# Thermal evolution of trans-Neptunian objects through observations of Centaurs with JWST

Javier Licandro, Noemí Pinilla-Alonso, Bryan Holler, Mário de Prá, Mario

Melita, Ana Carolina de Souza Feliciano, Rosario Brunetto, Aurélie

Guilbert-Lepoutre, Elsa Hénault, Vania Lorenzi, et al.

► To cite this version:

Javier Licandro, Noemí Pinilla-Alonso, Bryan Holler, Mário de Prá, Mario Melita, et al. Thermal evolution of trans-Neptunian objects through observations of Centaurs with JWST. Nature Astronomy, 2024, 9 (2), pp.245-251. 10.1038/s41550-024-02417-2. hal-04961632

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# Thermal evolution of trans-Neptunian objects through observations of Centaurs with JWST

Received: 13 November 2023

Accepted: 19 October 2024

Published online: 19 December 2024

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Javier Licandro Ⓔ¹,²✉, Noemí Pinilla-Alonso Ⓔ³, Bryan J. Holler Ⓔ⁴, Mário N. De Prá³, Mario Melita⁵,⁶,⁷, Ana Carolina de Souza Feliciano³, Rosario Brunetto Ⓔ⁸, Aurélie Guilbert-Lepoutre⁹, Elsa Hénault Ⓔ⁸, Vania Lorenzi Ⓔ¹,¹⁰, John A. Stansberry Ⓔ⁴,¹¹,¹², Charles A. Schambeau Ⓔ³,¹³, Brittany Harvison³, Yvonne J. Pendleton Ⓔ¹³, Dale P. Cruikshank Ⓔ¹³, Thomas Müller Ⓔ¹⁴, Lucas McClure¹¹, Joshua P. Emery Ⓔ¹¹, Nuno Peixinho Ⓔ¹⁵, Michele T. Bannister¹⁶ & Ian Wong Ⓔ¹⁷,¹⁸

Centaurs are small bodies orbiting between Jupiter and Neptune and behave as an intermediate population between trans-Neptunian-belt objects and Jupiter-family comets. As such, their surface composition and evolutionary processes are key to understanding the Solar System's history. However, the mechanisms driving their transformation and the impact of thermal processing on their surfaces remain open questions. Here we examined the surface properties of five Centaurs using the James Webb Space Telescope near-infrared spectrograph reflectance spectra (0.6–5.3 µm). They exhibit considerable diversity in surface composition. Our analysis indicates that Centaurs can be split into two main categories, which is also observed for trans-Neptunian objects: one group has surfaces composed of refractory materials with some water ice, whereas the other is dominated by carbon-based materials. Additionally, two of the five objects have primarily refractory surfaces with minimal volatiles, suggesting a high concentration of primitive, comet-like dust. We suggest that the observed Centaur surfaces reflect their transitional states, as they are shifting from being ice-rich bodies to progressively becoming more dominated by non-volatile materials as they approach the Sun. Such thermal processing may have changed the surface properties of other similar Solar System bodies, like comets, Jupiter trojans and D-type asteroids.

Centaurs are icy bodies, short-time residents (<10⁷ years) in the region between the orbits of Jupiter and Neptune, with semimajor axes $a \le 30.1$ AU and Tisserand invariant with respect to Jupiter $T_J > 3.05$ (ref. 1). Centaurs are scattered objects originating from the trans-Neptunian belt, and they contribute to a dynamical cascade that supplies Jupiter-family comets²⁻⁴. Being closer to the Sun than the less accessible trans-Neptunian objects (TNOs), yet not as processed as the rapidly evolving Jupiter-family comets, they are a key population in investigations of the origin of our Solar System.

Near-infrared spectroscopy of icy bodies provides critical information on surface composition. Ices, solid complex organic materials and silicates can be identified in the 1–5 µm region. Reflectance spectra were recently published for 54 TNOs⁵. This sample represents the diversity of the TNO population and was observed as part of the James Webb Space Telescope (JWST) GO-1 large programme 'Discovering the surface composition of the trans-Neptunian objects, icy embryos for planet formation' (DiSCo-TNOs, Programme ID 2418). Using the PRISM/CLEAR grating/filter pair of the integral field unit (IFU) of the
<table><caption>Table 1 | Orbital and known physical properties of the observed Centaurs</caption><thead><tr><th rowspan="2">Number</th><th rowspan="2">Name</th><th rowspan="2">Provisional designation</th><th>a</th><th>e</th><th>i</th><th>q</th><th>H</th><th>D</th><th>p<sub>v</sub></th><th>T<sub>q</sub></th><th>S'<sub>vis</sub></th></tr><tr><th>(AU)</th><th>(′)</th><th>(AU)</th><th>(mag)</th><th>(km)</th><th></th><th>(K)</th><th>(%/1,000 Å)</th></tr></thead><tbody><tr><td>32532</td><td>Thereus</td><td>2001 PT13</td><td>10.6</td><td>0.20</td><td>20.4</td><td>8.5</td><td>9.4</td><td>62<sup>+3</sup><sub>-3</sub></td><td>0.08<sup>+0.02</sup><sub>-0.01</sub></td><td>95</td><td>9.48±2.01</td></tr><tr><td>52872</td><td>Okyrhoe</td><td>1998 SG35</td><td>8.4</td><td>0.30</td><td>15.6</td><td>5.8</td><td>11.1</td><td>35<sup>+3</sup><sub>-3</sub></td><td>0.06<sup>+0.01</sup><sub>-0.01</sub></td><td>115</td><td>11.27±3.22</td></tr><tr><td>136204</td><td>–</td><td>2003 WL7</td><td>20.1</td><td>0.26</td><td>11.2</td><td>14.9</td><td>8.8</td><td>105<sup>+6</sup><sub>-7</sub></td><td>0.05<sup>+0.01</sup><sub>-0.01</sub></td><td>72</td><td>11.7±2.06</td></tr><tr><td>250112</td><td>–</td><td>2002 KY14</td><td>12.6</td><td>0.32</td><td>19.5</td><td>8.6</td><td>10.4</td><td>47<sup>+3</sup><sub>-4</sub></td><td>0.06<sup>+0.01</sup><sub>-0.01</sub></td><td>94</td><td>33.42±2.46</td></tr><tr><td>310071</td><td>–</td><td>2010 KR59</td><td>29.8</td><td>0.56</td><td>19.7</td><td>13.0</td><td>7.7</td><td>110<sup>+30</sup><sub>-30</sub></td><td>0.12<sup>+0.04</sup><sub>-0.04</sub></td><td>76</td><td>23.89±4.47</td></tr></tbody></table>

This table lists the orbital semimajor axis *a*, eccentricity *e*, inclination *i*, perihelion distance *q*, visible absolute magnitude *H*, diameter *D*, geometric albedo *p*<sub>v</sub>, surface temperature estimated at the perihelion distance $T_q = T_0(1 - A)^{1/4} \sqrt{1/2}$ (where *A* is the Bond albedo) and the spectral visible slope $S'_{vis}$. $S'_{vis}$ values for the first four objects are from ref. 37, and the values for 2010 KR59 are from ref. 38. *H*, *D* and *p*<sub>v</sub> values for the first four objects are from ref. 39, and the values for 2010 KR59 are from refs. 38,40–42.

near-infrared spectrograph (NIRSpec) onboard JWST<sup>6,7</sup>, low-resolution spectra ($30 < R < 300$) in the 0.6–5.3 µm range were acquired for all 54 objects (Methods). By using different clustering techniques, three spectral types of TNOs were identified<sup>5</sup>, each one named after the shape of the spectrum in the 3 µm region: bowl (25% of the sample), double-dip (43%) and cliff (32%). The spectra of TNOs<sup>5</sup> have evident signatures of CO<sub>2</sub> ice, and roughly half of them also have strong signatures of CO ice<sup>8</sup>.

There are three compositional groups in the trans-Neptunian region, each separated by the dominant chemical species in the surface: (1) Bowl TNOs are dominated by water and silicates. (2) Double-dip TNOs are dominated by CO<sub>2</sub> and CO ices. (3) Cliff TNOs are dominated by carbon ices and complex organics<sup>5</sup>. The three groups provide a picture of the ice retention lines in the Solar System that probably occurred in the outer protoplanetary disk, just before a major planetary migration<sup>5</sup>. Recent results strengthen this hypothesis<sup>9–11</sup>.

The surface compositions of objects in the trans-Neptunian belt predominantly evolve due to the competition of two resurfacing effects: irradiation and collisions<sup>12</sup>. In surfaces originally rich in molecular ices ($H_2O$, $CO_2$, $NH_3$, $CH_4$ and so on), long-term irradiation results in the formation of complex organic molecules that reside in an 'irradiation mantle'. Collisions erode this mantle and cover the surface with fresh (non-irradiated) materials. For Centaurs that venture closer to the Sun, another resurfacing process may occur due to increased insolation. This triggers phase transitions of their icy components and subsequent outgassing from their surfaces, which may entrain solid particles to produce a dust coma. A portion of the entrained dust particles will fail to reach escape velocity and fall back to the surface to form a 'dust mantle'. Some Centaurs (-10%) show comet-like activity, even at heliocentric distances $r > 5.2$ AU (ref. 13), where temperatures are such that water ice does not sublimate. This process is, thus, not a good driver for cometary activity among Centaurs. Other phase transitions are more probable, such as the crystallization of amorphous water ice, which is efficient up to 10–12 AU and releases trapped volatiles<sup>13,14</sup>.

In this paper, we analyse the reflectance spectra of the five Centaurs observed as part of DiSCo-TNOs: 52872 Okyrhoe, 32532 Thereus, 136204 (2003 WL7), 250112 (2002 KY14) and 310071 (2010 KR59). Their final reflectance spectra are shown in Extended Data Fig. 1, and details of the observations are listed in Extended Data Table 1. None of these Centaurs showed activity, so their uncontaminated reflectance spectra provide crucial information about their surface composition. Some of their orbital and known physical properties are presented in Table 1. We derived compositional information and compared them with the reflectance spectra of their precursors, the TNOs, with an unprecedented level of detail. Until now, only low-resolution spectra in the 0.4–2.4 µm range have been published for a dozen Centaurs. Spectra in this region with a rather poor signal-to-noise ratio seem to be largely featureless<sup>15</sup>. Only weak water-ice bands have been detected in a few objects and CH<sub>3</sub>OH only in Pholus<sup>16</sup>.

# Results

## Spectral characteristics and classes of Centaurs

Using the same clustering analysis used for TNOs<sup>5</sup>, we found that Thereus and 2003 WL7 are bowl type and that 2002 KY14 is cliff type, whereas the remaining two Centaurs, Okyrhoe and 2010 KR59, do not fit into any spectral class (Fig. 1). They have a unique spectrum that is different from the spectrum of any observed TNO. A principal component analysis using three components applied to TNO spectra show that in the PC1 versus PC2 plots, bowl types are extremely well separated from double-dip and cliff types whereas double-dip and cliff objects show a more continuous distribution<sup>5</sup>. Figure 2 shows PC1 and PC2 for the five Centaurs projected onto the graph for the TNOs in the DiSCo sample. Note that both misfit Centaurs are close to the bowl TNOs and are well separated from the other two classes and that the cliff Centaur is well separated from cliff TNOs.

Additionally, the spectra of four Centaurs in the same spectral region were obtained during the 1st cycle programme of Guaranteed Time Kuiper Belt Science with JWST (Proposal 1273, PI J. Stansberry). Two of these are cliff types, one is a bowl type and the last has a spectrum very much like that of Okyrhoe (J.L. et al., manuscript in preparation). Finally, the reflectance spectrum of the active Centaur C/2014 OG392 (PANSTARRS) (C. Schambeau, personal communication) is compatible with it as being cliff type. Despite the reduced sample size for the Centaur JWST spectra, our results suggest that there are significant differences between the spectral-class distributions of Centaurs and TNOs: (1) Although 43% of the TNOs in the TNO sample have a double-dip spectral class<sup>5</sup>, no observed Centaur exhibits this type of spectrum. (2) Three-tenths of the observed Centaurs have a spectral class that has not been observed in any TNO.

We next explored whether there are also differences between the observed cliff and bowl Centaurs compared to the cliff and bowl TNOs. Figure 1a compares the spectra of Thereus and 2003 WL7 with that of bowl-type TNOs, whereas Fig. 1c compares spectrum of 2002 KY14 with that of cliff-type TNOs. A first look at the Thereus and 2003 WL7 spectra show that their shapes are very much like that of bowl TNOs. The slope in the 0.7–2.0 µm range of both objects is in the 'reddish' range of the colours of the TNOs. There are some small differences. For instance, there is a hint of shallower water-ice absorption bands at 1.5 and 2.0 µm, although the bowl-shaped bands at 3 µm are of similar depths and widths to those of the other bowl TNOs. Although 2003 WL7 resembles the average bowl, Thereus shows some differences: the continuum at 3.6 µm is slightly smaller than that of TNOs and the wide band between 3.5 and 5.0 µm is slightly shallower. We also found small but not significant differences when we compared the depth of the CO<sub>2</sub> fundamental band at -4.27 µm for the Centaurs (Table 2) with means for the TNO classes<sup>8</sup>. The band depth of Thereus (34.2 ± 1.6) is close to the mean value for the group of bowl-type TNOs, 35.31 ± 8.20, and 2003 WL7 has a similar band (37.5 ± 5.5). Like most
<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Comparison of spectra of Centaurs and TNOs. a, Thereus (violet) and 2003 WL7 (blue) compared with the TNOs belonging to the bowl group. Light-grey lines are spectra of individual bowl TNOs. The black line is the mean bowl spectrum. b, Okyrhoe (green) and 2010 KR59 (light green) compared with the TNOs belonging to the bowl group, coloured as in a, c, 2002 KY14 (red) compared with the TNOs belonging to the cliff group. Grey lines are spectra of individual cliff TNOs, whereas the black line is the mean cliff spectrum.</figcaption></figure>

bowl TNOs, the spectra of Thereus and 2003 WL7 do not show the CO band at ~4.68 µm.

On the other hand, the spectrum of the cliff Centaur 2002 KY14 has important differences with respect to those of cliff TNOs. Its 3 µm band (attributed to -OH and -NH in complex organics, plus -OH of methanol and possibly some amorphous water) is shallower (the reflectance of 2002 KY14 at 3.05 µm is three times larger than that of cliff TNOs) but the band between 3.3 and 3.6 µm is at least as deep as those of cliff TNOs but wider, with a local maximum at 4.1 µm compared to 3.7 µm for the median cliff reflectance. The fundamental CO₂ band at 4.26 µm is also shallower (the CO₂ band depth is only 35.6 ± 1.1 whereas the mean band depth of the cliff TNOs is 76.76 ± 14.84). There is no clear evidence of the 4.68 µm CO band. We detected an absorption band around 4.6–4.7 µm that can be attributed to the C≡N stretching in complex organics. OCN has also been detected in cliff TNOs⁵. Moreover, 2002 KY14 has weak absorption bands at 2.27 and 2.34, which have also been observed in cliff TNOs and are attributed to the asymmetric and symmetric combination modes of the -CH₃ group due to the presence of methanol ice. The band at 4.9 µm attributed to OCS (carbonyl sulfide) in TNOs⁵ is slightly deeper in 2002 KY14. Finally, other weak bands detected in cliff TNOs between 1.5 and 2.0 µm are not seen in the spectrum of 2002 KY14.

Finally, the characteristics of the spectra of the 'misfit' Centaurs Okyrhoe and 2010 KR59 resemble that of bowl-type TNOs due to their red spectral slopes below 2.5 µm and a distinct bowl-shaped absorption at 3 µm. Notably, their spectra exhibit substantial differences from the median bowl-type TNO spectrum. Their 3 µm band is shallower. The water-ice bands at 1.5, 2.0 and 4.5 µm and the 3.10 µm Fresnel peak are weak. The CO₂ fundamental band is also weak (<10%). We classify

<figure><img src="image_4.png" /><figcaption>**Fig. 2** | Principal component analysis. First two principal components of the five Centaurs (rhombi) projected over the three spectral groups of TNOs. Light-blue circles are bowl objects, orange crosses are double-dip objects and red squares are cliff objects in the DiSCo dataset.</figcaption></figure>

the two misfits as being shallow type. Although both Centaurs have similar reflectances in the 3 µm band valley, the 2010 KR59 spectrum increases more at 3.6 µm. We show in Methods that the spectrum of 2010 KR59 is noisier than the other spectra due to a nearby background star (Methods), which especially introduces uncertainty in this region. Nevertheless, the observed difference from 3.3 to 3.6 µm may suggest compositional variations.

## Surface composition and effects of cometary-like activity

We studied the surface composition with a surface-scattering model using the optical constants of several materials expected to be on small, icy bodies (Extended Data Table 2 lists the optical constants used), which allowed us to fit their spectra, as described in 'Fitting reflectance spectra with scattering models'. Reasonably good fits were obtained for bowl and shallow Centaurs (Fig. 3) using H₂O and CO₂ ices, solid complex organics (tholins) and amorphous silicates (pyroxenes and olivines). The models are described in 'Fitting reflectance spectra with scattering models'. The obtained fractions, particle sizes and shifts of the materials used are listed in Extended Data Table 3. As with the cliff TNOs, we still did not find satisfactory models for cliff-type Centaur 2002 KY14.

The derived coarse surface composition of the bowl Centaurs shown in Fig. 3 is ~50% ices (water and CO₂ ice in ~5:3 to 4:1 ratio), ~30% amorphous silicates and ~10–20% complex organics ('Fitting reflectance spectra with scattering models'), in agreement with peak positions of the CO₂ band. These results indicate that CO₂ is in an intimate mixture with H₂O (ref. 8). Shallow-type Centaurs have no CO₂, and they have much more amorphous silicates (~70–80%), less water ice (~10–20%) and less complex organics (~7–8%) than bowls. The loss of CO₂ and the much larger amount of silicates in shallow-type Centaurs support the thermal surface weathering mechanism previously proposed (main text): activity due to sublimation of volatiles and the deposition of solid particles onto the surface to form a dust mantle.

Even if we cannot yet model cliff spectra, the information about the materials on the surface of 2002 KY14 and the comparison with TNO cliff spectra presented in 'Spectral characteristics and classes of Centaurs' suggest that a similar resurfacing mechanism occurred. First, 2002 KY14 has lower levels of CO₂, CO and CH₃OH compared to cliff TNOs. The depletion of these molecules is compatible with sublimation activity. Second, the species responsible for the 4.62 µm absorption feature in their spectra have remained, suggesting that the carrier is non-volatile. The latter point is consistent with the hypothesis that 'the 4.6 µm feature is most probably dominated by the -CN (triple bond) band of complex organics'⁵, and it provides further support
<table><caption>**Table 2 | Measured CO₂ and CO fundamental band parameters of the observed Centaurs**</caption><thead><tr><th rowspan="2">Number</th><th rowspan="2">Name</th><th rowspan="2">Provisional designation</th><th rowspan="2">Group</th><th>CO₂ depth</th><th rowspan="2">CO</th></tr><tr><th>(%)</th></tr></thead><tbody><tr><td>32532</td><td>Thereus</td><td>2001 PT13</td><td>Bowl</td><td>34.2±1.6</td><td>No</td></tr><tr><td>136204</td><td></td><td>2003 WL7</td><td>Bowl</td><td>37.5±5.5</td><td>No</td></tr><tr><td>250112</td><td></td><td>2002 KY14</td><td>Cliff</td><td>35.6±1.1</td><td>Maybe</td></tr><tr><td>52872</td><td>Okyrhoe</td><td>1998 SG35</td><td>Shallow</td><td>8.9±1.5</td><td>No</td></tr><tr><td>310071</td><td></td><td>2010 KR59</td><td>Shallow</td><td>9.6±2.7</td><td>No</td></tr><tr><td colspan="4">Mean for bowl TNOs</td><td>Bowl</td><td>35.31±8.20</td></tr><tr><td colspan="4">Mean for cliff TNOs</td><td>Cliff</td><td>76.76±14.84</td></tr></tbody></table>

CO₂ fundamental band parameter⁸ for the five Centaurs presented in this paper. Band depth is expressed as the drop in reflectance from a local continuum around the CO₂ band. It was extracted using a Gaussian fit. The values and error bars are, respectively, the average and the standard deviation obtained using different wavelength ranges for a linear baseline. Detections of CO based on a visual inspection are also listed. The last two lines are the means of the CO₂ fundamental band parameter for the bowl and cliff TNOs⁸.

for the entire interpretation of the cliff-type spectrum proposed⁵. Third, probably only a part of the 4.9 µm band is OCS, and the feature at 4.93 µm can be attributed to olivine, in agreement with the overall pattern. Finally, the shape of C–H stretching absorptions of aliphatic (chain-like), conjugated and aromatic hydrocarbons in the 3.33 to 3.6 µm range resembles (albeit being somewhat larger) that found in meteorites and cometary dust¹⁷. The hypothesis that 2002 KY14 has a weathered surface due to activity is also supported by the very low value of its geometric albedo $p_V = 0.06 \pm 0.01$, indicative of the presence of a larger abundance of dark materials (silicates or complex organics) than on the surface of the mean TNO cliff objects that have a mean geometric albedo of $p_V = 0.12 \pm 0.05$.

## Discussion

Our spectroscopic analysis of Centaurs and TNOs using JWST data revealed that two of the three compositional groups identified in the TNO population⁵, one with water-rich (bowl-type) objects and the other with carbon-rich (cliff-type) objects, are also present in the Centaur population. However, we observed important disparities in their spectral-class distribution compared to TNOs, which is indicative of variations in the composition of the upper layers of their surface. Notably, their distinct thermal evolution played a crucial role in shaping these disparities. The key findings from our analysis highlight a general trend. As Centaurs traverse the giant planet region, they experience increased surface temperatures that trigger phase transitions, such as the sublimation of volatiles like CO or CO₂, so that cometary activity can be sustained and their surface undergoes chemical transformations. Specifically, the surface of a Centaur seems to transition from an ice-rich state to one that is less icy and more refractory.

Cometary activity at large heliocentric distances beyond 15 AU supports the activation hypothesis. Activity has been observed in C/2014 UN271 (Bernardinelli–Bernstein) around 23–24 AU (refs. 18,19), in C/2010 U3 (Boattini) at ~26 AU (ref. 20) and in C/2017 K2 (PanSTARRS) at 23.7 AU (ref. 21). For C/2014 UN271, emissions from CO and CO₂ gas-phase vibrational bands were detected with JWST/NIRSpec at 18.2 AU (ref. 22).

It is important to address the presence of CO on the surfaces of TNOs and its potential significance in driving activity when they are scattered into the Centaur region. Although CO signatures have, indeed, been observed in TNOs, this CO is probably a result of irradiation processes rather than originating from primordial internal

<figure><img src="image_5.png" /><figcaption>**Fig. 3 | Surface compositional models.** *a, b*, Results of the scattering model fitting of the Centaurs analysed in this paper. *a*, Shallow-type Centaurs. *b*, Bowl Centaurs. Spectra of the labelled Centaurs are represented by grey dots with error bars (computed as the standard deviation of the four dithered exposures of each target). Modelled spectra are represented with grey solid lines. Fittings reasonably reproduce the general shape and most important spectral features observed, thus providing a reasonable and good, rough composition.</figcaption></figure>

sources⁸. Several studies have suggested that CO should have completely sublimated from the upper kilometre-scale layers of most TNOs²³⁻²⁶. This implies that the CO detected on TNO surfaces is superficial and may not be the primary driver of sustained activity within the Centaur region.

There are important differences between Centaurs and TNOs, especially cliff objects. Specifically, 2002 KY14 shows a notable absence of CO and a significantly reduced level of CO₂, suggesting that there was a rapid loss of volatiles when it entered the giant planet region. This suggests that any CO present as a thin surface layer would be rapidly depleted. In contrast, long-period comets, which originate far beyond the trans-Neptunian belt, may retain their initial CO upon first entering the planetary region, which possibly explains their activity at larger distances. The detection of CO in Centaur 2014 OG392 around 10 AU supports this (C. Schambeau, personal communication) and has been interpreted as being consistent with processes like crystallization or segregation rather than direct sublimation.

Scattering models for shallow-type Centaurs indicate a high presence of amorphous silicates, a key component of cometary dust. Remote mid-infrared studies of cometary comae dust commonly find spectral features in the 10 µm region dominated by amorphous silicates, with crystalline peaks contributing variably (20 to 75% crystal mass fractions²⁷), as in laboratory spectra of particles from comets or primitive asteroids. Amorphous silicates are also important components of the surface of Jupiter trojans²⁸. Cometary grains and primitive meteorites also contain opaque phases, like iron sulfides, oxides and carbon-rich materials, which are intimately mixed with silicates at sub-wavelength scales, suggesting that Centaurs share this trait. Hence, the high abundance of amorphous silicate detected in Centaurs suggests a high abundance of primitive, cometary-like dust grains. With surface activity causing Centaurs to become less icy and more refractory, similar spectral characteristics are expected in comets, Jupiter trojans, main-belt comets and D-type asteroids, due to their similarly evolved surfaces. This is exemplified by comparing the spectra of shallow-type Centaurs with those of comet 67P/Churyumov–Gerasimenko (ref. 29) and main-belt comet 238P/Read (ref. 30; Fig. 4). Both comets have
<figure><img src="image_6.png" /></figure>

**Fig. 4 | Comparison of spectra of shallow-type Centaurs and comets.** Spectra of shallow-type Centaurs Okyrhoe and 2010 KR59 spectra compared with those of comet 67P/Churyumov–Gerasimenko from ref. 29 and main-belt comet 238P/Read. The spectrum of 238P is the pure reflectance spectrum obtained after subtracting the observed one presented in Extended Data Fig. 3 of ref. 30. The thermal emission is also presented in ref. 30.

shallow, bowl-shaped 3 µm bands, slightly red infrared slopes and an absence of water-ice bands and the CO₂ fundamental band, akin to shallow-type Centaurs.

Linking the orbital histories of these objects to their spectral surface characteristics was more complex than anticipated (M.M. et al., manuscript in preparation), a challenge highlighted by the difficulty in precisely determining their individual dynamical histories due to the chaotic nature of their orbits. Additionally, the absence of double-dip Centaurs introduces further complexity. A plausible explanation is the rapid transformation of double-dip TNOs into shallow-type Centaurs upon entering the giant planets region. Double-dip TNOs are characterized by their rich CO and CO₂ surface content, which should make them more prone to activation at greater heliocentric distances. However, despite their richness in these volatiles, they contain fewer of the other essential molecules required for the formation of complex organics. This activation process results in the sublimation of CO and CO₂, leaving behind dust and water-ice particles on their surfaces.

The limited number of Centaurs observed within the DiSCo programme prevented us from reaching definitive conclusions regarding the nature of the recognized bimodal distribution of the colours of the Centaur population, categorized as ‘red’ and ‘less red’³¹⁻³³. As with TNOs, the cliff Centaur we studied is red and both bowl Centaurs are less red, which suggests a compositional cause for that bimodality. However, although shallow-type Okyrhoe is less red, like the bowls, 2010 KR59 is a red Centaur, with a colour like those observed in the population of double-dip TNOs. This suggests that 2010 KR59 may be a thermally evolved double-dip object. To validate this theory, further scrutiny of this specific Centaur and further Centaur data are imperative.

Finally, Okyrhoe is on the latest US National Academy of Sciences Planetary Decadal Survey list of Centaurs accessible for space missions. Its status as a Centaur featuring a thermally evolved surface further enhances its appeal as a target for exploration.

## Methods

### Observations, data reduction and reflectance spectra

The near-infrared spectral observations of the Centaurs presented in this work were obtained with JWST using the IFU of NIRSpec as part of the cycle 1 DiSCo-TNOs programme (Programme ID 2418, PI Pinilla-Alonso)⁶,⁷. For all objects, the PRISM/CLEAR grating/filter pair was used to acquire low-resolution spectra (R from 30 to 300) in the range 0.6–5.3 µm. Details of the observations are given in Extended Data Table 1. A detailed description of the data acquisition and reduction procedures can be found in DiSCo TNO paper³. The final reflectance spectra of the five Centaurs are shown in Extended Data Fig. 1. Note that we had to discard several exposures of 2010 KR59 due to a background star being too close to the target, so its final reflectance spectrum was noisier and with a higher uncertainty than the others.

Data were acquired using the NIRSpec IFU, which provided a spatial resolution within a 3" square field of view. The observations were performed using the four-point dither pattern to mitigate the impact of potential pointing errors and to maximize the signal-to-noise ratio. Each target was observed for approximately 1 h using the NRSIRS2RAPID read-out pattern to ensure the high-quality of the spectral data.

Data reduction was performed using the latest JWST calibration pipeline (v1.11.4) with extra custom steps to remove any residual artefacts, such as 1/f noise. The ‘de-striping’ procedure in the pipeline was applied to minimize systematic noise, and a custom fitting method for the point spread function was used to extract the spectra from the IFU data cubes. Solar correction was performed using a reference star observed under similar conditions, which ensured the accuracy of the reflectance spectra. The extracted spectra were then combined across the dither positions, and outliers were rejected to ensure the robustness of the final spectra. Finally, reflectance spectra were normalized at 2.6 µm. A detailed description of the data acquisition and reduction procedures is given in our DiSCo TNO paper⁵.

### Fitting reflectance spectra with scattering models

To obtain information on the surface composition of our sample of Centaurs, we used the model presented in ref. 34 using intimate mixtures, which emulates the scattering of light from particulate surfaces. This model requires laboratory measurements of the optical constants of different materials as an input, along with grain sizes and fractional abundances for each component. A list of the optical constants used in the modelling is provided in Extended Data Table 2. Another parameter, the porosity, had minor effects on the overall shape of the spectra and was assumed fixed at 0.5. Despite their limitations, models like Shkuratov’s are valuable tools for deriving the rough surface composition of icy bodies.

The spectra of mixtures that best fitted the spectra were obtained following the Bayesian framework used for TNOs⁸. To account for the influence of temperature on the band positions of water ice³⁵,³⁶, we incorporated another parameter that allowed for shifts of ±0.03 µm in the wavelength vector of each optical constant. We adopted a uniform prior distribution for this parameter to ensure that we comprehensively explored the possible wavelength shifts. By employing the materials whose fractions, particle sizes and shifts better fitted the spectra, as illustrated in Extended Data Table 3, reasonably good fits were achieved for the bowl-type and shallow-type objects within our sample, as depicted in Fig. 3. However, our attempts to replicate the spectral characteristics of the cliff object 2002 KY14 were unsuccessful. The main difficulty in modelling this object was reproducing the 2.5–4.0 µm region, where the complex organics dominate. Our inability to generate good fits to the whole spectral coverage can be attributed to the limited collection of optical constants available in the literature, especially of complex organics, and to the simplistic configuration of the intimate mixtures adopted here for the modelling. The acquisition of more data for optical constants in the laboratory and trials using more sophisticated models are highly encouraged but out of the scope of this work.

### Data availability

The JWST data used in this analysis are publicly available from the STScI MAST Archive (https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html).

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## Acknowledgements

The DiSCo-TNOs team would like to thank W. Eck and A. Henry of the Space Telescope Science Institute for their help in preparing the observations for execution. This work is based on observations made with the NASA/ESA/CSA JWST under the GO-1 programme 2418. Support for this programme was provided by NASA through a grant from the Space Telescope Science Institute. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. J.L. acknowledges support from the Agencia Estatal de Investigación del Ministerio de Ciencia e Innovación (Grant 'Hydrated Minerals and Organic Compounds in Primitive Asteroids', Ref. PID2020-120464GB-100). N.P. acknowledges funding from Fundação para a Ciência e a Tecnologia (Research Grant Nos. UIDB/04434/2020 and UIDB/04434/2020). R.B. and E.H. acknowledge support from CNES-France (JWST mission). A.G.-L. received funding from the European Research Council under the European Union's Horizon 2020 research and innovation programme (Grant Agreement No. 802699). J.A.S. acknowledges Lowell Observatory and Northern Arizona University, both in Flagstaff, AZ, for support during his sabbatical tenure there.
**Author contributions**

J.L., N.P.-A., V.L., M.N.D.P., B.J.H. and J.A.S. designed the observational programme. N.P.-A., M.N.D.P., R.B., J.L., Y.J.P., D.P.C., B.J.H., T.M., J.A.S. and J.P.E. conceived the science goals of DiSCo. J.L. and N.P.-A. conceived in particular the goals related to Centaurs. B.J.H., N.P.-A., I.W., A.C.d.S.F., M.N.D.P. and C.A.S. reduced and validated the data. J.L., N.P.-A., E.H., R.B., M.N.D.P., A.C.d.S.F., J.A.S. and T.M. performed the band identification and spectral characterization. R.B., M.N.D.P. and N.P.-A. performed the clustering and the study of the band areas. J.L., R.B., N.P.-A. and A.G.-L. elaborated on and proposed the scenario for interpreting the results. J.L. and N.P.-A. drafted the paper. All authors were involved in the discussion of the results and the finalization of the paper.

**Competing interests**

The authors declare no competing interests.

**Additional information**

Extended data is available for this paper at
https://doi.org/10.1038/s41550-024-02417-2.

**Correspondence and requests for materials** should be addressed to
Javier Licandro.

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2024

<footnote n="1">¹Instituto de Astrofísica de Canarias (IAC), La Laguna, Spain. ²Departamento de Astrofísica, Universidad de La Laguna, La Laguna, Spain. ³Florida Space Institute, University of Central Florida, Orlando, FL, USA. ⁴Space Telescope Science Institute, Baltimore, MD, USA. ⁵Instituto de Astronomía y Física del Espacio, UBA-CONICET, Buenos Aires, Argentina. ⁶Facultad de Ciencias Astronómicas y Geofísicas, UNLP, Buenos Aires, Argentina. ⁷Instituto de Tecnología e Ingeniería, UNAHUR, Villa Tesei, Argentina. ⁸Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, Orsay, France. ⁹Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, UMR5276 CNRS, UCBL, ENSL, Villeurbanne, France. ¹⁰Fundación Galileo Galilei-INAF, Tenerife, Spain. ¹¹Northern Arizona University, Flagstaff, AZ, USA. ¹²Lowell Observatory, Flagstaff, AZ, USA. ¹³Department of Physics, University of Central Florida, Orlando, FL, USA. ¹⁴Max-Planck-Institut für extraterrestrische Physik, Garching, Germany. ¹⁵Instituto de Astrofísica e Ciências do Espaço, Departamento de Física, Universidade de Coimbra, Coimbra, Portugal. ¹⁶School of Physical and Chemical Sciences – Te Kura Matū, University of Canterbury, Christchurch, New Zealand. ¹⁷NASA Goddard Space Flight Center, Greenbelt, MD, USA. ¹⁸American University, Washington DC, USA. ✉e-mail: jlicandr@iac.es</footnote>
<figure><img src="image_7.png" /></figure>

**Extended Data Fig. 1** | Reflectance spectra of the five Centaurs presented in this paper. Reflectance spectra of the five Centaurs presented in this paper shifted in the y-axis by 1.0. In red is represented the Centaur with a Cliff-type spectrum defined in<sup>s</sup>, in blue those with a Bowl-type spectrum, and in green the

two ones that belong to a spectral class that is unique between the Centaurs.
Error bars (computed as the standard deviation of the four dithered exposures of
each target) are shown in light-grey.
Extended Data Table 1 | Description of the observations

<table><thead><tr><th>Name</th><th>Date</th><th>r mag</th><th>Groups /Int.</th><th>Int.</th><th>Tot. Int. (4p Dit.)</th><th>Tot.Exp. (sec)</th></tr></thead><tbody><tr><td>2002 KY14</td><td>2022/10/30</td><td>21.4</td><td>24</td><td>1</td><td>4</td><td>1459</td></tr><tr><td>Okyrhoe</td><td>2022/11/03</td><td>21.2</td><td>24</td><td>1</td><td>4</td><td>1459</td></tr><tr><td>2003 WL7</td><td>2022/11/23</td><td>21.2</td><td>24</td><td>1</td><td>4</td><td>1459</td></tr><tr><td>Thereus</td><td>2023/01/17</td><td>19.9</td><td>14</td><td>1</td><td>4</td><td>875</td></tr><tr><td>2010 KR59</td><td>2023/03/02</td><td>20.9</td><td>20</td><td>1</td><td>4</td><td>1226</td></tr></tbody></table>

From left column to the right: name of the object, date of the observation, r-SDSS magnitude at the moment of observation, number of groups per integration, number of integrations, total number of integrations, considering a 4 points dither, and total exposure time in seconds.
<table><caption>Extended Data Table 2 | Optical constants used for the models in this work</caption><thead><tr><th>Optical Constant</th><th>Phase</th><th>Temperature</th><th>Ref</th></tr></thead><tbody><tr><td>Carbon dioxide</td><td>-</td><td>12.5 K</td><td>40</td></tr><tr><td>Water Ice</td><td>amorphous</td><td>40 K</td><td>35,36</td></tr><tr><td>Water Ice</td><td>crystalline</td><td>40 K</td><td>35,36</td></tr><tr><td>Titan tholin</td><td>-</td><td>170 K</td><td>41</td></tr><tr><td>Tritan tholin</td><td>-</td><td>170 K</td><td>41</td></tr><tr><td>Pyroxene (P4)</td><td>-</td><td>-</td><td>42</td></tr><tr><td>Olivine</td><td>-</td><td>-</td><td>42</td></tr></tbody></table>
Extended Data Table 3 | Properties of the materials used in the models

<table><thead><tr><th rowspan="2">Material</th><th colspan="3">2003 WL7</th><th colspan="3">Thereus</th><th colspan="3">2010 KR59</th><th colspan="3">Okyrhoe</th></tr><tr><th>fraction</th><th>size (μm)</th><th>shift (μm)</th><th>fraction</th><th>size (μm)</th><th>shift (μm)</th><th>fraction</th><th>size (μm)</th><th>shift (μm)</th><th>fraction</th><th>size (μm)</th><th>shift (μm)</th></tr></thead><tbody><tr><td>CO2</td><td>0.21</td><td>0.15</td><td>0.0</td><td>0.10</td><td>0.25</td><td>0.0</td><td>0.00</td><td>-</td><td>-</td><td>0.00</td><td>-</td><td>-</td></tr><tr><td>Ice Tholin</td><td>0.14</td><td>1.57</td><td>0.0</td><td>0.21</td><td>1.89</td><td>0.0</td><td>0.07</td><td>1.92</td><td>0.01</td><td>0.08</td><td>9.39</td><td>0.0</td></tr><tr><td>Olivine 0</td><td>0.02</td><td>7.97</td><td>0.0</td><td>0.0</td><td>-</td><td>-</td><td>0.0</td><td>-</td><td>-</td><td>0.02</td><td>394.91</td><td>0.0</td></tr><tr><td>Olivine 1</td><td>0.0</td><td>-</td><td>-</td><td>0.0</td><td>-</td><td>-</td><td>0.36</td><td>18.87</td><td>-0.01</td><td>0.0</td><td>-</td><td>-</td></tr><tr><td>Pyroxene 2</td><td>0.27</td><td>1.73</td><td>0.0</td><td>0.22</td><td>1.39</td><td>0.0</td><td>0.36</td><td>3.39</td><td>0.0</td><td>0.74</td><td>0.34</td><td>0.0</td></tr><tr><td>Pyroxene 6</td><td>0.02</td><td>497.5</td><td>0.0</td><td>0.07</td><td>190.5</td><td>0.0</td><td>0.0</td><td>-</td><td>-</td><td>0.07</td><td>18.2</td><td>0.0</td></tr><tr><td>H<sub>2</sub>O-ice Amo.</td><td>0.1</td><td>8.34</td><td>0.0</td><td>0.14</td><td>5.53</td><td>0.0</td><td>0.20</td><td>2.39</td><td>0.01</td><td>0.02</td><td>5.54</td><td>0.0</td></tr><tr><td>H<sub>2</sub>O-ice Crys.</td><td>0.25</td><td>1.18</td><td>0.0</td><td>0.25</td><td>2.63</td><td>0.0</td><td>0.01</td><td>483.5</td><td>-0.02</td><td>0.07</td><td>2.02</td><td>-0.01</td></tr></tbody></table>

Fraction, particle size and shifts of the different materials used in the models shown in Fig. 3.