# Spectral Diversity of DiSCo's TNOs Revealed by JWST: Early Sculpting and Late Irradiation

R. Brunetto, E. Hénault, S. Cryan, N. Pinilla-Alonso, J P Emery, A.

Guilbert-Lepoutre, B J Holler, L T Mcclure, T G Müller, Y. Pendleton, et al.

► To cite this version:

R. Brunetto, E. Hénault, S. Cryan, N. Pinilla-Alonso, J P Emery, et al., Spectral Diversity of DiSCo's TNOs Revealed by JWST: Early Sculpting and Late Irradiation. The Astrophysical Journal Letters, 2025, 982 (1), pp.L8. 10.3847/2041-8213/adb977. hal-05353624

HAL Id: hal-05353624

https://cnrs.hal.science/hal-05353624v1

Submitted on 7 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
OPEN ACCESS

<figure><img src="image_2.png" /></figure>

CrossMark

# Spectral Diversity of DiSCo's TNOs Revealed by JWST: Early Sculpting and Late Irradiation

R. Brunetto<fnref n="1" />, E. Hénault<fnref n="1" />, S. Cryan<fnref n="1" />, N. Pinilla-Alonso<fnref n="2" />,<fnref n="3" />, J. P. Emery<fnref n="4" />, A. Guilbert-Lepoutre<fnref n="5" />, B. J. Holler<fnref n="6" />, L. T. McClure<fnref n="4" />, T. G. Müller<fnref n="7" />, Y. Pendleton<fnref n="3" />, A. C. de Souza-Feliciano<fnref n="8" />, J. Stansberry<fnref n="6" />, W. Grundy<fnref n="4" />,<fnref n="9" />, N. Peixinho<fnref n="10" />, G. Strazzulla<fnref n="11" />, M. T. Bannister<fnref n="12" />, D. Cruikshank<fnref n="3" />, B. Harvison<fnref n="8" />, J. Licandro<fnref n="13" />,<fnref n="14" />, V. Lorenzi<fnref n="15" />, M. N. de Prá<fnref n="8" />,<fnref n="16" />, and C. Schambeau<fnref n="8" />

Received 2024 November 28; revised 2025 February 21; accepted 2025 February 21; published 2025 March 12

## Abstract

The spectral diversity of trans-Neptunian objects (TNOs) is crucial for understanding the processes that led to the formation and evolution of planetesimals in the outer solar system. Using near-IR (NIR) spectra obtained by the James Webb Space Telescope (JWST) as part of the DiSCo-TNOs large program, we report the detection of well-clustered subgroups of TNOs. A first subgroup has strong NIR features with contributions from H₂O, CO₂, CO, CH₃OH, and other organic molecules. The 2.27 µm band area, commonly attributed to methanol, decreases with increasing eccentricity, which is compatible with a late destruction of CH₃OH by cosmic ion irradiation at the edge of the heliosphere. The absence of the strongest CH₃OH bands in the JWST spectra is compatible with an irradiation-induced surface stratification, with CH₃OH abundance increasing with increasing depth. A second subgroup has much weaker NIR bands, and these cannot be explained by a late irradiation scenario. This group is further divided into two subgroups (cold classical TNOs and objects with low perihelion) that are spectrally very similar except for their CO₂ band area. We propose two possible interpretations. In one scenario, the TNO subgroups sampled a similar molecular inventory in the protoplanetary disk, after which early surface processes, such as primordial sublimation or irradiation from the young Sun, sculpted the two groups before planetary migration occurred. In a second scenario, the subgroups formed in different locations of the disk where molecules were available in different abundances. A combination of both scenarios is also possible.

*Unified Astronomy Thesaurus concepts: Trans-Neptunian objects (1705); Classical Kuiper belt objects (250); Scattered disk objects (1430); Detached objects (376); Kuiper belt (893); Infrared spectroscopy (2285); Ice spectroscopy (2250); James Webb Space Telescope (2291); Carbon dioxide (196); Complex organic molecules (2256); Molecular spectroscopy (2095); Protoplanetary disks (1300)*

## 1. Introduction

The spectral diversity of outer solar system small bodies is a tool to access both their primordial compositions and their evolutionary histories. The variety of their reflectance spectra can arise from differences in the solar nebula and protoplanetary disk materials from which they formed, shaped by gradients in temperature, radiation, and chemical environment (a concept known as the “heritage” scenario, P. Caselli & C. Ceccarelli 2012). At the same time, these spectra also reflect the ensemble of processes that have altered their surfaces since formation, including irradiation by solar and cosmic

particles, collisions, and thermal evolution (“nurture” scenario; R. Brunetto et al. 2006; R. L. Hudson et al. 2008; M. E. Brown et al. 2011; C. M. Dalle Ore et al. 2011). By analyzing spectral features linked to ices and organic refractories, we can potentially disentangle the relative contributions of primordial composition and post-formation processing.

In this context, methanol (CH₃OH) is a key molecule. It is present in different astrophysical environments, from the interstellar medium (ISM) to the solar system, offering unique insights into the physical conditions within these regions and the processes that govern molecular evolution in space. Methanol can be a reactant and/or a product in numerous chemical reactions. In the ISM, it can form through hydrogenation of CO (H. M. Cuppen et al. 2009; G. W. Fuchs et al. 2009) or via a pathway starting with CH₄ and OH (D. Qasim et al. 2018). Methanol can evolve through radiation-driven chemical processing, leading to its destruction and

<figure><img src="image_3.png" /></figure>

Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

<footnote n="1">¹ Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, Orsay, France; rosario.brunetto@universite-paris-saclay.fr</footnote>

<footnote n="2">² Institute of Space Science and Technology of Asturias (ICTEA), Universidad de Oviedo, Asturias, Spain</footnote>

<footnote n="3">³ University of Central Florida, Department of Physics, Orlando, FL 32816, USA</footnote>

<footnote n="4">⁴ Northern Arizona University, Flagstaff, AZ 86011, USA</footnote>

<footnote n="5">⁵ Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, CNRS, UCBL, ENSL, Villeurbanne, France</footnote>

<footnote n="6">⁶ Space Telescope Science Institute, Baltimore, MD 21218, USA</footnote>

<footnote n="7">⁷ Max-Planck-Institut für extraterrestrische Physik, Garching, Germany</footnote>

<footnote n="8">⁸ Florida Space Institute, University of Central Florida, Orlando, FL 32826, USA</footnote>

<footnote n="9">⁹ Lowell Observatory, Flagstaff, AZ 86001, USA</footnote>

<footnote n="10">¹⁰ Instituto de Astrofísica e Ciencias do Espaço, Universidade de Coimbra, Portugal</footnote>

<footnote n="11">¹¹ INAF-Osservatorio Astrofísico di Catania, Catania, Italy</footnote>

<footnote n="12">¹² University of Canterbury, School of Physical and Chemical Sciences - Te Kura Matu, Christchurch, New Zealand</footnote>

<footnote n="13">¹³ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain</footnote>

<footnote n="14">¹⁴ Universidad de La Laguna, Departamento de Astrofísica, La Laguna, Tenerife, Spain</footnote>

<footnote n="15">¹⁵ Fundación Galileo Galilei-INAF, La Palma, Santa Cruz de Tenerife, Spain</footnote>

<footnote n="16">¹⁶ Observatório Nacional do Rio de Janeiro, Rio de Janeiro, Brazil</footnote>
the formation of secondary molecules (H₂O, CO, CO₂, etc.; M. E. Palumbo et al. 1999). Methanol is also a precursor to complex organic molecules (COMs) through processes like UV photolysis and ion irradiation (P. A. Gerakines et al. 1996; K. I. Öberg et al. 2009; S. Maity et al. 2015), bridging simple molecules and COMs essential for prebiotic chemistry (C. Zhang et al. 2024).

Solid methanol has been observed in dense molecular clouds in the ISM, in star-forming regions, and toward young stellar objects (YSOs; R. J. A. Grim et al. 1991; E. Dartois et al. 1999; A. C. A. Boogert et al. 2022). In the solar system, methanol ice has been detected by ground-based observations of the Centaur Pholus (D. P. Cruikshank et al. 1998) and the Plutino 2002 VE95 (F. Merlin et al. 2012), and on Arrokoth, a cold classical Kuiper Belt object observed by New Horizons (W. M. Grundy et al. 2020). Methanol is also present in comet nuclei, where it is released alongside water and other volatiles as they approach the Sun (N. Biver et al. 2019; M. Lippi et al. 2021).

In the ISM and in comets, the methanol/water ratio varies considerably, typically from 0.2% to 10% (C. Knez et al. 2005; M. J. Mumma & S. B. Charnley 2011; A. C. A. Boogert et al. 2015; D. Bockelée-Morvan & N. Biver 2017; M. Lippi et al. 2024), and is highest (≈30%) in massive YSOs (A. C. A. Boogert et al. 2015). Using the James Webb Space Telescope (JWST), M. K. McClure et al. (2023) estimated a ratio of methanol/water of about 4%–10% in molecular cloud ices. The gas-phase methanol/water ratios in hot corinos<fnref n="17" /> and cold envelopes of YSOs are similar to those of comets (P. Caselli & C. Ceccarelli 2012), suggesting continuity in the composition of ices from the ISM through the protoplanetary disk phase to comet formation (“heritage”). In all these environments, water ice is the dominant species.

In contrast, the composition observed on the surfaces of certain outer solar system bodies differs from this heritage model. On Pholus, the methanol/water ratio is about 1 (D. P. Cruikshank et al. 1998), and on Arrokoth, it exceeds 1 (W. M. Grundy et al. 2020). Such high methanol/water ratios suggest that outer solar system bodies have undergone alteration after formation (“nurture”) by secondary processes, such as irradiation, differentiation, or primordial sublimation (M. E. Brown et al. 2011). However, clear evidence pointing to specific post-formation processes is still lacking.

More recently, three main spectral types of trans-Neptunian objects (TNOs) were discovered with JWST as part of the large Cycle 1 program 2418 DiSco-TNOs (“Discovering the Surface Composition of TNOs”; N. Pinilla-Alonso et al. 2025). These new observations have revealed that, while CO₂ is widespread in all observed TNOs and Centaurs (M. N. De Prá et al. 2025), methanol is not. One compositional group of TNOs (nick-named “Cliff”-type) shows several bands indicative of methanol and organics from 2.2 to 2.6 µm, while another group (nicknamed “Double-Dip”-type) shows much weaker bands in the same region but displays prominent features of CO₂ and CO ices. Other compositional groups, named “Bowl”-type and “Shallow”-type (the latter found only in Centaurs; J. Licandro et al. 2025), show water ice bands but no signs of methanol at the current detection limits in the JWST data. A full spectral description of these compositional groups and their nuances can be found in N. Pinilla-Alonso et al. (2025).

In this study, we examine the distribution and properties of Cliff-type and Double-Dip-type TNOs, in order to provide insights into the early and late evolutionary processes affecting TNOs that formed in the outer disk, and contribute to a broader understanding of the formation and alteration of icy bodies in the outer solar system.

## 2. Observations, Experiments, and Methods

The spectral data set and classification system used in this study are presented by N. Pinilla-Alonso et al. (2025), with additional improvements to the data reduction and extraction processes. Briefly, JWST observations used the integral field unit (IFU) and the PRISM mode of the Near-InfraRed Spectrograph (NIRSpec; T. Böker et al. 2022) to observe 59 TNOs and Centaurs. Raw data were processed locally using the JWST calibration pipeline, and the spectra were extracted using a custom point-spread function fitting procedure (A. C. Souza-Feliciano et al. 2024; M. N. De Prá et al. 2025; N. Pinilla-Alonso et al. 2025). Improvements since the publication of N. Pinilla-Alonso et al. (2025) are focused on the standardization of detector pattern noise removal and the reduction of unphysical scatter between dithers. In particular, the data were processed with version 1.17.1 of the pipeline and *jwst_1322.pmap*. Additionally, the raw data were no longer corrected for the 1/f pattern noise via an in-house algorithm. Instead, the 1/f pattern noise was removed via the nondefault NSClean algorithm in the JWST calibration pipeline (B. J. Rauscher 2024). In addition, in the dither combination step, the dithers were first normalized to the median of the four dithers before taking the median in order to reduce the unphysical dispersion between dithers (due to uncorrected variations across the IFU field of view). Following taking the median, the normalization value was multiplied back in to recover the original flux level. We considered relative reflectance spectra for 16 Cliff-TNOs and 22 Double-Dip-TNOs, normalized at 2.6 µm (see Table A1 in Appendix A for parameters). Spectral types were defined using unsupervised clustering methods (k-means, hierarchical clustering, principal component analysis (PCA)) and confirmed by independent analysis of diagnostic spectral features. Full data set details on acquisition, calibration, and classification are in N. Pinilla-Alonso et al. (2025).

We used band areas reported by N. Pinilla-Alonso et al. (2025) for the 2.7 µm band of CO₂ ($\nu_1 + \nu_3$ combination mode; S. A. Sandford & L. J. Allamandola 1990). In N. Pinilla-Alonso et al. (2025), the 2.27, 2.34, and 2.5 µm bands were integrated together and collectively attributed to CH₃OH (combination of the C–H asymmetric stretching and deformation modes, combination of the C–H symmetric stretching and deformation modes, and C–H combination mode, respectively; L. G. Weyer 1985; D. P. Cruikshank et al. 1998), and/or to C–H, N–H, and possibly C≡N bonds in complex organics. Here we integrated the 2.27 µm band and the 2.5 µm bands separately (in the range 2.23–2.29 µm and 2.44–2.56 µm, respectively), in order to investigate relative variations that would be informative of the molecular carriers. In particular, there is a large consensus on using the 2.27 µm band as the best proxy to detect methanol on TNOs and Centaurs (D. P. Cruikshank et al. 1998; M. A. Barucci et al. 2006; F. Merlin et al. 2012; A. C. Souza-Feliciano et al. 2018; S. A. Stern et al. 2019; W. M. Grundy et al. 2020; R. G. Urso et al. 2020).

We considered a large 3.4 µm band (integration range 3.36–3.56 µm), which can include the different C–H stretching modes of methanol, notably the 3.53 symmetric stretch (M. E. Palumbo et al. 1999), as well as aliphatic C–H stretching modes in more complex organics, such as the ones

<footnote n="17">¹⁷ A warm (>100 K) and compact (<100 au) molecular clump found in the inner envelope of a Class 0 protostar (P. Caselli & C. Ceccarelli 2012).</footnote>
reported by M. C. De Sanctis et al. (2017) for Ceres, H. H. Kaplan et al. (2021) for Bennu, or L. P. Keller et al. (2006) from cometary samples returned by Stardust.

In addition, we calculated new band areas (see Figure 1): the large 3 µm band, which can include the methanol O–H stretching mode at 3.1 µm (M. E. Palumbo et al. 1999), contributions from O–H in other organics and water ice, and N–H in N-bearing species; and the 2 µm band, which can have contributions from O–H combination in water and/or methanol ices and also from different combination modes in complex organics (N. Pinilla-Alonso et al. 2025). We also estimated the 2 µm band position by extracting the reflectance minimum between 1.8 and 2.2 µm in the smoothed spectra after a baseline correction (error bars were calculated as standard deviations of values obtained using five different smoothing functions from 11 to 27 points).

For each band, areas were obtained by averaging calcula-
tions using two or three baselines (linear, second-order, and
third-order polynomial functions). For some TNOs, local
continuum shapes occasionally yielded negative areas, which
we interpret as nondetections. Error bars reflect the standard
error from these baseline calculations.

We compare the TNO spectra with some laboratory
measurements of methanol ice performed at IAS-Orsay in
France using the INGMAR setup (R. G. Urso et al. 2020) and
at INAF-Catania in Italy (R. Brunetto et al. 2005). Measure-
ments in Orsay and Catania were performed before and after
irradiation with 30 and 200 keV H⁺, respectively, to simulate
the irradiation of TNO surfaces in space (see R. Brunetto et al.
2006; R. Brunetto & T. L. Roush 2008; E. Hénault et al. 2025,
for more details of the experimental procedures).

Finally, some synthetic spectra of icy surfaces were
produced using the Hapke scattering model, with assumptions
similar to those reported by D. P. Cruikshank et al. (1998).

3. Results

The collection of Cliff-TNO spectra is shown in Figure 1(a).
Five Cliff-TNOs (ID13 2002 TC<sub>302</sub>, ID15 2010 VZ<sub>98</sub>, ID19 2004
PF<sub>115</sub>, ID29 2007 JF<sub>43</sub>, and ID39 1995 TL<sub>8</sub>) show clear bands at
2.27, 2.34, and 2.5 µm. We define them as Cliff1. The remaining
Cliff-TNOs, defined as Cliff2, show much weaker signatures in
these regions. Cliff2-TNOs include two dynamical subgroups: six
cold classical TNOs (Cliff2-ColdCl; with eccentricity <0.1), and
four TNOs (Cliff2-Eccentric, *e* > 0.2), which are spectrally very
similar to Cliff2-ColdCl, except for the slightly stronger 2.7 µm
CO<sub>2</sub> bands in the Cliff2-ColdCl (Figure 2(a)).

Cliff1-TNOs also show a stronger contribution of CO₂
(at 2.7 and 4.25 µm), ¹³CO₂ (4.38 µm), H₂O (1.5, 2.0, 3.0 µm,
and a large absorption from 3.7 to 4.7 µm), and CO (4.68 µm)
features than Cliff2. These relative absorption depths are
illustrated by the ratio of the average Cliff1 to the average
Cliff2 spectrum (Figure 1(b)).

The IR band areas vary considerably among the considered TNOs (Figure 2). Cliff1 can be clearly distinguished from both Cliff2 and the Double-Dip-TNOs in the plots involving the 2, 2.27, 2.5, 2.7, and 3 µm band areas (Figures 2(a), (b), (d), (e), (f)), with most Cliff1 having 1.5× to 5× stronger bands than Cliff2. We use these plots as a defining tool for the Cliff1-TNOs and Cliff2-TNOs. Note that in Figure 2(e) there is no overlap between Cliff1, Cliff2, and Double-Dip groups, except for ID20 2004 PG115 and ID3 Huya (see below), and that there is no correlation between the 2.27 and 2.7 µm band areas, except for a strong negative correlation within Cliff1-TNOs (Appendix B). We also note that the 2 µm

band is blueshifted by about 40 nm with respect to most Bowl-
TNOs and to pure crystalline water ice (Figure 2(f)).

A marked linear relationship between the 2.27 and 2.5 µm bands (Figure 2(d) and Appendix B) suggests that these bands originate either from the same molecule or from carriers that are very closely related. On the contrary, the aliphatic C–H bands at 3.36–3.56 µm only moderately correlate with the 2.27 µm band, and they do not separate Cliff1 from Cliff2 as significantly as the other bands do (Figure 2(c) and Appendix B).

The Cliff-TNO 2004 PG<sub>115</sub> (ID20) has 2, 2.27, and 2.5 µm band areas that identify it as a Cliff1-TNO, yet its CO<sub>2</sub> and 3 µm bands behave like those of Double-Dip-TNOs. Conversely, the Double-Dip-TNO Huya (ID3) behaves as a Cliff2-Eccentric in terms of CO<sub>2</sub> band areas.

A significant difference between Cliff1 and Cliff2 objects is evident in both the overall spectral shape and their “continua,” as confirmed by PCA (Figure 3(a)) and distinct near-IR (NIR) spectral slopes (Figure 3(b), SIR1 and SIR2 in the 0.7–1.2 µm and 1.15–2.42 µm ranges, respectively; N. Pinilla-Alonso et al. 2025). Both PCA and spectral slopes distinguish Cliff1 from Cliff2, with Cliff1 showing an especially redder profile in SIR2, where Cliff2-TNOs generally exhibit a negative slope (Figure 1(a)).

**4. Discussion**

4.1. *Spectral Diversity*

Our spectral analysis uncovers two subgroups within the Cliff-
TNO population. Cliff1-TNOs and Cliff2-TNOs, the latter
composed of Cliff2-ColdCl and Cliff2-Eccentric, are spectrally
distinct from one another while remaining unique from Double-
Dip-TNOs. The strong 2.7 µm CO₂ band area in Cliff1-TNOs
(Figures 2(a) and (e)), as well as the deep bands due to the
4.25 µm feature of CO₂, ¹³CO₂ (4.38 µm), and CO (4.68 µm;
Figure 1), suggests an elevated abundance of ices on Cliff1-TNOs
compared to Cliff2-TNOs. The larger 2, 2.27, 2.5, and 3 µm band
areas in Cliff1-TNOs (Figures 2(b) and (d)) also point toward a
surface possibly richer in CH₃OH and H₂O ices, though these
same bands can also be influenced by other organics. The weaker
ice signatures observed in Cliff2 may be due to an elevated
contribution of other components, such as silicate dust or tholin-
like organics, or a unique spatial assembly of ices and dust that
diminishes the detectability of icy features.

A Kolmogorov–Smirnov (K-S) test supports the separation
between Cliff1 and Cliff2 (Appendix B). The band areas at 2.27,
2.5, 2.7, and 3 µm show that Cliff1 and Cliff2 are sampled from
different distributions with the highest significance (*p* < 0.001),
suggesting that CH₃OH, CO₂, H₂O, and possibly other organics
have very different abundances in the two subgroups. The band
areas due to aliphatic CH and CO do not distinguish Cliff1 and
Cliff2 as significantly, likely because C–H stretching is a property
of several organic molecules nonunique to a specific TNO
subgroup and CO has a secondary origin (E. Hénault et al. 2025).

The spectral slopes also vary among the compositional groups of TNOs (Figure 3(b)), with Cliff1 and Cliff2 belonging to statistically distinct (*p* < 0.001) distributions based on the SIR2 K-S test (Appendix B). This is further evidence that Cliff1 and Cliff2 have different compositions. If Cliff1-TNOs are indeed richer in ices than Cliff2-TNOs, then the latter must contain other materials with different coloring agents (e.g., carbons, silicates, minerals, or organic residues). While neither SIR1 nor SIR2 shows a strong correlation with the spectral bands investigated in this study (Figure 3 and Appendix B), the wavelength region above
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) The collection of DiSCo Cliff-type spectra, compared to Huya, Double-Dip Varda, and Bowl 2012 DR30, normalized at 2.6 µm and offset by 0.5 for clarity. In all figures, the TNOs are labeled by their ObsID (Table A1). The spectrum of ID49 2001 QY297 was impacted by a NIRSpec electrical short and is unexploitable beyond 2.7 µm, but the NIR spectral slopes classify it as a Cliff2-TNO (Figure 3(b)). (b) Three example Cliff1, Cliff2-Eccentric, and Cliff2-ColdCl objects compared to the spectrum of a tholin rich in C–H and N–H groups (D. Nna-Mvondo et al. 2013), a Hapke model of a mixture of methanol and amorphous carbon, a laboratory transmission spectrum of pure methanol, a mixture of H₂O:CH₃OH:CO₂ = 0.9:1.4:1 at 67K reported by P. Ehrenfreund et al. (1999), a lab spectrum of ion-irradiated methanol (dose of 50 ± 10 eV/CH₃OH molecule), and a model of a stratified surface with a decreasing fraction of irradiated methanol from the surface down to 1.5 mm; see Section 4. We saturated the optical depth of the lab measurements to highlight the different molecular absorptions. The ratio of the average Cliff1/Cliff2 is also shown (excluding 2004 PG115), offset by −0.2. All other spectra are offset by 0.5 for clarity. (c) Expansion of the short wavelength range. Vertical lines in panels (b) and (c) guide the eye at some features of interest.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** The areas of (a) the 2.7 µm band (CO₂), (b) the large 3 µm band (O–H, N–H), (c) the large 3.4 µm band (C–H), and (d) the 2.5 µm band for Double-Dip and Cliff-TNOs, as a function of the 2.27 µm band area (CH₃OH and other organics). (e) The 3 µm band area as a function of the 2.7 µm band area. (f) The 2 µm band area against its peak position, with vertical gray areas showing the peak position of pure amorphous and crystalline water ice (from R. M. Mastrapa et al. 2008) and water produced after irradiation of methanol (from R. Brunetto et al. 2006 and this work). TNOs are labeled by their ObsID (Table A1). TNO ID49 2001 QY₂₉₇ is not included because its spectrum was too noisy to perform reliable calculations of band areas. ID3 Huya and ID20 2004 PG₁₁₅ are represented by gray data points. Note that all panels except panel (c) can significantly discriminate between Cliff1 and Cliff2. Panels (a), (d), and (e) can also marginally discriminate Cliff2-Eccentric from Cliff2-ColdCl (see also Figure 4). Panel (f) also includes the Bowl-TNOs to show that the position of Cliff1 is blueshifted.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3.** (a) PCA performed in the 0.78–4.45 µm range (the original spectra covered 0.6–5.3 µm; here we focus on the range of higher signal-to-noise ratio) on the complete DiSCo data set, including Bowl-types. (b) The SIR2 spectral slope (1.15–2.42 µm range) vs. the SIR1 spectral slope (0.7–1.2 µm range), including Bowl-types. This panel also includes ID49 2001 QY<sub>297</sub>, as its slopes indicate it as a Cliff2-TNO, like the other cold classicals. (c, d) SIR1 slope vs. the 2.27 µm band (CH<sub>3</sub>OH and other organics) and the 3 µm band (O–H, N–H). (e, f) Same as panels (c) and (d), but for SIR2. TNOs are labeled by their ObsID (Table A1). Slopes are measured in %/1000 Å. Note that these plots cannot discriminate between Cliff2-Eccentric and Cliff2-ColdCl.</figcaption></figure>
4.4 µm shows tentative evidence of N-bearing species, which may help explain the difference in spectral slopes. These spectral features will be explored in a forthcoming study (Cryan et al. 2025, in preparation). Further, dedicated spectral modeling efforts will be necessary to estimate relative abundances of the different components and thus complete the picture.

## 4.2. The Case of Methanol

Double-Dip-TNO and Cliff-TNO spectra show several bands compatible with a methanol contribution. In particular, methanol can contribute to the bands at 2.27 and 2.34 µm (combinations of the CH stretching and deformation modes), 2.5 µm (CH combination mode), and the broad 3 µm absorption based on its fundamental 3.1 µm O-H stretch. Cliff1-TNOs show especially high band areas at these wavelengths. However, other strong methanol bands seen in laboratory spectra, like the 3.38, 3.53, 3.9, and 4.48 µm features (Figure 1(b)), are weak in Cliff1-TNOs. The lack of strong 3.53 and 3.9 µm bands, which have high values of the imaginary refractive index ($k \approx 0.045$ and $\approx 0.01$, respectively, vs. $k \approx 2E-3$ at 2.27 µm and $k \approx 1E-3$ at 2.5 µm; R.H. Brown, personal communication), is especially intriguing. This could be explained by a surface stratification where the methanol abundance increases with depth, as strong k-value bands probing the surface are subdued, while weaker k-value bands, probing deeper, are more pronounced.

In addition, the short-wavelength region (Figure 1(c)) shows that pure methanol does not match a typical Cliff1 spectrum. The Cliff1 2.34–2.27 µm relative band depth is 2–3 times larger than for pure methanol, suggesting that some additional species are contributing in this region, and/or that methanol is mixed with other species that modify its 2.34–2.27 µm relative band strength.<fnref n="18" />

These evidences are compatible with a scenario of surface irradiation causing (1) selective destruction of surface CH₃OH, producing stratification, and (2) the production of secondary CO₂, H₂O, and CO, which are by-products of ion-irradiated CH₃OH (M. E. Palumbo et al. 1999; E. Hénault et al. 2025). This scenario aligns with calculations of the dose released by solar and cosmic ion irradiation on TNOs (E. Quirico et al. 2023). Additional stratification may result from diffusion and segregation of various molecular by-products with different thermodynamic properties (A. Parhi & D. Prialnik 2023).

To illustrate the potential effect of an irradiation-induced surface stratification, we use a simple spectral model with spectra of unirradiated and proton-irradiated methanol as end-members. According to the Beer–Lambert law, photons inside a material are significantly attenuated at depth $\delta \approx \lambda/(4\pi k)$, where $\lambda$ is the wavelength and $k$ is the imaginary part of the refractive index. This expression indicates that methanol bands probe depths from 1 µm to 1 mm matching typical solar and cosmic ion penetration depths on TNOs (E. Quirico et al. 2023). We produced a “stratified” model (Figure 1(b)) assuming a linear decrease of irradiation with depth from 1 µm to 1.5 mm. First, we calculated an effective absorption coefficient, $\alpha_{\text{eff}}$, by averaging coefficients of unirradiated, CH₃OH<sub>unirr</sub>, and proton-irradiated methanol, CH₃OH<sub>irr</sub>, end-members. From this absorption coefficient we estimated an effective photon penetration depth as $\delta_{\text{eff}} = 1/\alpha_{\text{eff}}$. Finally, at

each wavelength we calculated the stratified spectrum as CH₃OH<sub>unirr</sub> + (1 - $\delta_{\text{eff}}$/1500)*(CH₃OH<sub>irr</sub> - CH₃OH<sub>unirr</sub>), placing a threshold for $\delta_{\text{eff}}$ at 1.5 mm, which implies that the composition below 1.5 mm is unirradiated methanol. While this simple model can mimic many features of a Cliff-spectrum (the presence of H₂O, CO₂, and CO by-products; subdued 3.38, 3.53, 3.9, and 4.48 µm features; a 2.34–2.27 µm relative band depth that is closer to the observed one; etc.), it is still far from reproducing the Cliff-spectra entirely. In particular, water produced from the irradiation of methanol (R. Brunetto et al. 2006) cannot fully explain the shape and position of the 2 µm band observed in Cliff1-TNOs (Figures 1(c) and (f)). An additional contribution from water or other OH-, NH-, or CH-bearing species seems necessary to reproduce the 1.5 and 2.0 µm bands.<fnref n="19" /> Cliff2-TNOs appear to have limited surface methanol, which may indicate either greater irradiation than in Cliff1 or an initially lower methanol content. In summary, pure methanol is not observed on any TNO, but irradiated methanol and its by-products probably play a major role in establishing Cliff-spectra. Future spectral models could quantify the layering and relative abundances of CH₃OH and its by-products.

## 4.3. Trends of Late Irradiation and Evolution

Independent support for the irradiation scenario comes from the relationship between spectral and orbital parameters. For Cliff1-TNOs, the 2.27 µm band areas decrease, while CO₂ band areas increase, with increasing eccentricity (Figure 4), semimajor axis, and aphelion (Figure A1), consistent with a late irradiation scenario. Cliff1-TNOs with higher eccentricity spend long periods far from the Sun, exposed to high ion fluxes (J. F. Cooper et al. 2003). Specifically, TNOs ID13 2002 TC₃₀₂, ID20 2004 PG₁₁₅, and ID15 2010 VZ₉₈ have aphelions of 73, 145, and 287 au, respectively. In the past billion years, they probably spent most of their time near or beyond the termination shock (about 80–90 au), accumulating subsurface irradiation doses 5–10 × higher (G. Strazzulla et al. 2003) than cold classical TNOs. Laboratory experiments (M. H. Moore et al. 1996; M. E. Palumbo et al. 1999) showed that these dose differences can partially destroy CH₃OH and accumulate secondary CO₂, as observed in Cliff1-TNOs. Of course, this does not imply that all observed CO₂ is secondary. A supporting trend of secondary CO being produced on Cliff-TNOs by the irradiation of CH₃OH is discussed by E. Hénault et al. (2025). Similarly, an irradiation trend with increasing heliocentric distance can explain the relative variations of CH₄ and its radiolysis by-products on Quaoar, Gonggong, and Sedna (J. P. Emery et al. 2024). In contrast, Cliff2-TNOs show no clear irradiation-related trends.

The band areas of Cliff2-TNOs are difficult to reconcile with the Cliff1 irradiation trend in their current dynamical state. Cliff2-ColdCl reside in a region of the solar system where irradiation doses are much lower than at high semimajor axes (J. F. Cooper et al. 2003), so if they started with the same original composition as Cliff1, current irradiation conditions cannot explain their much weaker ice bands.

A curious feature of Figure 4 is that the 2.27 µm/2.7 µm band area ratio seems to be the only parameter (along with eccentricity) capable of discriminating between Cliff2-

<footnote n="18">¹⁸ Differences between the prism and lab spectra can only be partially attributed to the low spectral resolution and sampling of the JWST spectra in this range.</footnote>

<footnote n="19">¹⁹ A contribution from NH₃ ice or NH₃ hydrates at 2 µm is also possible; however, they also have bands at 2.23 and 2.21 µm, respectively (M. H. Moore et al. 2007), which are not detected on Cliff1-TNOs and in any case not compatible with the bands at 2.27 and 2.5 µm.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) The 2.27 µm band area (CH₃OH and other organics) and (b) 2.7 µm band area (CO₂) as a function of eccentricity. (c) The 2.27 µm/2.7 µm band area ratio as a function of eccentricity. (d) The CO₂ band area as a function of perihelion distance. A vertical dashed line is drawn at 31.3 au to guide the eye. Panels (a), (b), and (c) include an asymptotic exponential fit for Cliff1-TNOs to guide the eye.</figcaption></figure>

Eccentric and Cliff2-ColdCl, mainly due to weaker CO₂ bands in Cliff2-Eccentric. Cliff2-Eccentric all have low perihelion. For perihelion less than 31 au (and increasing temperature; Table A1), there is a lack of objects rich in CO₂, both in Cliff-TNOs and Double-Dip-TNOs (Figure 4(d)), potentially linked to CO₂ loss (see Appendix C). CO₂ loss can also explain the low CO₂ band area of Huya (ID3), the only Double-Dip-TNO with perihelion below 31 au,<fnref n="20" /> and the paucity of Double-Dip spectra observed in Neptune Trojans (L. Markwardt et al. 2023) and in Centaurs (J. Licandro et al. 2025), where the only Double-Dip Centaur observed so far is active and losing CO₂ (N. Pinilla-Alonso et al. 2024). Thus, a late evolution process may explain the minor difference between Cliff2-Eccentric and Cliff2-ColdCl. This issue will have to be addressed in further studies considering the thermal and orbital evolutions of TNOs and a larger sample of objects with high perihelion temperatures.

### 4.4. Early Sculpting

If late evolution cannot easily explain the differences between Cliff1-TNOs and Cliff2-TNOs, then the differences are probably caused by processes that took place before TNO orbits (other than the cold classicals) were perturbed by the giant planet instability, or even before their accretion.

It is generally considered that methanol forms via hydrogenation reactions of CO molecules at the surface of icy grains (G. W. Fuchs et al. 2009). Astrochemical simulations of disks around young stars show that CO can be converted into other species only in the cold parts of the disk below 30 K, and with relatively high cosmic-ray ionization rates (A. D. Bosman et al. 2018). Those simulations also indicate that the volatile carbon composition of the icy planetesimals forming in the midplane can be dominated by CH₃OH, CO₂, and/or hydrocarbons. Thus, the existence of a primordial CO “snowline” in the early solar nebula (C. Qi et al. 2013) probably played a key role in establishing the early conditions that later produced the Cliff-type TNOs.

A crucial observation in our data is that all six of the cold classicals observed in DiSCo belong to the Cliff2 category (Figure 5). If the cold classicals were composed of a similar proportion of spectral groups to the entire data set (11.3% Cliff1, 20.7% Cliff2, 43.4% Double-dip, 24.5% Bowl), the probability of

<footnote n="20">²⁰ Huya was classified as Double-Dip based on the clustering (Figure 3(a)) and on the slopes (Figure 3(b)); however, all the other spectral and orbital parameters would tend to classify it as a Cliff2-Eccentric. Huya is probably a transition object between Cliff2-TNOs and Double-Dip-TNOs, as 2004 PG 115 is a transition object between Cliff1-TNOs and Double-Dip-TNOs. The causes of that will have to be investigated in a dedicated study.</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Histogram of the DiSCo spectral groups split by dynamical classes. Scat: scattering disk objects. Resonant: see Table A1 for the resonances. HC: hot classical TNOs. CC: cold classical TNOs.</figcaption></figure>

observing only Cliff2 out of six cold classicals would be
$0.207^6 = 0.008\%$. This presents a very strong indication that (1)
cold classicals, even beyond our data set, do not include TNOs
from all the DiSCo groups in the same proportion as the general
TNO population, and (2) there is a significant association between
cold classicals and Cliff2.<fnref n="21" /> Cold classical TNOs formed in the
outer part of the protoplanetary disk (H. F. Levison &
A. Morbidelli 2003; K. Batygin et al. 2011; D. Nesvorný
2015; D. Nesvorný et al. 2020). Thus, we can reasonably
hypothesize that Cliff2-TNOs are the ones that formed farthest
from the Sun. We consider two possible scenarios.

In the first scenario (post-accretional sculpting), Cliff1-TNOs and
Cliff2-TNOs formed very close to each other in the protoplanetary
disk, or, in any case, close enough so that they accreted very similar
molecular ingredients. However, their surfaces were sculpted by
early processes before planet migrations. Cliff1-TNOs are on
average somewhat larger than Cliff2-TNOs (Table A1), but neither
diameter nor albedo discriminates between the two groups
(Table B1 in Appendix B); thus, the process that shaped their
spectra is probably relatively independent of the size (at least in this
size range). One possibility is primordial evaporation, when the
disk is cleared out and the young TNO surfaces undergo an era of
sublimation, setting “ice retention lines” in the disk of planetesimals
(E. L. Schaller & M. E. Brown 2007; M. E. Brown et al. 2011;
A. Parhi & D. Prialnik 2023). N. Pinilla-Alonso et al. (2025)
proposed that Bowl-TNOs, Double-Dip-TNOs, and Cliff-TNOs
formed beyond the retention lines of H₂O, CO₂, and CH₃OH,
respectively. Following this scenario, Cliff1 and Cliff2 must have
formed beyond the surface retention line of different species. In
addition, if the early sublimation of volatiles, such as CO, was more
efficient on Cliff1, it might have brought more refractory icy grains
to their surface. This scenario suggests that the differences between
Cliff1 and Cliff2 may be primarily surface level rather than
indicative of distinct bulk compositions.

In the second scenario, Cliff1-TNOs and Cliff2-TNOs formed in
different locations from different molecular ingredients. In

particular, Cliff1-TNOs must have accreted more ice, in particular
methanol. This scenario implies that the disk was rather
inhomogeneous in its outer parts, with some regions richer in
ices (Cliff1) than others (Cliff2). Conditions for a local
accumulation of CH3OH ice on pebbles located beyond 40 au
are possible in the protoplanetary disk (S. Krijt et al. 2020), a
scenario that deserves further investigation. A combination of both
scenarios is also possible.

5. Conclusions

Our findings provide compelling evidence of the existence of
a group of TNOs (Cliff1) whose spectra are rich in bands from
several species (CH3OH, H2O, CO2, CO, organics). These
objects are distinct from the ones that formed and always
remained in the outermost part of the disk (Cliff2). While some
effects of late evolution (e.g., surface irradiation, loss of
surficial CO2) are probably seen in the spectra ("nurture"), the
identification of several TNO subgroups offers new evidence of
early heterogeneities among planetesimals, providing a window
into the conditions of the outer solar nebula and, by extension,
other protoplanetary disks. The spectral signatures suggest that
some of these planetesimals were formed at or beyond the
CH3OH ice line in the protoplanetary disk, offering an
unprecedented opportunity to probe primordial disk chemistry
and icy grain formation ("heritage"). The IR spectral slopes can
distinguish the compositional groups of TNOs observed
by DiSCo, potentially unlocking the relationship between
TNO colors and compositional gradients in the protoplanetary
disk (M. A. Barucci et al. 2005; D. Nesvorný et al. 2020;
M. Marsset et al. 2023).

More generally, JWST spectra of TNOs provide a valuable
framework for interpreting other ongoing JWST observations
(Neptune Trojans, Centaurs, asteroids, protoplanetary disks,
etc.) and future missions targeting icy bodies, as well as
precious inputs for modeling the chemical and physical
conditions of protoplanetary disks and the early dynamical
evolution of our system.

Acknowledgments

We thank an anonymous reviewer for useful comments and
feedback. This work is based on observations made with the
NASA/ESA/CSA James Webb Space Telescope. The JWST
data presented in this Letter were obtained from the Mikulski
Archive for Space Telescopes (MAST) 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. These observations are
associated with program 2418. The specific observations
analyzed can be accessed via doi: 10.17909/r2zp-r280.
Support for program 2418 was provided by NASA through a
grant from the Space Telescope Science Institute. E.H. and R.
B. acknowledge support from the CNES-France (JWST
mission). N.P.-A. acknowledges funding through the ATRAE
program of the Ministry of Science, Innovation, and Uni-
versities (MICIU) and the State Agency for Research (AEI) in
Spain. N.P. acknowledges funding by Fundação para a Ciência
e a Tecnologia (FCT) through the research grants UIDB/
04434/2020 and UIDP/04434/2020.

Facility: JWST (NIRSpec).

Software: Python.

<footnote n="21">²¹ A detailed comparison of cold classical Arroloth with the Cliff subgroups will be performed in a dedicated study (Grundy et al. 2025, in preparation).</footnote>
Appendix A
Supplementary Material

Appendix A contains the physical and spectral parameters that we considered for the TNOs investigated in this study (Table A1) as well as supplementary evidence for the late irradiation scenario presented in the main text (Figure A1).

<table><caption>Table A1
Relevant Physical and Spectral Parameters for Cliff1-TNOs, Cliff2-TNOs, and Double-Dip-TNOs</caption><thead><tr><th>Number</th><th>Prov. Des.</th><th>Name</th><th>ID</th><th>Class</th><th>a<br>(au)</th><th>q<br>(au)</th><th>Q<br>(au)</th><th>e</th><th>i</th><th>D<br>(km)</th><th>pV</th><th>T<sub>a</sub><br>(K)</th><th>H<sub>2</sub>O 2.0 µm<br>(1E-3 µm)</th><th>M-OH 2.27 µm<br>(1E-3 µm)</th><th>M-OH 2.5µm<br>(1E-3 µm)</th><th>CO<sub>2</sub> 2.7µm<br>(1E-3 µm)</th><th>O-H 3µm<br>(1E-3 µm)</th><th>C-H 3.4µm<br>(1E-3 µm)</th><th>SIR1<br>(%/1000 Å)</th><th>SIR2<br>(%/1000 Å)</th></tr></thead><tbody><tr><td colspan="15">Cliff1</td></tr><tr><td>48639</td><td>1995 TL8</td><td></td><td>39</td><td>HC</td><td>52.98</td><td>40.19</td><td>65.83</td><td>0.24</td><td>0.2</td><td>244</td><td>0.23</td><td>42.6</td><td>7.45 ± 0.42</td><td>1.15 ± 0.08</td><td>1.77 ± 0.34</td><td>2.23 ± 0.30</td><td>470.4 ± 7.2</td><td>24.17 ± 6.21</td><td>8.37 ± 0.22</td><td>1.11 ± 0.14</td></tr><tr><td>84522</td><td>2002 TC302</td><td></td><td>13</td><td>5:2 Res</td><td>55.63</td><td>39.20</td><td>72.06</td><td>0.30</td><td>35.0</td><td>584.1</td><td>0.12</td><td>43.9</td><td>7.12 ± 0.58</td><td>0.74 ± 0.02</td><td>1.39 ± 0.15</td><td>1.94 ± 0.30</td><td>394.3 ± 18.9</td><td>28.11 ± 4.80</td><td>7.2 ± 0.18</td><td>0.65 ± 0.12</td></tr><tr><td>175113</td><td>2004 PF115</td><td></td><td>19</td><td>HC</td><td>39.25</td><td>36.80</td><td>41.60</td><td>0.06</td><td>13.4</td><td>468.2</td><td>0.12</td><td>45.2</td><td>8.58 ± 0.63</td><td>1.65 ± 0.12</td><td>2.69 ± 0.22</td><td>1.10 ± 0.28</td><td>479.7 ± 31.8</td><td>31.80 ± 5.79</td><td>8.62 ± 0.15</td><td>1.01 ± 0.09</td></tr><tr><td>307982</td><td>2004 PG115</td><td></td><td>20</td><td>Scat</td><td>92.11</td><td>36.48</td><td>147.75</td><td>0.60</td><td>16.3</td><td>334</td><td>0.1</td><td>45.6</td><td>6.93 ± 0.68</td><td>0.68 ± 0.06</td><td>0.70 ± 0.16</td><td>5.85 ± 0.41</td><td>260.7 ± 20.2</td><td>19.83 ± 3.47</td><td>7.5 ± 0.16</td><td>1.09 ± 0.09</td></tr><tr><td>444745</td><td>2007 JF43</td><td></td><td>29</td><td>3:2 Res</td><td>39.34</td><td>32.01</td><td>46.65</td><td>0.19</td><td>15.1</td><td>388</td><td>0.08</td><td>48.8</td><td>7.82 ± 0.48</td><td>1.11 ± 0.07</td><td>1.73 ± 0.25</td><td>2.27 ± 0.33</td><td>422.7 ± 9.3</td><td>29.31 ± 6.98</td><td>8.82 ± 0.16</td><td>0.74 ± 0.09</td></tr><tr><td>445473</td><td>2010 VZ98</td><td></td><td>15</td><td>Scat</td><td>158.30</td><td>34.37</td><td>282.23</td><td>0.78</td><td>4.5</td><td>367</td><td>0.12</td><td>46.8</td><td>6.89 ± 0.26</td><td>0.59 ± 0.06</td><td>1.35 ± 0.19</td><td>2.59 ± 0.38</td><td>407.5 ± 31.6</td><td>20.03 ± 3.05</td><td>8.99 ± 0.14</td><td>1.57 ± 0.09</td></tr><tr><td colspan="15">Cliff2</td></tr><tr><td>79360</td><td>1997 CS29</td><td>Sila-N.</td><td>43</td><td>CC</td><td>43.67</td><td>43.41</td><td>43.94</td><td>0.01</td><td>2.3</td><td>343</td><td>0.09</td><td>41.9</td><td>2.84 ± 0.27</td><td>0.45 ± 0.03</td><td>0.71 ± 0.14</td><td>0.89 ± 0.16</td><td>263.7 ± 12.4</td><td>14.38 ± 1.96</td><td>6.22 ± 0.16</td><td>-0.5 ± 0.09</td></tr><tr><td>44594</td><td>1999 OX3</td><td></td><td>104</td><td>Scat</td><td>32.73</td><td>17.60</td><td>47.77</td><td>0.46</td><td>2.6</td><td>135</td><td>0.08</td><td>65.8</td><td>2.05 ± 0.06</td><td>0.47 ± 0.03</td><td>1.11 ± 0.10</td><td>-0.01 ± 0.12</td><td>269.1 ± 14.1</td><td>16.64 ± 2.54</td><td>7.12 ± 0.17</td><td>-0.5 ± 0.1</td></tr><tr><td>66652</td><td>1999 RZ253</td><td>Borasisi</td><td>56</td><td>CC</td><td>44.12</td><td>40.19</td><td>48.06</td><td>0.09</td><td>0.6</td><td>163</td><td>0.24</td><td>42.5</td><td>2.09 ± 0.38</td><td>0.23 ± 0.04</td><td>0.27 ± 0.11</td><td>1.09 ± 0.16</td><td>295.0 ± 8.8</td><td>13.61 ± 1.36</td><td>5.31 ± 0.15</td><td>-0.7 ± 0.09</td></tr><tr><td>47171</td><td>1999 TC36</td><td>Lempo</td><td>6</td><td>3:2 Res</td><td>39.81</td><td>30.57</td><td>48.96</td><td>0.23</td><td>8.4</td><td>393.1</td><td>0.08</td><td>50</td><td>2.89 ± 0.29</td><td>0.27 ± 0.05</td><td>0.26 ± 0.12</td><td>0.20 ± 0.16</td><td>266.1 ± 4.9</td><td>21.14 ± 4.62</td><td>7.47 ± 0.14</td><td>0.07 ± 0.08</td></tr><tr><td>523591</td><td>2001 QD298</td><td></td><td>59</td><td>CC</td><td>42.95</td><td>40.51</td><td>45.50</td><td>0.06</td><td>5.0</td><td>233</td><td>0.07</td><td>43.5</td><td>-0.46 ± 0.53</td><td>0.24 ± 0.05</td><td>-0.05 ± 0.19</td><td>0.89 ± 0.25</td><td>277.7 ± 1.8</td><td>29.23 ± 3.67</td><td>5.63 ± 0.17</td><td>-0.61 ± 0.11</td></tr><tr><td>275809</td><td>2001 QY297</td><td></td><td>49</td><td>CC</td><td>44.17</td><td>40.60</td><td>47.73</td><td>0.08</td><td>1.5</td><td>229</td><td>0.15</td><td>42.9</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>6.85 ± 0.4</td><td>-1.16 ± 0.23</td></tr><tr><td>385437</td><td>2003 GH55</td><td></td><td>58</td><td>CC</td><td>43.93</td><td>40.66</td><td>47.30</td><td>0.08</td><td>1.1</td><td>178</td><td>0.15</td><td>42.9</td><td>2.72 ± 0.46</td><td>0.25 ± 0.02</td><td>0.20 ± 0.14</td><td>0.73 ± 0.23</td><td>291.8 ± 20.2</td><td>18.10 ± 4.34</td><td>7.33 ± 0.14</td><td>-0.34 ± 0.09</td></tr><tr><td>469705</td><td>2005EF298</td><td>Kágára</td><td>57</td><td>CC</td><td>43.77</td><td>40.06</td><td>47.41</td><td>0.08</td><td>2.9</td><td>174</td><td>0.16</td><td>43.1</td><td>2.51 ± 0.17</td><td>0.10 ± 0.06</td><td>0.05 ± 0.25</td><td>0.59 ± 0.14</td><td>312.9 ± 17.0</td><td>21.30 ± 2.56</td><td>6.07 ± 0.19</td><td>-0.84 ± 0.1</td></tr><tr><td>469750</td><td>2005 PU21</td><td></td><td>40</td><td>Scat</td><td>183.99</td><td>29.30</td><td>338.66</td><td>0.84</td><td>6.2</td><td>283</td><td>0.06</td><td>51.2</td><td>2.51 ± 0.02</td><td>0.51 ± 0.12</td><td>0.70 ± 0.18</td><td>0.72 ± 0.16</td><td>282.8 ± 3.7</td><td>13.35 ± 1.00</td><td>5.24 ± 0.14</td><td>-1.35 ± 0.08</td></tr><tr><td>341520</td><td>2007 TY430</td><td>Mors</td><td>45</td><td>3:2 Res</td><td>39.61</td><td>28.85</td><td>50.25</td><td>0.27</td><td>11.3</td><td>102</td><td>&gt;0.12</td><td>&lt;51.2</td><td>3.05 ± 0.31</td><td>0.33 ± 0.07</td><td>0.70 ± 0.16</td><td>0.21 ± 0.12</td><td>278.7 ± 10.2</td><td>19.02 ± 6.16</td><td>6.71 ± 0.22</td><td>-0.83 ± 0.11</td></tr><tr><td>341520</td><td>2007 TY430</td><td>Somnus</td><td>45</td><td>3:2 Res</td><td>39.61</td><td>28.85</td><td>50.25</td><td>0.27</td><td>11.3</td><td>97</td><td>&gt;0.12</td><td>&lt;51.2</td><td></td><td></td><td></td><td></td><td></td><td></td><td>7.06 ± 0.22</td><td>-0.61 ± 0.13</td></tr><tr><td colspan="15">D.Dip</td></tr><tr><td>26375</td><td>1999 DE9</td><td></td><td>21</td><td>5:2 Res</td><td>55.01</td><td>36.43</td><td>77.73</td><td>0.42</td><td>7.6</td><td>311</td><td>0.16</td><td>48.1</td><td>1.68 ± 0.07</td><td>0.46 ± 0.09</td><td>0.04 ± 0.07</td><td>3.81 ± 0.20</td><td>205.8 ± 0.9</td><td>13.28 ± 3.55</td><td>4.55 ± 0.16</td><td>-0.21 ± 0.09</td></tr><tr><td>38628</td><td>2000 EB173</td><td>Huya</td><td>3</td><td>3:2 Res</td><td>39.28</td><td>28.54</td><td>50.19</td><td>0.28</td><td>15.5</td><td>458</td><td>0.08</td><td>51.7</td><td>2.62 ± 0.25</td><td>0.75 ± 0.12</td><td>0.93 ± 0.15</td><td>3.60 ± 0.37</td><td>265.6 ± 11.1</td><td>10.22 ± 0.93</td><td>4.78 ± 0.17</td><td>-0.29 ± 0.09</td></tr><tr><td>138537</td><td>2000 OK67</td><td></td><td>51</td><td>HC</td><td>47.01</td><td>40.04</td><td>53.98</td><td>0.15</td><td>4.9</td><td>164</td><td>0.17</td><td>43.1</td><td>1.86 ± 0.12</td><td>0.17 ± 0.03</td><td>0.00 ± 0.05</td><td>5.55 ± 0.33</td><td>118.4 ± 8.1</td><td>15.51 ± 4.90</td><td>5.11 ± 0.16</td><td>0.18 ± 0.11</td></tr><tr><td>82155</td><td>2001 FZ173</td><td></td><td>47</td><td>Scat</td><td>84.25</td><td>32.39</td><td>136.08</td><td>0.62</td><td>12.7</td><td>212</td><td>0.12</td><td>48.2</td><td>1.37 ± 0.06</td><td>0.13 ± 0.04</td><td>-0.13 ± 0.06</td><td>2.63 ± 0.17</td><td>226.4 ± 7.8</td><td>11.34 ± 3.00</td><td>3.74 ± 0.21</td><td>-0.22 ± 0.12</td></tr><tr><td>55637</td><td>2002 UX25</td><td></td><td>7</td><td>HC</td><td>42.99</td><td>36.80</td><td>49.05</td><td>0.14</td><td>19.4</td><td>697.2</td><td>0.11</td><td>45.4</td><td>2.10 ± 0.13</td><td>0.16 ± 0.04</td><td>-0.10 ± 0.04</td><td>2.71 ± 0.26</td><td>230.7 ± 12.7</td><td>10.66 ± 2.35</td><td>4.38 ± 0.16</td><td>-0.11 ± 0.09</td></tr><tr><td>120132</td><td>2003 FY128</td><td></td><td>116</td><td>Detach</td><td>49.05</td><td>36.95</td><td>61.11</td><td>0.25</td><td>11.8</td><td>460</td><td>0.08</td><td>45.4</td><td>1.13 ± 0.10</td><td>0.07 ± 0.02</td><td>-0.03 ± 0.01</td><td>5.22 ± 0.25</td><td>162.2 ± 15.0</td><td>11.92 ± 3.72</td><td>3.89 ± 0.19</td><td>-0.26 ± 0.1</td></tr><tr><td>174567</td><td>2003 MW 12</td><td>Varda</td><td>10</td><td>15:8 Res</td><td>45.79</td><td>39.04</td><td>52.47</td><td>0.15</td><td>21.5</td><td>792</td><td>0.1</td><td>44</td><td>2.98 ± 0.33</td><td>0.20 ± 0.07</td><td>-0.23 ± 0.10</td><td>4.75 ± 0.33</td><td>201.0 ± 4.4</td><td>13.38 ± 2.90</td><td>5.1 ± 0.15</td><td>0.41 ± 0.09</td></tr><tr><td>523601</td><td>2003 UY413</td><td></td><td>153</td><td>HC</td><td>47.64</td><td>37.57</td><td>57.72</td><td>0.21</td><td>20.7</td><td>293</td><td>0.14</td><td>44.7</td><td>2.14 ± 0.06</td><td>-0.05 ± 0.01</td><td>0.00 ± 0.06</td><td>4.04 ± 0.24</td><td>202.1 ± 7.8</td><td>15.08 ± 4.07</td><td>4.11 ± 0.17</td><td>0.09 ± 0.1</td></tr><tr><td>120348</td><td>2004 TY364</td><td></td><td>12</td><td>HC</td><td>39.09</td><td>36.59</td><td>41.60</td><td>0.06</td><td>24.8</td><td>512</td><td>0.11</td><td>45.5</td><td>1.33 ± 0.07</td><td>0.09 ± 0.02</td><td>-0.13 ± 0.03</td><td>3.71 ± 0.31</td><td>208.9 ± 5.6</td><td>16.23 ± 2.43</td><td>4.34 ± 0.18</td><td>0.13 ± 0.1</td></tr><tr><td>144897</td><td>2004 UX10</td><td></td><td>114</td><td>HC</td><td>39.25</td><td>37.68</td><td>40.91</td><td>0.04</td><td>9.5</td><td>398.1</td><td>0.14</td><td>44.6</td><td>2.21 ± 0.12</td><td>-0.01 ± 0.03</td><td>-0.47 ± 0.05</td><td>4.30 ± 0.28</td><td>208.4 ± 2.0</td><td>8.59 ± 2.04</td><td>2.89 ± 0.17</td><td>-0.37 ± 0.09</td></tr></tbody></table>
<table><caption>Table A1
(Continued)</caption><thead><tr><th>Number</th><th>Prov. Des.</th><th>Name</th><th>ID</th><th>Class</th><th>a<br>(au)</th><th>q<br>(au)</th><th>Q<br>(au)</th><th>e</th><th>i</th><th>D<br>(km)</th><th>pV</th><th>T<sub>g</sub><br>(K)</th><th>H<sub>2</sub>O 2.0 µm<br>(1E-3 µm)</th><th>M-OH 2.27 µm<br>(1E-3 µm)</th><th>M-OH 2.5µm<br>(1E-3 µm)</th><th>CO<sub>2</sub> 2.7µm<br>(1E-3 µm)</th><th>O-H 3µm<br>(1E-3 µm)</th><th>C-H 3.4µm<br>(1E-3 µm)</th><th>SIR1<br>(%/1000 Å)</th><th>SIR2<br>(%/1000 Å)</th></tr></thead><tbody><tr><td>230965</td><td>2004 XA192</td><td></td><td>9</td><td>Detach</td><td>47.16</td><td>35.46</td><td>58.86</td><td>0.25</td><td>38.2</td><td>339</td><td>0.26</td><td>45</td><td>2.80 ± 0.36</td><td>0.45 ± 0.13</td><td>0.15 ± 0.19</td><td>5.25 ± 0.30</td><td>187.7 ± 5.8</td><td>14.66 ± 3.47</td><td>4.21 ± 0.14</td><td>0.02 ± 0.09</td></tr><tr><td>303775</td><td>2005 QU182</td><td></td><td>133</td><td>Scat</td><td>114.18</td><td>37.12</td><td>191.99</td><td>0.68</td><td>14.0</td><td>416</td><td>0.33</td><td>43.4</td><td>3.37 ± 0.35</td><td>0.19 ± 0.04</td><td>-0.02 ± 0.03</td><td>3.94 ± 0.23</td><td>217.0 ± 1.6</td><td>15.72 ± 4.20</td><td>3.66 ± 0.18</td><td>0.12 ± 0.11</td></tr><tr><td>145452</td><td>2005 RN43</td><td></td><td>11</td><td>HC</td><td>41.85</td><td>40.57</td><td>43.12</td><td>0.03</td><td>19.2</td><td>679</td><td>0.11</td><td>43.2</td><td>2.38 ± 0.10</td><td>0.14 ± 0.12</td><td>0.20 ± 0.21</td><td>5.32 ± 0.36</td><td>208.6 ± 2.0</td><td>18.25 ± 4.56</td><td>3.85 ± 0.16</td><td>-0.04 ± 0.08</td></tr><tr><td>145480</td><td>2005 TB190</td><td></td><td>25</td><td>Detach</td><td>76.89</td><td>46.22</td><td>107.56</td><td>0.40</td><td>26.4</td><td>464</td><td>0.15</td><td>40.2</td><td>2.86 ± 0.13</td><td>-0.15 ± 0.07</td><td>-0.28 ± 0.12</td><td>4.69 ± 0.28</td><td>206.7 ± 10.1</td><td>11.36 ± 3.47</td><td>4.42 ± 0.21</td><td>-0.29 ± 0.1</td></tr><tr><td>229762</td><td>2007 UK126</td><td>G!kun.</td><td>2</td><td>Detach</td><td>73.87</td><td>37.63</td><td>110.11</td><td>0.49</td><td>23.4</td><td>599</td><td>0.17</td><td>44.4</td><td>3.52 ± 0.06</td><td>0.65 ± 0.04</td><td>1.08 ± 0.04</td><td>3.40 ± 0.29</td><td>245.5 ± 10.0</td><td>19.62 ± 4.41</td><td>4.21 ± 0.16</td><td>-0.41 ± 0.09</td></tr><tr><td>470596</td><td>2008 NW4</td><td></td><td>36</td><td>HC</td><td>45.55</td><td>36.57</td><td>54.53</td><td>0.20</td><td>23.1</td><td>297</td><td>0.1</td><td>45.6</td><td>3.13 ± 0.22</td><td>0.11 ± 0.05</td><td>0.38 ± 0.09</td><td>4.65 ± 0.27</td><td>215.5 ± 2.6</td><td>12.18 ± 2.93</td><td>5.14 ± 0.21</td><td>-0.49 ± 0.11</td></tr><tr><td>471143</td><td>2010 EK139</td><td>Dziew.</td><td>5</td><td>7:2 Res</td><td>69.11</td><td>32.46</td><td>105.76</td><td>0.53</td><td>29.5</td><td>433</td><td>0.3</td><td>46.7</td><td>2.88 ± 0.15</td><td>0.32 ± 0.04</td><td>-0.06 ± 0.13</td><td>5.58 ± 0.32</td><td>188.2 ± 0.8</td><td>20.76 ± 5.14</td><td>4.34 ± 0.16</td><td>-0.19 ± 0.08</td></tr><tr><td>552555</td><td>2010 ER65</td><td></td><td>46</td><td>Detach</td><td>96.65</td><td>39.97</td><td>153.30</td><td>0.59</td><td>21.3</td><td>341</td><td>0.12</td><td>43.4</td><td>3.60 ± 0.34</td><td>0.17 ± 0.02</td><td>-0.18 ± 0.10</td><td>5.22 ± 0.27</td><td>232.4 ± 0.3</td><td>14.18 ± 2.49</td><td>5.85 ± 0.24</td><td>0.4 ± 0.13</td></tr><tr><td>471137</td><td>2010 ET65</td><td></td><td>44</td><td>Detach</td><td>61.67</td><td>39.60</td><td>83.74</td><td>0.36</td><td>30.7</td><td>338</td><td>0.12</td><td>43.6</td><td>2.64 ± 0.02</td><td>-0.02 ± 0.09</td><td>-0.58 ± 0.19</td><td>3.54 ± 0.22</td><td>204.2 ± 0.6</td><td>13.51 ± 3.60</td><td>4.07 ± 0.17</td><td>-0.59 ± 0.09</td></tr><tr><td>471210</td><td>2010 VW11</td><td></td><td>31</td><td>Detach</td><td>50.98</td><td>36.56</td><td>65.40</td><td>0.28</td><td>27.8</td><td>304</td><td>0.13</td><td>45.4</td><td>2.21 ± 0.06</td><td>0.39 ± 0.06</td><td>0.12 ± 0.08</td><td>3.91 ± 0.22</td><td>200.2 ± 17.1</td><td>17.54 ± 4.59</td><td>3.88 ± 0.19</td><td>-0.33 ± 0.12</td></tr><tr><td>523722</td><td>2014 LV28</td><td></td><td>42</td><td>Scat</td><td>69.95</td><td>34.24</td><td>105.65</td><td>0.51</td><td>10.7</td><td>268</td><td>0.13</td><td>46.9</td><td>3.77 ± 0.51</td><td>0.26 ± 0.04</td><td>0.17 ± 0.06</td><td>5.07 ± 0.42</td><td>206.3 ± 1.6</td><td>14.25 ± 2.98</td><td>4.94 ± 0.18</td><td>0.09 ± 0.11</td></tr><tr><td>523777</td><td>2014 YF50</td><td></td><td>30</td><td>13:3 Res</td><td>78.87</td><td>35.71</td><td>122.02</td><td>0.55</td><td>18.2</td><td>315</td><td>0.12</td><td>46</td><td>3.14 ± 0.13</td><td>0.37 ± 0.14</td><td>0.29 ± 0.22</td><td>0.42 ± 0.10</td><td>272.7 ± 7.2</td><td>17.32 ± 4.97</td><td>5.66 ± 0.17</td><td>-0.48 ± 0.17</td></tr></tbody></table>

**Note.** Observation ID, dynamical classification (HC: hot classical; CC: cold classical; Scat: scattered disk object; R: resonant object; Detach: detached object), semimajor axis *a*, perihelion *q*, Aphelion *Q*, eccentricity *e*, inclination *i*, diameter *D*, geometric V-band albedo *pV*, temperature at perihelion *T*, areas and errors for different bands (M-OH: methanol, with possible contribution from other organics), slopes SIR1 and SIR2 in the 0.7–1.2 µm and 1.15–2.42 µm ranges, respectively. *q*Sila-N.: Sila-Nunam; G!kun.: G!kun//’homdima; Dziew.: Dziewanna. The two components of the resolved binary Mors-Somnus are both Cliff-types with very similar spectral properties (A. C. Souza-Feliciano et al. 2024; N. Pinilla-Alonso et al. 2025). However, the spectrum of Somnus is too noisy to perform reliable calculations of band areas, so we excluded it from our analysis. The temperatures were calculated using the equilibrium temperature (lower than the maximum temperatures discussed in Appendix C) at TNO distance *d* (here the object’s perihelion distance): $T_d = T_{Sun} \times (1 - A)^{1/4} \times \sqrt{R_{Sun}/(2d)}$, where $T_{Sun}$ and $R_{Sun}$ are given; *A* is the bond albedo, with $A = pV \times q$; *pV* is the geometric V-band albedo; and *q* is the phase integral, calculated via $q = 0.336 \times pV + 0.479$ (Brucker et al. 2009).
<figure><img src="image_9.png" /><figcaption>**Figure A1.** The 2.27 µm band area (methanol and organics) as a function of (a) semimajor axis and (b) aphelion. (c, d) Same as panels (a) and (b), but for CO₂.</figcaption></figure>

## Appendix B
Statistical Analyses

We calculated the Pearson correlation coefficient (PCC) between the band areas investigated in Cliff1-TNOs (Figure B1). In employing PCC, we evaluated only linear relationships and considered that the band areas are normally distributed among Cliff1-TNOs. We excluded the Cliff-TNO 2004 PG115 (ID20) from our statistical analyses given its outlying behavior in many of the features considered.

The band area at 2.27 µm (CH₃OH and organics) shows a strong positive correlation ($r \ge 0.8$) with the band areas at 2.5 µm (CH₃OH and organics) and 3 µm (O–H, N–H), a moderate positive correlation ($0.6 \le r < 0.8$) with the 3.36–3.56 µm band area (C–H), and a strong negative correlation ($r \le -0.8$) with the 2.7 µm band area (CO₂). Accordingly, the 2.5 µm band area (CH₃OH and organics) also shows a strong positive correlation with the 3 µm band area (O–H, N–H), a moderate positive correlation with the 3.36–3.56 µm band area (C–H), and a strong negative correlation with the 2.7 µm band area (CO₂). The similar correlation trends for the 2.27 and 2.5 µm band areas provide further evidence that the carriers of these spectral features are either the same or closely related. The

3 µm band area (O–H, N–H) exhibits a moderate positive correlation with the 4.68 µm band area (CO), and the 3.36–3.56 µm band area (C–H) shows a strong negative correlation with the 2.7 µm band area (CO₂) and a moderate negative correlation with SIR2. SIR1 and SIR2 themselves exhibit a moderate positive correlation.

To further support the separation between Cliff1 and Cliff2, we performed a K-S test (Table B1), a nonparametric statistical method used to assess the null hypothesis that two groups originate from the same parent distribution. The band areas at 2.27 µm (CH₃OH and organics), 2.5 µm (CH₃OH and organics), 2.7 µm (CO₂), and 3 µm (O–H, N–H), as well as SIR2, most effectively distinguish Cliff1 from Cliff2, with $p < 0.001$. The band area at 4.68 µm (CO) and SIR2 also separate Cliff1 and Cliff2 below the $p < 0.05$ significance level. In contrast, Cliff1 and Cliff2 originate from the same parent distribution based on their 3.36–3.56 µm (C–H) band area, diameters, and geometric albedos ($p_v$) ($p > 0.05$). We did not control for the increase in family-wise error rate when performing multiple statistical tests on correlated variables. As such, our interpretation of the K-S test results is limited and should be viewed with caution.
<figure><img src="image_10.png" /><figcaption>Figure B1. The PCC ($-1 \le r \le 1$) between the 2.27, 2.5, 2.7, 3, 3.36–3.56, and 4.68 µm band areas, as well as SIR1 and SIR2, only for Cliff1-TNOs.</figcaption></figure>

<table><caption>Table B1
Kolmogorov–Smirnov Test</caption><thead><tr><th rowspan="2">Feature</th><th colspan="2">Cliff1-Cliff2</th></tr><tr><th>D-statistic<br>df = 2</th><th>p-value</th></tr></thead><tbody><tr><td>2.27 µm</td><td>1.00</td><td>&lt;.001<sup>c</sup></td></tr><tr><td>2.5 µm</td><td>1.00</td><td>&lt;.001<sup>c</sup></td></tr><tr><td>2.7 µm</td><td>1.00</td><td>&lt;.001<sup>c</sup></td></tr><tr><td>3 µm</td><td>1.00</td><td>&lt;.001<sup>c</sup></td></tr><tr><td>3.36–3.56 µm</td><td>0.689</td><td>.056</td></tr><tr><td>4.68 µm</td><td>0.778</td><td>.028<sup>a</sup></td></tr><tr><td>SIR1</td><td>0.800</td><td>.014<sup>a</sup></td></tr><tr><td>SIR2</td><td>1.00</td><td>&lt;.001<sup>c</sup></td></tr><tr><td>Diameter</td><td>0.689</td><td>.056</td></tr><tr><td>p<sub>v</sub></td><td>0.356</td><td>.714</td></tr></tbody></table>

Notes. $p_v$: geometric albedo.

a $p < 0.05$.

b $p < 0.01$.

c $p < 0.001$.

## Appendix C
Sublimation Rates at Perihelion

To estimate whether sublimation of surficial CO₂ might be responsible for the trend we observe in Figures 4(c) and (d), we use the standard model for comet production rates. In essence, it gives the equilibrium temperature and sublimation rate per unit area for a sublimating gray body at a function of heliocentric distance. Considering CO₂ and the subsolar point for the maximum temperature (typically higher than the isothermal temperatures mentioned in Table A1), we need to solve

$$ \frac{(1 - A) F_{\odot}}{d_{H}^{2}} = \varepsilon \sigma T_{\text{eq}}^{4} + P_{\text{CO}_{2}} \sqrt{\frac{m_{\text{CO}_{2}}}{2\pi k_{\text{B}} T_{\text{eq}}}} \Delta H_{\text{CO}_{2}}, \quad (\text{C1}) $$

with $A=0.1$ the Bond albedo, $F_{\odot}=1367 \text{ Wm}^{-2}$ the solar constant, $d_H$ the object's heliocentric distance in au, $\varepsilon=0.9$ the emissivity, $\sigma$ the Stefan–Boltzmann constant, and $T_{\text{eq}}$ the temperature in K. For the sublimation rate, $m_{\text{CO}_2}$ is the mass of a CO₂ molecule in kg, $k_B$ is the Boltzmann constant, and $\Delta H_{\text{CO}_2}$ is the latent heat of sublimation in J kg⁻¹. The gas pressure $P_{\text{CO}_2}$ in Pa is the saturated vapor pressure computed from the Clausius–Clapeyron equation.

The sublimation rate is a steep function of temperature (N. Fray & B. Schmitt 2009). Consequently, sublimation over geological timescales is dominated by the transient maximum temperatures. At 31 au, where we find a change in the surface composition (Figure 4(d)), the rate of sublimation is 3 orders of magnitude higher than at 41 au for example: sublimating one gram of CO₂ ice under those maximum temperature conditions would take approximately 3600 yr. Objects do not spend much time at perihelion, so it will take roughly 1 Myr to sublimate the top layer of CO₂ from the surface (considering that it takes several thousand years to sublimate at maximum temperature, that objects spend a few years at perihelion, and that their orbital periods are several hundred years).

What is important to keep in mind here is that only the uppermost optically active layer needs to sublimate, given the strong absorptions of CO₂. From these considerations, we can say that CO₂ begins to sublimate at extremely low rates, yet probably sufficient to affect the CO₂ spectral signatures on the characteristic timescales of the considered TNOs. If only a small amount of CO₂ needs to be lost to produce significant spectral differences (e.g., comparing a typical Double-Dip spectrum to Huya; N. Pinilla-Alonso et al. 2025), then the absence of CO₂-rich Double-Dip objects below 31 au suggests that Double-Dip surfaces are probably (1) highly stratified, with a thin, superficial, and pure CO₂ layer that is easily lost; and (2) difficult to sustain in the Centaur region. This aligns with the absence of Double-Dip features in Centaurs reported by J. Licandro et al. (2025).

## ORCID iDs

R. Brunetto https://orcid.org/0000-0003-3001-9362

E. Hénault https://orcid.org/0009-0001-3076-0073

S. Cryan https://orcid.org/0009-0007-3966-1980

N. Pinilla-Alonso https://orcid.org/0000-0002-2770-7896

J. P. Emery https://orcid.org/0000-0001-9265-9475

A. Guilbert-Lepoutre https://orcid.org/0000-0003-2354-0766

B. J. Holler https://orcid.org/0000-0002-6117-0164

L. T. McClure https://orcid.org/0000-0002-8041-0573

T. G. Müller https://orcid.org/0000-0002-0717-0462

Y. Pendleton https://orcid.org/0000-0001-8102-2903

A. C. de Souza-Feliciano https://orcid.org/0000-0002-2414-1460

J. Stansberry https://orcid.org/0000-0003-2434-5225

W. Grundy https://orcid.org/0000-0002-8296-6540

N. Peixinho https://orcid.org/0000-0002-6830-476X

G. Strazzulla https://orcid.org/0000-0003-1412-4023

M. T. Bannister https://orcid.org/0000-0003-3257-4490

D. Cruikshank https://orcid.org/0000-0002-0541-5569

B. Harvison https://orcid.org/0009-0006-0624-5385

J. Licandro https://orcid.org/0000-0002-9214-337X

V. Lorenzi https://orcid.org/0000-0002-1958-9930

M. N. de Prá https://orcid.org/0000-0001-8746-6468

C. Schambeau https://orcid.org/0000-0003-1800-8521
References

Barucci, M. A., Belskaya, I. N., Fulchignoni, M., & Birlan, M. 2005, AJ, 130, 1291

Barucci, M. A., Merlin, F., Dotto, E., Doressoundiram, A., & de Bergh, C. 2006, A&A, 455, 725

Batygin, K., Brown, M. E., & Fraser, W. C. 2011, ApJ, 738, 13

Biver, N., Bockelée-Morvan, D., Hofstadter, M., et al. 2019, A&A, 630, A19

Bockelée-Morvan, D., & Biver, N. 2017, RSPTA, 375, 20160252

Böker, T., Arribas, S., Lützgendorf, N., et al. 2022, A&A, 661, A82

Boogert, A. C. A., Brewer, K., Brittain, A., & Emerson, K. S. 2022, ApJ, 941, 32

Boogert, A. C. A., Gerakines, P. A., & Whittet, D. C. B. 2015, ARA&A, 53, 541

Bosman, A. D., Walsh, C., & van Dishoeck, E. F. 2018, A&A, 618, A182

Brown, M. E., Schaller, E. L., & Fraser, W. C. 2011, ApJL, 739, L60

Brunetto, R., Baratta, G. A., Domingo, M., & Strazzulla, G. 2005, Icar, 175, 226

Brunetto, R., Barucci, M. A., Dotto, E., & Strazzulla, G. 2006, ApJ, 644, 646

Brunetto, R., & Roush, T. L. 2008, A&A, 481, 879

Caselli, P., & Ceccarelli, C. 2012, A&ARv, 20, 56

Cooper, J. F., Christian, E. R., Richardson, J. D., & Wang, C. 2003, EM&P, 92, 261

Cruikshank, D. P., Roush, T. L., Bartholomew, M. J., et al. 1998, Icar, 135, 389

Cuppen, H. M., van Dishoeck, E. F., Herbst, E., & Tielens, A. G. G. M. 2009, A&A, 508, 275

Dalle Ore, C. M., Fulchignoni, M., Cruikshank, D. P., et al. 2011, A&A, 533, A98

Dartois, E., Schutte, W., Geballe, T. R., et al. 1999, A&A, 342, L32

De Prá, M. N., Hénault, E., Pinilla-Alonso, N., et al. 2025, NatAs, 9, 252

De Sanctis, M. C., Ammannito, E., McSween, H. Y., et al. 2017, Sci, 355, 719

Ehrenfreund, P., Kerkhof, O., Schutte, W. A., et al. 1999, A&A, 350, 240

Emery, J. P., Wong, I., Brunetto, R., et al. 2024, Icar, 414, 116017

Fray, N., & Schmitt, B. 2009, P&SS, 57, 2053

Fuchs, G. W., Cuppen, H. M., Ioppolo, S., et al. 2009, A&A, 505, 629

Gerakines, P. A., Schutte, W. A., & Ehrenfreund, P. 1996, A&A, 312, 289

Grim, R. J. A., Baas, F., Geballe, T. R., Greenberg, J. M., & Schutte, W. A. 1991, A&A, 243, 473

Grundy, W. M., Bird, M. K., Britt, D. T., et al. 2020, Sci, 367, aay3705

Hudson, R. L., Palumbo, M. E., Strazzulla, G., et al. 2008, in The Solar System Beyond Neptune, ed. M. A. Barucci et al. (Tucson, AZ: Univ. of Arizona Press), 507

Hénault, E., Brunetto, R., Pinilla-Alonso, N., et al. 2025, A&A, 694, A126

Kaplan, H. H., Simon, A. A., Hamilton, V. E., et al. 2021, A&A, 653, L1

Keller, L. P., Bajt, S., Baratta, G. A., et al. 2006, Sci, 314, 1728

Knez, C., Boogert, A. C. A., Pontoppidan, K. M., et al. 2005, ApJL, 635, L145

Krijt, S., Bosman, A. D., Zhang, K., et al. 2020, ApJ, 899, 134

Levison, H. F., & Morbidelli, A. 2003, Natur, 426, 419

Licandro, J., Pinilla-Alonso, N., Holler, B. J., et al. 2025, NatAs, 9, 245

Lippi, M., Podio, L., Codella, C., et al. 2024, ApJL, 970, L5

Lippi, M., Villanueva, G. L., Mumma, M. J., & Faggi, S. 2021, AJ, 162, 74

Maity, S., Kaiser, R. I., & Jones, B. M. 2015, PCCP, 17, 3081

Markwardt, L., Holler, B. J., Lin, H. W., et al. 2023, arXiv:2310.03998

Marssset, M., Fraser, W. C., Schwamb, M. E., et al. 2023, PSJ, 4, 160

Mastrapa, R. M., Bernstein, M. P., Sandford, S. A., et al. 2008, Icar, 197, 307

McClure, M. K., Rocha, W. R. M., Pontoppidan, K. M., et al. 2023, NatAs, 7, 431

Merlin, F., Quirico, E., Barucci, M. A., & de Bergh, C. 2012, A&A, 544, A20

Moore, M. H., Ferrante, R. F., Hudson, R. L., & Stone, J. N. 2007, Icar, 190, 260

Moore, M. H., Ferrante, R. F., & Nuth, J. A. I. 1996, P&SS, 44, 927

Mumma, M. J., & Charnley, S. B. 2011, ARA&A, 49, 471

Nesvorný, D. 2015, AJ, 150, 73

Nesvorný, D., Vokrouhlický, D., Alexandersen, M., et al. 2020, AJ, 160, 46

Nna-Mvondo, D., de la Fuente, J. L., Ruiz-Bermejo, M., Khare, B., & McKay, C. P. 2013, P&SS, 85, 279

Öberg, K. I., Garrod, R. T., van Dishoeck, E. F., & Linnartz, H. 2009, A&A, 504, 891

Palumbo, M. E., Castorina, A. C., & Strazzulla, G. 1999, A&A, 342, 551

Parhi, A., & Prialnik, D. 2023, MNRAS, 522, 2081

Pinilla-Alonso, N., Brunetto, R., De Prá, M. N., et al. 2025, NatAs, 9, 230

Pinilla-Alonso, N., Licandro, J., Brunetto, R., et al. 2024, A&A, 692, L11

Qasim, D., Chuang, K. J., Fedoseev, G., et al. 2018, A&A, 612, A83

Qi, C., Öberg, K. I., Wilner, D. J., et al. 2013, Sci, 341, 630

Quirico, E., Bacmann, A., Wolters, C., et al. 2023, Icar, 394, 115396

Rauscher, B. J. 2024, PASP, 136, 015001

Sandford, S. A., & Allandamola, L. J. 1990, ApJ, 355, 357

Schaller, E. L., & Brown, M. E. 2007, ApJL, 659, L61

Souza-Feliciano, A. C., Alvarez-Candal, A., & Jiménez-Teja, Y. 2018, A&A, 614, A92

Souza-Feliciano, A. C., Holler, B. J., Pinilla-Alonso, N., et al. 2024, A&A, 681, L17

Stern, S. A., Weaver, H. A., Spencer, J. R., et al. 2019, Sci, 364, aaw9771

Strazzulla, G., Cooper, J. F., Christian, E. R., & Johnson, R. E. 2003, CRPhy, 4, 791

Urso, R. G., Baklouti, D., Djouadi, Z., Pinilla-Alonso, N., & Brunetto, R. 2020, ApJL, 894, L3

Weyer, L. G. 1985, ApSRv, 21, 1

Zhang, C., Leyva, V., Wang, J., et al. 2024, PNAS, 121, e2320215121