributions are subtracted from the observed PACS 100 and 160 μm lightcurve data. The triaxial Quaoar model solutions (at 100 μm in green and at 160 μm in orange) are shown on top. The boxes represent the mean PACS values.

How to cite: Kiss, C., Müller, T., Marton, G., Szakáts, R., Pál, A., Molnár, L., Vilenius, E., Rengel, M., Ortiz, J.-L., and Fernandez-Valenzuela, E.: The visible and thermal light curve of the large Kuiper belt object (50000) Quaoar, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-104, https://doi.org/10.5194/epsc2024-104, 2024.

Introduction

Bienor is an inactive centaur with an orbital period of 67.34 years and a rotation period of approximately 9.14 hours [1]. Upon studying the color-albedo distribution of over 100 transneptunian objects and centaurs, two distinct clusters emerged: one composed of dark-neutral objects, and another of bright-red objects [2]. This distinction is particularly evident among centaurs [3,4,5], with median albedos of approximately ~5% for the dark-neutral group, and ~8.4% for the bright red group [6]. Bienor falls within the dark-neutral group.

Regarding shape and orientation, extreme light curve amplitudes suggest that Bienor is a highly elongated body [7]. According to [8], there are two possible rotational pole orientations: β = 50° ± 3°, λ = 35° ± 8° (prograde rotation), or β = −50° ± 3°, λ = 215° ± 8° (retrograde rotation). Assuming Bienor is a triaxial ellipsoid under hydrostatic equilibrium, [8] also constrained the b/a axial ratio to 0.45 ± 0.05.

Some controversy surrounds this object due to the inconsistency in size and albedo measurements obtained through various techniques [9]. Thermal measurements using data from WISE provided a diameter of (187.5 ± 15.5) km and a geometric albedo of (5.0 ± 1.9) % [3]. Radiometric measurements from the HST and SST resulted in a diameter of  (198 ± 7) km and a geometric albedo of (4.3 ± 1.6) % [10]. Subsequently, observations from SST, HST, and ALMA produced a diameter range of 179 –184 ± 6 km and a geometric albedo of (5.0 –  5.3) ± 1.8 % [9]. However, the stellar occultation technique estimated an area-equivalent diameter of 150 ± 20 km [11], smaller than the thermal diameters and suggestive of a possible satellite or binary nature. Moreover, [11] reported a strong irregularity in one of the minima of the rotational light curve, but no features compatible with rings or satellites within their data. 

Methods

Predictions of stellar occultations by Bienor were initially made using the Numerical Integration of the Motion of an Asteroid (NIMA) solution [12]. Subsequent observational campaigns [11] led to updates and refinements of Bienor's orbit. These efforts resulted in the successful prediction and observation of three positive occultations on February 6, 2022, December 26, 2022, and February 14, 2023. Moreover, to determine Bienor's rotational phase during these occultations, photometric data were collected in 2021 and 2023 using the 1.5 m telescope at the Sierra Nevada Observatory (OSN) and the 1.23 m telescope at the Calar Alto Observatory (CAHA).

Results

Thanks to the analysis of rotational light curves (Fig. 1), we obtained a refined rotation period of Bienor and confirmed the presence of a clear asymmetry in the rotational light curve with distinct absolute and relative minima.

Then, we successfully observed the three predicted stellar occultations by Bienor from Japan, Eastern Europe, and the USA, obtaining a total of 11 chords. The data acquired allowed to determine the projected shape of Bienor for the occultation that took place on December 26, 2022, obtaining a projected ellipse with a major semi-axis of 107.01 ± 3.72 km and a minor semi-axis of 51.97 ± 2.13 km (Fig. 2a). Assuming this solution, we calculated the projected area of Bienor during the date of the observations that led [9] to determine an effective area of 179–184 ± 6 km. We obtained a Bienor area-equivalent diameter of (158 ± 16) km, lower than the thermal estimation.

Fig. 1. Rotational light curves using (a) 2021 and (b) 2023 observational data. The vertical line indicates the rotational phase during the three occultations. Dashed lines show the uncertainty after propagating the 0.0002 h. At the bottom, the difference (residual) between the modeled curve and the observational data is shown.

The projected area on December 26, 2022, along with the absolute magnitude value during the occultation (7.72 ± 0.04), enabled us to determine a geometric albedo of 6.5 ± 0.5%, once again larger than previous thermal estimates. We also determined the absolute values of the triaxial ellipsoid axes.

Fig. 2. a) Occultation on December 26, 2022. The dashed line describes the ellipse that best fits the points. The dashed arrow in the upper right corner indicates the direction of the shadow motion. b) Bienor simulation with our software seen from Earth during the occultation of December 26, 2022. The white arrow indicates the determined rotation pole solution.

In addition, we developed software to simulate synthetic light curves from a three-dimensional shape model and using photometric models (Fig. 2b). With this software, we validated the rotational pole solution of β = 50°, λ = 35°, which aligns with observations spanning 22 years. Finally, we discuss several possibilities that could account for the asymmetries observed in the rotational light curves, including irregular shapes, the possibility that Bienor is a contact binary, the presence of equatorial regions with much lower albedo, and eventually the possibility of a satellite. This last option is the most plausible as it could explain not only the asymmetries but also the missing area that would account for the discrepancies in albedo.

References

[1] Ortiz, J. L., et al. 2002, A&A, 388, 661. [2] Lacerda, P., et al. 2014, ApJ, 793, L2. [3] Bauer, J. M., et al. 2013, ApJ, 773, 22. [4] Duffard, R., et al. 2014, A&A, 564, A92. [5] Tegler, S. C., et al. 2016, AJ, 152, 210. [6] Müller, T., et al. 2020, in The Trans-Neptunian Solar System. [7] DeMeo, F. E., et al. 2009, A&A, 493, 283. [8] Fernández-Valenzuela, E., et al. 2017, MNRAS, 466, 4147. [9] Lellouch, E., et al. 2017, A&A, 608, A45. [10] Duffard, R., et al. 2014, A&A, 568, A79. [11] Fernández-Valenzuela, E., et al. 2023, A&A, 669, A112. [12] Desmars, J., et al. 2015, A&A, 584, A96.

How to cite: Rizos Garcia, J. L., Fernández-Valenzuela, E., and Ortiz, J. L. and the The Lucky Star and the Bienor occultation campaigns team.: A study of centaur (54598) Bienor from multiple stellar occultations and rotational light curves, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-530, https://doi.org/10.5194/epsc2024-530, 2024.

How to cite: Sicardy, B. and Salo, H.: The dense resonance meshes around the ringed Chariklo, Haumea and Quaoar, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-123, https://doi.org/10.5194/epsc2024-123, 2024.