# Methanol on red TNOs: A link between early composition and irradiation history

E Hénault, D Baklouti, R Brunetto, Z Djouadi, R.G Urso, P

Benoit-Lamaitrie, J Bourçois, O Mivumbi, C.M Dalle Ore, A Ricca

► To cite this version:

E Hénault, D Baklouti, R Brunetto, Z Djouadi, R.G Urso, et al. Methanol on red TNOs: A link between early composition and irradiation history. Icarus, 2025, 441, pp.116669. 10.1016/j.icarus.2025.116669. hal-05153559

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Research Paper

# Methanol on red TNOs: A link between early composition and irradiation history

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

E. Hénault<sup>a,∗</sup>, D. Baklouti<sup>a,†</sup>, R. Brunetto<sup>a,‡</sup>, Z. Djouadi<sup>a,§</sup>, R.G. Urso<sup>b,¶</sup>, P. Benoit-Lamaitrie<sup>c</sup>, J. Bourçois<sup>c</sup>, O. Mivumbi<sup>a</sup>, C.M. Dalle Ore<sup>d,e †</sup>, A. Ricca<sup>d,e ‡</sup>

## ARTICLE INFO

Dataset link: https://www.sshade.eu/data/EXPERIMENT_ZeD_20250609_01, https://www.sshade.eu/data/EXPERIMENT_ZeD_20250228_01

Keywords:

Trans-Neptunian objects (1705)

Small Solar System bodies (1469)

Surface composition (2115)

Surface processes (2116)

Laboratory astrophysics (2004)

Astrochemistry (75)

## ABSTRACT

Solid methanol is detected on small icy bodies of the outer solar system, namely trans-Neptunian objects (TNOs). Interestingly, all objects with methanol detections also display a strong positive spectral slope in the visible to near-infrared range, commonly attributed to complex organics, that could be formed by space weathering of methanol ice. We question the relationship between this red color and methanol content by performing irradiation experiments on methanol ices, pure and mixed with water, to simulate the ion bombardment of TNO surfaces. To compare the experimental spectral evolution under irradiation to observations, we focused our investigations on the normalized reflectivity gradient between the J and R photometric bands (JR slope) and the methanol to water abundance ratio post-irradiation. We show that ion irradiation triggers the destruction of methanol and the spectral reddening of the ices. Moreover, we find that JR slopes depend on both irradiation dose and initial ice composition. To account for red slopes alongside detectable methanol ice, we conclude that an irradiation gradient needs to be present at the surface of methanol-rich objects. However, methanol needs to be at least as abundant as water in the initial icy mixture, which suggests an early segregation of the icy species. We provide an estimation of the irradiation doses received at different depths of methanol-rich TNO surfaces, consistent with surface exposures of ~4.5 Gyrs.

## 1. Introduction

Solid methanol (CH<sub>3</sub>OH) is detected at the surface of various small icy bodies, specifically trans-Neptunian objects (TNOs). Methanol ice is identified through its 2.27 and 2.34 µm absorptions on the cold-classical Arrokoth, target of the extended New Horizons mission (Stern et al., 2019; Grundy et al., 2020). Methanol had been similarly detected, alongside water ice (H<sub>2</sub>O), at the surface of the Centaur Pholus (Cruikshank et al., 1998) and the plutino 2002 VE<sub>95</sub> (Barucci et al., 2006; Merlin et al., 2012). Methanol is also tentatively attributed to a ~2.3 µm absorption feature on other small Solar System objects (Barkume et al., 2008; Guilbert et al., 2009; Barucci et al., 2011; Souza-Feliciano et al., 2018). These tentative detections show the interest towards methanol as a key molecule stable against sublimation for Gyrs at the surfaces of TNOs (Lisse et al., 2021), but also show the difficulty of obtaining ground-based spectroscopic data with signal to noise ratio

appropriate for clear band identification. Recently, the DiSCo-TNOs large program for the James Webb Space Telescope (JWST) revealed the existence of a compositional group of objects rich in methanol ice (Pinilla-Alonso et al., 2024).

Interestingly, all objects with confirmed methanol detection are very red in the visible to near-infrared range (increase of reflectance with increasing wavelength). These red slopes are commonly attributed to the presence of “complex organics” and reproduced in light scattering models by using the optical constants of some type of laboratory analogue, namely nitrogen-rich tholins (Wilson et al., 1994; Cruikshank et al., 2005). These organics could be directly inherited from protoplanetary dust or formed by the continuous energetic processing of TNO surfaces (Dalle Ore et al., 2011).

Potential links between color and ice composition were previously investigated and debated. Based on the experimental evidence that irradiated methanol produces very red slopes (Brunetto et al.,

<footnote><sup>a</sup> Université Paris-Saclay, CNRS, Institut d'Astrophysique Spatiale, Orsay, 91405, France</footnote>

<footnote><sup>b</sup> INAF-Osservatorio Astrofisico di Catania, via Santa Sofia 78, Catania, 95123, Italy</footnote>

<footnote><sup>c</sup> ICJLab, Université Paris-Saclay, CNRS/IN2P3, Institut de Physique des 2 Infinis Irène Joliot-Curie, Orsay, 91405, France</footnote>

<footnote><sup>d</sup> NASA Ames Research Center, Moffett Field, CA 94035-1000, USA</footnote>

<footnote><sup>e</sup> SETI Institute, 339 Bernardo Ave., Suite 200, Mountain View, CA 94043, USA</footnote>

<footnote>* Corresponding author.
E-mail address: elsa.henault@universite-paris-saclay.fr (E. Hénault).</footnote>
2006), Brown et al. (2011) hypothesized that red objects were formed beyond the methanol retention line while objects richer in water and CO₂ would showcase flatter spectra. Merlin et al. (2017) found that the mean spectra of the reddest taxonomic group displays an absorption above 2.2 µm, consistent with methanol or hydrocarbons. But a survey of color and ice composition on 40 objects shows no correlation between red slopes and detectable methanol content (Peixoto et al., 2012). Previously, Barkume et al. (2008) showed that there was no correlation between NIR features of water ice and taxonomic groups based on colors. Dalle Ore et al. (2015) observed some of the reddest TNOs with the 3.55 and 4.5 µm photometric bands to show that they could all contain either methanol, methane (CH₄), carbon monoxide (CO) or carbon dioxide (CO₂) and that these ices may be the precursors of a spectrally red organic material formed by irradiation. Fernández-Valenzuela et al. (2021) extended the photometric study of 100 TNOs beyond 2.2 µm using a “compositional clock”, a method for compositional identification using a color-color diagram where materials (silicates, organics, ices) occupy different regions. They found correlations between color taxonomy and organics detection but no links to molecular ices. Now, the DiSCo-TNOs program, bringing both chemical composition from data above 2.5 µm and spectral slopes in the visible to near-infrared (Pinilla-Alonso et al., 2024), has the potential to unlock correlations between colors and ice composition.

TNOs are generally assumed to be among the most primitive objects in the Solar System, as frozen remnants of the building blocks of the early planetesimals (Nesvorný, 2018). However, various processes like collisions and radiogenic decay are thought to have influenced their composition since their formation. More importantly when considering remote sensing is the fact that their atmosphereless surfaces are exposed to energetic processing by solar wind (SW), solar energetic particles (SEPs) and galactic cosmic rays (GCRs) (Strazzulla et al., 2003).

Various laboratory experiments have been carried out to study the effect of ion irradiation on methanol ice, pure and mixed with water ice. They focused on identifying and quantifying irradiation products such as CO, CO₂, CH₄, H₂O and H₂CO (Baratta et al., 1994; Moore et al., 1996; Palumbo et al., 1999; de Barros et al., 2014; Quirico et al., 2023). These experiments were performed in transmission and in the mid-infrared range, most appropriate for chemical identification.

Fewer experiments have been probed by near-infrared reflectance spectroscopy. Brunetto et al. (2005) and Urso et al. (2020a) focused on the changes in the 2.27 µm and 2.34 µm methanol bands upon irradiation. Brunetto et al. (2006) showed that irradiation of C-bearing ices, specifically methanol, methane and benzene, produces red slopes that can fit the diversity of the colors observed for TNOs. Poston et al. (2018) irradiated complex icy mixtures containing methanol to show that the slope was particularly affected by the presence of sulfur compounds. Zhang et al. (2023) matched visible colors of irradiated methane and acetylene to a portion of TNOs. More recently, Zhang et al. (2024) studied the irradiation of methanol to shed light on the origin of Arroloth's red slope.

Considering the confirmed detections of methanol on very red objects, some alongside water ice, we questioned the role of methanol abundance in the evolution of the surfaces of TNOs and Centaurs upon irradiation. We aimed at extending the Brunetto et al. (2006) study by irradiating CH₃OH:H₂O ice mixtures at 60 K and 35 K in proportions 1:0, 4:1 and 1:1 with 30 keV H⁺ up to doses of 275 eV/16u. We monitored the irradiation-induced changes by reflection spectroscopy, to extract the spectral slopes, and by transmission spectroscopy to derive the relative abundance of methanol with respect to water ice. Comparing these experimental spectral parameters to observed ones allowed us to investigate the irradiation history of methanol-rich surfaces and explore the link between red color and ice composition.

## 2. Methods

### 2.1. Experimental set-up

We used the Irradiation de Glaces et Météorites Analysées par Réflectance (INGMAR, Lantz et al., 2017; Urso et al., 2020a) experimental setup to irradiate ices. INGMAR is a high-vacuum (P > 10⁻⁸ mbar) chamber connected to the SIDONIE ion separator (MOSAIC platform) of IJCLab at Orsay, France (Chauvin et al., 2004).

A closed cycle He-cryocooler (12–300 K) is used for the cooling of a substrate down to temperatures relevant to TNOs, here 60 and 35 K chosen as representative of the temperature variations of Centaurs (De Sanctis et al., 2000) and TNOs (Umurhan et al., 2022; Hénault et al., 2025). Ice deposits were formed by condensation of a gas, either pure CH₃OH gas or CH₃OH mixed with H₂O, on a cooled substrate. The CH₃OH/H₂O ratio in the gaseous mixture was fixed by the molar fraction of a prepared binary liquid mixture using the water-methanol binary diagram (Ilia, 2016). Preparing the mixture in the liquid phase allows for a continuous and reproducible deposition of a gas of fixed proportions. The ice mixture ratio was also checked by estimating molecular abundances from IR absorptions and comparing to literature spectra (Palumbo et al., 1999) during blank experiments that also allowed us to check the absence of contaminants. The liquid preparations [Aldrich, HPLC grade] were purified of dissolved gases by cycles of pumping. The deposition rate ranged from 10 nm min⁻¹ to 34 nm min⁻¹, depending on the experiment, and was controlled by a Swagelok stainless-steel low-flow inlet valve. The inlet is far enough from the substrate to perform background deposition which is known to form isotropic ice films, more porous than those grown by direct deposition at normal incidence (Dohnálek et al., 2003).

The ice samples are irradiated by an ion beam arriving at 10° with respect to the surface normal. We used 30 keV H⁺ ions to simulate energetic processing of TNO surfaces. This energy allowed us to investigate the physico-chemical changes produced mainly in a regime of inelastic energy loss (S<sub>e</sub>/S<sub>n</sub> ~ 200), which is thought to dominate energy deposition on TNO surfaces (Quirico et al., 2023). This regime generates excitation and ionization of the target molecules along the ion track, with the production of secondary electrons of lower energy which contribute to chemical reactions (Rothard et al., 2017). H⁺ is also a reactive ion which is incorporated by implantation into the irradiated ices (Garozzo et al., 2008) and thus may compensate for some of the dehydrogenation of the irradiation process (Strazzulla et al., 1991). According to Stopping and Range of Ions in Matter (SRIM) simulations (Ziegler et al., 2010), 30 keV H⁺ ions penetrate up to ~ 600 nm in CH₃OH:H₂O mixtures with a density of 0.8 g cm⁻³. In order to produce fully irradiated deposits thick enough for reflectance spectroscopy, we performed a simultaneous deposition and irradiation of ices, a common technique used in previous studies (e.g. Baratta et al., 1994). In fact, keeping the ion flux constant for all experiments, but varying the deposition rate of the ices from one experiment to the other, allowed us to process samples at different irradiation doses. We obtained µm-thick ices which are homogeneously irradiated in depth (within 10%, as the first and last layers will be irradiated, respectively, more and less than the bulk of the ice). The ion beam was rastered to ensure a homogeneous irradiation across the ice surface (ø 18 mm). The ion current was continuously monitored and typically kept between 700 to 800 nA cm⁻² to avoid macroscopic heating of the ice sample. Fluences ranged from 5 × 10¹⁶ ions cm⁻² to 1 × 10¹⁷ ions cm⁻².

### 2.2. Transmission mode experiments

The infrared light beam arrived at 10° with respect to the surface normal, opposite of the ion beam. The infrared detector is positioned to measure direct transmission. Ices are formed by condensation of a gaseous mixture on a cooled MgF₂ substrate (Crystran). A copper cylinder is placed behind the MgF₂ window to act as a cold trap
that will prevent the gas from condensing on the rear side of the MgF₂ window. This avoids the deposition of un-irradiated ices that would otherwise contribute to the transmission spectra. At 60 and 35 K, H₂O and CH₃OH condense in an amorphous state. The deposited molecularly-mixed ices result in thin films with a surface parallel to the substrate, as indicated by the presence of interference fringes produced by the light source continuum. The ice thickness (l) is determined by measuring the difference between interference fringes using the following equation (Godard, 2011):

$$l = \frac{1}{2n\Delta v_i \cos(\alpha_{\text{refracted}})} \quad (1)$$

where n is the estimated refractive index of the ice mixture, Δvᵢ the difference in wavenumber between two consecutive maxima or minima of the interference pattern and α<sub>refracted</sub> = sin⁻¹(sin(a<sub>incident</sub>)/n) is the angle of the refracted light which depends on the incident angle of the NIR beam, here 10°. The refractive index of ice mixtures depends on the properties of the ices, which in turn depend on the type of deposition (direct or diffuse) (Dohnálek et al., 2003), deposition temperature (Luna et al., 2018) and mixing ratios (Luna et al., 2012). We estimated the value of the refractive index for water-methanol ice mixtures with different ratios by weighing the refractive index of pure water (1.235 at 60 K from Kofman et al., 2019) and pure methanol (1.271 at 60 K from Luna et al., 2018) which are good approximates of the real part of the refractive index over our wavelength range. A linear regression between the ice thickness and the elapsed time since beginning the deposition gives the deposition rate, which was found to be stable throughout the entire experiment (< 15% variation). Transmission experiments allowed us to calibrate the deposition rate for reflection experiments and infer their final thicknesses.

From transmittance spectra, we obtain the relative abundance of water and methanol. Using relative abundances, instead of absolute, allows us to compare between different experiments and to compare with abundances derived from TNO spectra. We express the relative abundance of methanol with respect to water as :

$$N(\text{CH}_3\text{OH})/N(\text{H}_2\text{O}) = \frac{a(\text{CH}_3\text{OH})A(\text{H}_2\text{O})}{A(\text{CH}_3\text{OH})a(\text{H}_2\text{O})} \quad (2)$$

with A(X) being the band strength of molecule X and a(X) being the integrated optical depth of the associated vibration of molecule X. For methanol, we used its 3.53 µm band, associated to the –CH₃ symmetric stretching, with a band strength of 4.89 × 10⁻¹⁸ cm molecule⁻¹ from Luna et al. (2018). To estimate the abundance of water, only the broad –OH stretching mode at ~ 3 µm is available in our wavelength range. In the spectra of irradiated methanol ices (see Fig. 3 in the results section), the absorption at 3 µm contains contribution from both the remaining methanol ice and the abundantly formed water ice (Palumbo et al., 1999; Quirico et al., 2023). Other O–H bearing compounds are produced during irradiation of methanol (e.g. Hudson and Moore, 2000) but they are minor products, probably around 0.1% with respect to methanol (Öberg et al., 2009). Knowing the abundance of methanol from the 3.53 µm band, we estimate its contribution to the 3 µm band area with a band strength of 1.5 × 10⁻¹⁶ cm molecule⁻¹. The remaining band area is then attributed to O–H bonds in water ice and we use a band strength value of 2 × 10⁻¹⁶ cm molecule⁻¹ to estimate the column density of H₂O (Mastrapa et al., 2009). Although the band strength of water can change when mixed with other compounds (Öberg et al., 2007; Bouwman et al., 2007), we obtain a good approximate estimate with this set of values.

## 2.3. Reflection mode experiments

The light source is at an angle of 10° with respect to the surface normal, and the emission angle is also 10° but on a different plane to avoid collecting a strong specular reflection. The spot size diameter (4 mm) of the collected light on the sample is smaller than the one of the illuminating light source (12 mm). The reflected light is collected

<figure><img src="image_4.png" /><figcaption>Fig. 1. Spectra of pure CH₃OH irradiated to 79 eV/16u with increasing thickness as the ice is deposited and irradiated simultaneously. The irradiated ice thickness is given next to the corresponding spectra. Some overlap is shown between the visible and infrared detectors near 1 µm. Note that the reflectance values are greater than one, indicating that the growing ice is more diffusive than the substrate, hence only normalized reflectance are used in the remainder of this work.</figcaption></figure>

by an optical fiber and connected either to the visible (0.6 µm–1.05 µm) or near infrared (0.9–2.5 µm) detector. The observation geometry was identical in the visible and NIR range which ensures a good continuity of the reflectance between vis and NIR detectors (Rubino et al., 2020).

Ices condense on a copper disk coated with gold by electrodeposition at Institut d'Astrophysique Spatiale. The surface was scratched to introduce defects that make the gold layer more diffusive. Still, the collected light can gather a specular component depending on the orientation of the scratches with regards to the illuminating and collecting angles. The background for reflectance spectra is measured on the substrate which underwent a short period of ion irradiation (≤ 5 min) before ice deposition. This period is necessary for checking the ion beam's alignment and calibration. It also allows to cancel out any changes in spectral properties of the substrate due to irradiation and to sputter eventual contaminants from the substrate's surface. During ice deposition and irradiation, almost all impinging ions are implanted into the ice and not into the substrate. Thus, the irradiated ice reflectance spectra are free from contribution of potential changes in the optical properties of the substrate during ion bombardment.

From the reflectance spectra of each irradiated ice at final thickness, we determine the normalized reflectivity gradient, as defined by Jewitt (2002), between the J and R broad-band filters, respectively at 1.25 µm and 0.68 µm. This is referred to as JR slope and is a simple parameter to describe the dose-dependent behavior of the reflectance spectra in this featureless region.

Following a similar procedure to that of Poston et al. (2018), we deposited the irradiated ices until the reflectance spectra no longer changed, at least in the range from which we extract the JR slope. Poston et al. (2018) refer to this state as “spectral equilibrium” where reflectance spectra are no longer impacted by the increasing thickness of the ice and a diffuse reflectance regime is fully attained. Fig. 1 shows the spectral evolution of an irradiated methanol ice with increasing thickness. As the thickness increases, the interference pattern is lost, starting from the shortest wavelengths. From thicknesses ranging from 4 to 6 µm, the value of the JR slope is the same within error margins. The spectra at final thicknesses are as close as possible to a diffuse reflectance regime in our experimental set-up.

## 2.4. Estimation of irradiation doses

We quantified the irradiation effects by using the dose expressed in eV/16u, i.e., the total energy deposited per reference molecule of 16
<caption>**Table 1**</caption>

Summary of key parameters of the ion irradiation experiments: initial mixture composition, sample temperature, final sample thickness, average irradiation dose, the spectroscopic mode used where T stands for transmission spectroscopy and R for reflection spectroscopy. Each experiment allowed for the determination of molecular ratios or JR slopes at the single average dose applied to the entire final thickness of the ice sample. All ice mixtures were irradiated by 30 keV H⁺ ions.

<table><thead><tr><th>Initial ice<br>CH<sub>3</sub>OH:H<sub>2</sub>O</th><th>Temperature<br>(K)</th><th>Thickness<br>(µm)</th><th>Dose<br>(eV/16u)</th><th>Mode</th><th>N(CH<sub>3</sub>OH)/N(H<sub>2</sub>O)<br>in the irradiated ice</th><th>JR slope<br>(%/1000 Å)</th></tr></thead><tbody><tr><td>(1:0)</td><td>60</td><td>2.6 ± 0.7</td><td>107 ± 29</td><td>T</td><td>0.171 ± 0.030</td><td></td></tr><tr><td>(1:0)</td><td>60</td><td>6.4 ± 0.5</td><td>79 ± 6</td><td>R</td><td></td><td>7.75 ± 0.58</td></tr><tr><td>(4:1)</td><td>60</td><td>4.9 ± 0.5</td><td>114 ± 26</td><td>T</td><td>0.091 ± 0.011</td><td></td></tr><tr><td>(4:1)</td><td>60</td><td>3.6 ± 0.5</td><td>154 ± 21</td><td>R</td><td></td><td>7.88 ± 0.34</td></tr><tr><td>(1:1)</td><td>60</td><td>8.6 ± 0.6</td><td>58 ± 4</td><td>T</td><td>0.052 ± 0.009</td><td></td></tr><tr><td>(1:1)</td><td>60</td><td>7.3 ± 0.5</td><td>69 ± 5</td><td>R</td><td></td><td>3.50 ± 1.81</td></tr><tr><td>(1:1)</td><td>60</td><td>1.9 ± 0.1</td><td>79 ± 4</td><td>T</td><td>0.083 ± 0.018</td><td></td></tr><tr><td>(1:1)</td><td>35</td><td>5.4 ± 0.5</td><td>85 ± 8</td><td>T</td><td>0.131 ± 0.021</td><td></td></tr><tr><td>(1:1)</td><td>35</td><td>5.4 ± 0.6</td><td>83 ± 9</td><td>R</td><td></td><td>3.85 ± 1.51</td></tr><tr><td>(1:1)</td><td>60</td><td>4.5 ± 0.5</td><td>112 ± 12</td><td>T</td><td>0.054 ± 0.007</td><td></td></tr><tr><td>(1:1)</td><td>60</td><td>4.5 ± 0.5</td><td>111 ± 12</td><td>R</td><td></td><td>5.17 ± 0.89</td></tr><tr><td>(1:1)</td><td>60</td><td>2.8 ± 0.5</td><td>215 ± 38</td><td>T</td><td>0.043 ± 0.005</td><td></td></tr><tr><td>(1:1)</td><td>60</td><td>3.9 ± 0.5</td><td>275 ± 35</td><td>R</td><td></td><td>9.29 ± 0.32</td></tr></tbody></table>

atomic mass unit (u), typically CH₄ (Strazzulla et al., 1991). Although irradiation doses are more commonly expressed in eV/molecule or eV/atom, we here used this “normalized” unit to allow comparison between irradiated ices of different initial compositions.<fnref n="1" /> The irradiation dose is typically obtained by multiplying the fluence E (ions cm⁻²) by the stopping power (S<sub>total</sub> = S<sub>electronic</sub> + S<sub>nuclear</sub>) obtained using the SRIM code (Ziegler et al., 2010) for a given ice density ρ. In our experiments, as we irradiate while growing the ice, we assume that all the ions will deposit their energy E (30 keV) in the final thickness l with S<sub>total</sub> = E/l (eV cm⁻<fnref n="1" />). We thus calculate the dose as :

$$Dose = \frac{FS_{total}}{N_a \rho / M} \qquad (3)$$

with N<sub>a</sub> (mol⁻<fnref n="1" />) being Avogadro's number and the molar mass, M, set to 16 g mol⁻<fnref n="1" />. Errors in the dose (reported in Table 1) are estimated by propagating the error in the thickness of the ice, which is the most influential parameter to the relative error.

As done by Urso et al. (2020a), we assumed a density ρ of 0.8 g cm⁻³ as a mean value considering the different parameters affecting the density in our experiment: deposition rate (Bossa et al., 2015), temperature (Isokoski et al., 2014; Luna et al., 2018), mixture (Bar-Nun and Laufer, 2003) and irradiation induced compaction (Palumbo, 2006; Mejía et al., 2015). Assuming an ice density of 0.7 g cm⁻³ (or 0.9 g cm⁻³) would shift all dose estimations ~ 10% percent higher (or lower) than at 0.8 g cm⁻³ with no impact on the relative trends.

## 2.5. Summary of studied ice sample

Table 1 summarizes the experiments conducted in transmission and reflection modes where we varied the composition, temperature, thickness and thus dose, of the CH₃OH:H₂O mixture injected in the INGMAR chamber for deposition and irradiation. The final two columns of Table 1 are the spectral parameters derived from each experiment.

## 2.6. Selected observations

Here, we present the objects with confident methanol detection from the literature. We report the spectra, displayed in Fig. 2, used to calculate an object’s JR slope and how the relative abundance of methanol with respect to water is determined.

* Pholus is a ~ 310 km × 160 km × 150 km very red Centaur, homogeneous in color over its rotational period (Tegler et al., 2005). Its JR slope is calculated from spectra presented in Cruikshank et al. (1998). Using Hapke modeling, Cruikshank et al. (1998) found that the composition of Pholus consists of 15% of both H₂O and CH₃OH. Merlin et al. (2012) found proportions of 19% H₂O and 11% CH₃OH and of 4% H₂O and 11% CH₃OH with later observations. As the derived abundances depend on grain size, on the use of intimate or geographical mixture, and on the particular model used, other estimations exist. In particular, the Shkuratov model for Pholus spectra indicates that water could be four times more abundant than methanol (Poulet et al., 2002).

* 2002 VE<sub>95</sub> is a ~ 250 km wide red plutino. Its JR slope was estimated from spectra presented in Barucci et al. (2012). The Hapke model of its surface gives 12% amorphous H₂O and 10% CH₃OH and, likewise, the Shkuratov model gives 13% amorphous H₂O and 12% CH₃OH (Barucci et al., 2006). Hapke modeling of two later observations gave 7% amorphous H₂O for 10% CH₃OH and 4% amorphous H₂O for 12% CH₃OH (Barucci et al., 2012).

* Arrokoth is a ~ 35 km long bi-lobal cold-classical object that shows a homogeneous red color across its surface (Stern et al., 2019). Arrokoth is thought to be a very primitive object, inherited from the protosolar nebula (McKinnon et al., 2020). Crater counting estimated the age of its surface to date back to the end of disk accretion (Spencer et al., 2020). The collisions are very scarce for the cold-classical objects thus we can assume that the observed surface underwent prolonged energetic processing but minimum resurfacing events. The JR slope for Arrokoth is estimated from the composite spectra presented in Stern et al. (2019). Grundy et al. (2020) applied a Hapke model to quantify the chemical composition of Arrokoth, lacking the clear 1.5 and 2 µm features of Pholus and 2002 VE<sub>95</sub>, attributed to H₂O ice (Fig. 2). They found that the spectra can accommodate only up to 3% of H₂O while CH₃OH makes up the dominant component alongside tholins and amorphous carbon. As they do not provide a percentage for CH₃OH, we speculate that it is between 10 and 15% by analogy with the Hapke models of Pholus and 2002 VE<sub>95</sub>. Thus, we consider a methanol to water abundance ratio between 3.3 and 5. As the contribution of H₂O ice is very tenuous, these ratio estimates can be regarded more as lower limits rather than as a fixed range.

* From the DiSCo-TNOs large JWST program, we selected six objects (1995 TL<sub>8</sub>, 2002 TC<sub>302</sub>, 2004 PF<sub>115</sub>, 2004 PG<sub>115</sub>, 2007 JF<sub>43</sub> and 2010 VZ<sub>98</sub>) with the deepest 2.27 µm absorption as evidenced by visual inspection of the spectra and by the band area values provided by Pinilla-Alonso et al. (2024). Brunetto et al. (2025) show that these objects constitute a specific methanol-rich subgroup of TNOs (nicknamed Cliff1), and that they do not include the Cold Classical TNOs. These Cliff1-TNOs have additional bands

<footnote n="1">¹ As a reference point, 1 eV/16u is equivalent to 6 MGy with Gy being the gray, a standard unit for radiation dose absorption.</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Spectra of methanol-rich TNOs and Centaurs. Dotted vertical lines mark the H₂O and CH₃OH bands detected in this wavelength range. Spectra were normalized to unity at 2.2 µm and are shifted vertically for clarity. Left panel. Spectra (circle points) and model (dashed lines) of 2002 VE95 and Pholus are from Barucci et al. (2012) and Cruikshank et al. (1998) respectively. The composite spectrum of Arrokoth is from Stern et al. (2019) and the model at wavelengths > 1.2 µm is from Grundy et al. (2020). Right panel. Spectra (lines) of six objects from the DiSCo-TNO JWST program identified as rich in methanol by Pinilla-Alonso et al. (2024) and Brunetto et al. (2025).</figcaption></figure>

of H₂O, CO₂, and CO, all irradiation byproducts of methanol. Two of these objects are hot classical, two are scattering and two are resonant. Their diameters range from ~ 244 km to ~ 584 km. Spectra were obtained from Pinilla-Alonso et al. (2024) to calculate the JR slope. As the range of the observations did not attain 0.68 µm, we used the reflectance value at 0.7 µm. This has a minor impact on the values as the slope is relatively constant within the ~ 0.6 to 1.25 µm range. No estimates of molecular abundances from light scattering models are currently available. But a comparison of their spectra below 2.5 µm to the ones of Pholus and 2002 VE95 indicates that their methanol to water ratio should be on the same order of magnitude. Indeed, we see from Fig. 2 that methanol and water bands are of roughly similar depths in DiSCo objects, as is the case for Pholus and 2002 VE95.

## 3. Results

In Fig. 3 we present laboratory spectra of an irradiated pure methanol ice. On the left, we show the reflectance spectra obtained in the 0.6 to 2.5 µm range for a methanol ice at 60 K irradiated to 79 eV/16u. On the right panel, we show the transmittance spectra obtained in the 2.5 to 5 µm range for a methanol ice at 60 K irradiated to 107 eV/16u.

Radiolysis of methanol leads to the formation of various chemical compounds with the most abundant ones being CO₂, CO, CH₄, H₂CO and H₂O (Baratta et al., 1994; Moore et al., 1996; Palumbo et al., 1999). In our wavelength range, only CO₂ and CO have absorption bands that clearly stand out from the remaining CH₃OH absorptions. The evolution of CO and CO₂ as a function of irradiation dose and initial composition is addressed in a separate work (Hénault et al., 2025). Water ice, both as a product and as an initial component in the case of the irradiated mixtures, contributes to the broad 3 µm absorption. From the 3.4 µm C–H stretching absorptions, we cannot distinguish the contribution of CH₄ or that of more complex organics.

### 3.1. Molecular evolution under irradiation

In Fig. 4, we present the evolution of N(CH₃OH)/N(H₂O) as a function of irradiation dose. For comparison, we show previous data

obtained at 10 K and for lower irradiation doses by Palumbo et al. (1999). We find a good general match between our experiments and theirs, despite several differing factors (experimental set-up, temperature and bands used for the abundance estimation). Still, a discrepancy can be noted between our (1:1) point at 58 eV/16u and the rest of the (1:1) data, both from our measurements and from the literature. This is likely due to the important thickness of this irradiated ice sample compared to the others (Table 1), possibly saturating the 3 µm band from which the molecular ratio is extracted. Changing the temperature between 10, 35 and 60 K has no significant impact on N(CH₃OH)/N(H₂O) since both molecules are refractory at these temperatures.

Fig. 4 shows that N(CH₃OH)/N(H₂O) decreases with increasing irradiation dose. This decrease is steeper for initially pure CH₃OH than for CH₃OH:H₂O mixtures. This highlights the efficient destruction of CH₃OH while H₂O is produced and is also more resistant to radiolysis. Initially pure CH₃OH ice reaches ratios where H₂O becomes at least as abundant as CH₃OH above ~ 40 eV/16u. Ice mixtures in all proportions will tend towards a ratio between 0.01 and 0.2 at high doses, likely reaching a balance between the destruction and partial re-formation of methanol. At ~ 100 eV/16u, more than 90% of the initial methanol is destroyed and transformed into the products (Urso et al., 2022).

In the right panel of Fig. 4, we place for comparison the abundance ratios determined by Hapke modeling on Arrokoth, Pholus and 2002 VE95 as explained in Section 2.6. The six methanol-rich objects observed within the DiSCo-TNOs program probably have similar N(CH₃OH)/N(H₂O) than Pholus and 2002 VE95. When looking at the general trend for these nine objects, we find that the observed amounts of CH₃OH can only be sustained for irradiation doses lower than ~ 50 eV/16u and for initial ices where CH₃OH is at least as abundant as water.

### 3.2. Color evolution under irradiation

In Fig. 5, we report the experimental JR slopes as a function of the irradiation dose. They are plotted alongside previous experiments carried on pure methanol by Brunetto et al. (2006). There is a very good
<figure><img src="image_6.png" /><figcaption>Fig. 3. Spectra of pure CH<sub>3</sub>OH ice, un-irradiated (gray) and irradiated (black) to a dose of 79 ev/16u in the reflectance spectra (**left panel**) and to a dose of 107 eV/16u in the transmittance spectrum (**right panel**). Dashed lines mark the attribution of absorption bands to the corresponding chemical species. R and J reference points are marked as a visual aid in the estimation of the JR slope. In the reflectance spectra, some overlap is shown between the visible (dotted lines) and infrared (plain lines) detector in the region around 1 µm. The reflectance spectra of the un-irradiated methanol ice were corrected for the interference pattern from which no reliable JR slope can be extracted.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Left panel. Evolution of the relative abundance of methanol with respect to water as a function of irradiation dose for different initial ice mixtures. An empirical trend derived from the pure methanol data points is shown to guide the eye. Right panel. Range of relative abundances on Arrokoth, Pholus and 2002 VE<sub>95</sub> derived from Hapke modeling taken from references presented in Section 2.6.</figcaption></figure>

agreement between the data points from this study and ours, despite
significant differences in the spectroscopic set-up and thickness of the
ices. We also confirm that temperature does not affect the JR slope, at
least in the range explored here below ~ 80 K.

Fig. 5 shows that the JR slope increases with increasing irradiation
dose. As done by Brunetto et al. (2006), we can fit the experimental
points with $S' = C(1 - \exp(-\sigma \times \text{dose}))$ where C is the asymptotic
value (%/1000 Å) reached and σ is the cross section (16 µeV). The
JR slope produced after irradiation of pure methanol increases with
irradiation until reaching 26 ± 7%/1000 Å. The slope produced after
irradiation of the (1:1) mixture increases as well with irradiation but it
reaches a lower asymptote of 13.3 ± 0.3%/1000 Å. The irradiation of the
mixture where methanol dominates (4:1) fits well in between the two
previous trends. Data suggest that increasing the initial methanol rela-
tive abundance will increase the JR slope produced by irradiation. The
initial methanol content determines the ability of an H<sub>2</sub>O–CH<sub>3</sub>OH ice
to efficiently redden.

In the right panel of Fig. 5, we present the JR slope of objects with
strong CH<sub>3</sub>OH absorption. Objects from the DiSCo-TNOs program have
much lower uncertainties on their JR slope as they are derived from
spectra with far better signal to noise ratio than for Arrokoth, Pholus
and 2002 VE<sub>95</sub>. All objects have relatively similar red colors within

error margins, although all DiSCo-TNOs objects are on the lower range,
probably due to the correction of the solar contribution.

From Fig. 5, we find that observed JR slopes are consistent with
CH<sub>3</sub>OH:H<sub>2</sub>O (1:1) irradiated to at least ~ 250 eV/16u or with pure
CH<sub>3</sub>OH irradiated to at least ~ 100 eV/16u. We can generalize to a
broader estimate of at least ~ 150 eV/16u on methanol-rich ices, where
methanol is at least as abundant a water.

4. Discussion

4.1. *Physical and chemical effects in relation to the spectral slope*

We find from Fig. 5 that spectral reddening depends on the irradi-
ation dose and on the initial abundance of methanol with respect to
water. These dependencies are likely explained by both physical and
chemical effects in the irradiated ices.

Physical effects include ice disorder (Carrascosa et al., 2021) and
the creation of scattering centers like cracks, although these would be
more prevalent in much thicker ices, and sub-micron carbon particles,
as suggested by Poston et al. (2018). Strazzulla et al. (1988) irradiated
pure H₂O and pure CH₄ and showed an increase in light scattering
(at 632.8 nm) correlated to the detection of H₂ in the gas phase
which created a “fluffy” texture in the ices when desorbing. Since
loss of H was also observed to increase with increasing irradiation
dose (Strazzulla and Baratta, 1992; Ferini et al., 2004; Urso et al.,
2020b), an increase in scattering properties could contribute to the
trend of increasing JR slope with increasing irradiation dose.

Chemical effects are more studied in the literature and include
the formation of compounds which absorb in the visible. Detailed
characterization of the produced chemical species is more easily per-
formed on the refractory organic residue leftover from heating of the
irradiated ices back to room temperature. Quirico et al. (2023) showed
the formation of C=C and C=O bonds by irradiation of pure CH<sub>3</sub>OH,
as well as their stability during the temperature increase leading to
some organic residue formation. The UV-visible characterization of the
residue solubilized in methanol showed that visible absorption involves
π transitions in conjugated C=C bonds. With mass-spectrometry analysis
of residues of irradiated methanol ice mixtures, Urso et al. (2020b)
showed that the degree of unsaturation increases when increasing the
irradiation dose from ~ 30 eV/16u to ~ 100 eV/16u.

Characterizing the visible absorption in the irradiated ices as well
as simultaneously identifying the functional groups potentially re-
sponsible for the visible absorption is experimentally more challeng-
ing. Sakakibara et al. (2020) studied N<sub>2</sub>–He plasma irradiation of
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Left panel. Evolution of the JR slope as a function of irradiation dose. Trends derived for pure CH₃OH and CH₃OH:H₂O (1:1) irradiated ices are presented to guide the eye. Right panel. JR slope observed on methanol-rich TNO surfaces. Gray vertical lines represent the uncertainties surrounding the determination of the JR slope, which is calculated from local noise variations.</figcaption></figure>

CH₃OH:H₂O ices at 85 K and found that visible transmittance spectra showed an increased absorption with increasing irradiation time. They also showed that the red coloring is stable at low temperature but starts to fade at 120 K and disappears at 150 K, temperatures of water ice crystallization, which is know to expel trapped volatiles. They argued that color might be majorly determined by smaller and more volatile molecules than the macro-molecular component of tholins or residues. Ide et al. (2023) added an infrared characterization to identical experiments and found that ice coloration disappears alongside a decrease of the infrared signatures of unsaturated bonds. Zhang et al. (2024) irradiated pure methanol at 40 K with 5 keV electrons and performed spectral characterization both in the visible and mid infrared range. They found that the slope at 0.55 µm and the band area of C=O and C=C bonds both increase with increasing irradiation dose. They thus argue that conjugated C=O and C=C are likely the chromophores responsible of the reddening of the irradiated ices. Mass-spectrometry analysis of their residue showed a richness of unsaturated compounds like polycyclic aromatic hydrocarbons (PAHs) which could contribute to reddening, although their formation is not evidenced at low temperature.

Based on all the aforementioned elements, it becomes increasingly clear that the chemical origin of red spectral slopes in our laboratory analogs and on methanol-rich TNOs are unsaturated bonds of organic molecules, typically carbonyls (C=O) and C=C conjugated bonds, including but not limited to aromatic structures. We see from Fig. 5 that increasing the initial water content will produce less red slopes. We propose that the added water molecules will cause the formation of lower amounts of complex organics (dilution effect) and of molecules that absorb less efficiently in this spectral range (chemical effect). We find that the initial ice mixture must contain at least as much methanol as water to match the colors observed on methanol-rich TNOs.

## 4.2. Irradiation induced chemical variation within TNO surfaces

We see from Fig. 4 that the methanol to water abundance ratio observed on TNOs is compatible with irradiation doses lower than ~50 eV/16u. But we found from Fig. 5 that the JR slope observed on TNOs is compatible with irradiation doses higher than ~150 eV/16u. To reconcile the results obtained at the shortest wavelengths with the JR slope and at longer wavelengths with the N(CH₃OH)/N(H₂O) ratio, we have to consider the different penetration ranges of photons within the surface of TNOs. Indeed, photons probe different depths depending on their wavelength and on the optical and physical properties of the surface's material. This can be approximated using the attenuation

$λ/4πk$ where k is the imaginary part of the refractive index of the material at a given wavelength $λ$. Methanol ice has k values of $1.6×10^{-3}$ for the 2.27 µm band and $6.1 × 10^{-4}$ for the 2.34 µm band (Gerakines and Hudson, 2020) which correspond to penetration depths of ~115 and ~315 µm respectively. At 0.68 µm (R band) and at 1.25 µm (J band) values of k for complex organic mixtures like residues (Baratta et al., 2015) and tholins (Jovanović et al., 2021) are higher and give penetration depths ranging from ~5 to ~25 µm. Therefore, the JR slope in remote sensing observations probes the uppermost layers of TNO surfaces while the 2.27 and 2.34 µm methanol bands probe layers more than 10 times deeper, as represented in the left panel of Fig. 6. By comparing the observed parameters to the constraints from irradiation experiments, we find that top layers are likely irradiated to more than ~150 eV/16u, while deeper layers received less than ~50 eV/16u. Methanol-rich TNO surfaces are thus consistent with compositional variations induced by an irradiation gradient. As represented in Fig. 6, top layers are rich in coloring organic species formed by irradiation while deeper layers are rich in the precursor colorless ices.

An irradiation gradient is predicted by numerical models aiming to reproduce the accumulated irradiation doses at the surface of TNOs for ~4.5 Gyrs (Hudson et al., 2008; Loeffler et al., 2020; Quirico et al., 2023). Different estimates exist and are presented in the right panel of Fig. 6. They all find irradiation dose variations within the first meter of the surface, with the uppermost surface being the most irradiated and deeper layers being mostly preserved. The dose variation stems from the different penetration ranges of ions within the surface material. This penetration range depends on the nature and energy of the ion, and thus differs for the different family of ions defined as SW, SEPs and GCRs, but also on the nature of the surface material. The amount of energy deposited is where the different models vary. For example, Quirico et al. (2023) finds estimates much lower than those of Hudson et al. (2008) because they took into account sputtering which rejuvenates the surface by exposing fresher sub-surface layers. The estimates of Loeffler et al. (2020) are on the same order of magnitude as Quirico et al. (2023) but are a little lower, probably because they did not take into account the contribution of heavy ions. Still, models are in general agreement with each other.

By comparing observational parameters to the constraints of laboratory experiments, we obtained an independent confirmation of this irradiation gradient and an upper limit for the irradiation dose received at depths greater than ~115 µm. The good match between our dose estimates and the numerical estimations accumulated for ~4.5 Gyrs provides a long time-frame for the reddening of TNO surfaces.
Surface irradiation gradient derived from:

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Schematic representation of the irradiation gradient at the surface of methanol-rich TNOs. Left panel. Estimation of the accumulated irradiation doses obtained by comparing the spectral parameters of methanol-rich TNOs to ion irradiation experiments on methanol ices. Right panel. Estimations of the accumulated irradiation doses from different numerical calculations (Quirico et al., 2023; Loeffler et al., 2020; Hudson et al., 2008) based on measured and extrapolated ion fluxes in the outer solar system, integrated to represent ~ 4.5 Gyrs of surface exposure to ion bombardment.</figcaption></figure>

Other indicators also testify of irradiation processes. Urso et al. (2020a) compared the band profiles of the 2.27 and 2.34 µm methanol bands on Pholus and Arrokoth to irradiation experiments and found a good match with irradiated methanol for doses between 20 and 40 eV/16u. More recently, Hénault et al. (2025) showed that the CO discovered on TNO surfaces by the DiSCo-TNOs program (De Prá et al., 2024) was formed by irradiation, and specifically formed by irradiation of methanol on the objects whose JR slopes are shown in Fig. 5. With a complementary study of the 3.53 µm methanol band of the DiSCo-TNO targets presented here, Brunetto et al. (2025) also find that these TNO surfaces are best represented by a stratified surface where methanol abundance increases with depth, as irradiation dose decreases with depth. An implication of this irradiation scenario is that water ice currently observed on methanol-rich surfaces can be, at least partly, secondary as an irradiation product of methanol. We now have a growing body of evidence that the irradiation of methanol plays a determining role in the shaping of red TNO surfaces starting from methanol-rich ices.

### 4.3. Implications for the early surface composition of TNOs

Arrokoth, Pholus, 2002 $VE_{95}$ and the methanol-rich objects from the DiSCo program share similar spectral properties yet different physical and dynamical properties. Their similar surface composition suggests that they likely formed and evolved in a similar region of the protoplanetary disk before planetary migration (Pinilla-Alonso et al., 2024). On Pholus and 2002 $VE_{95}$, methanol ice is detected alongside water ice in comparable amounts (Cruikshank et al., 1998; Merlin et al., 2012) while on Arrokoth, methanol seems to dominate the icy component as water ice features are too tenuous to be firmly detected (Grundy et al., 2020). Interestingly, in cometary comae the abundance of methanol with respect to water ranges from < 0.2% to 4.8% (Dello Russo et al., 2016). In interstellar medium (ISM) ices, methanol abundance relative to water can be higher, in the 0.2% to 31% range (Boogert et al., 2015), but water ice is always the dominating ice. The current over-abundance of methanol on the objects studied here suggests that they underwent processes that virtually enriched their surface compared to their bulk and initial composition. This divergence from ISM and cometary abundances has to have happened relatively early in the history of TNOs so that the methanol-rich ices are set for prolonged irradiation,

efficiently coloring the surfaces up to the extent observed today. Early ice segregation is one of the processes that could have helped shaping the surfaces of the different compositional groups revealed by DiSCo-TNOs (Pinilla-Alonso et al., 2024) and potentially involve retention mechanisms at the surface of planetesimals (Brown et al., 2011). Early segregation followed by prolonged irradiation are enough to explain the surface characteristics of methanol-rich TNOs.

## 5. Conclusion

Methanol ice is detected on Arrokoth, Pholus, 2002 $VE_{95}$ and on at least six objects observed within the DiSCo-TNOs JWST program. All these objects show very red spectral slopes. We investigated an irradiation link between methanol content and color by performing ion irradiation experiments on methanol ices, pure and mixed with water. Methanol is efficiently destroyed by irradiation while water is also a product of methanol irradiation. The relative abundance of methanol with respect to water decreases with increasing irradiation dose.

* We find that the spectral slope of irradiated ices depends on the initial ice composition and on irradiation dose. Specifically, initially pure methanol ices redden more efficiently than methanol mixed with water ice.

* By comparing the spectral parameters obtained in the laboratory to those of methanol-rich TNOs, we find evidence for an irradiation gradient at their surface. JR slopes show that methanol-rich TNO surfaces likely received at least ~ 150 eV/16u within the first ~ 25 µm of the surface. Relative abundances of methanol show that methanol-rich TNO surfaces likely received an irradiation dose lower than ~ 50 eV/16u in layers deeper than ~ 115 µm.

* Methanol-rich TNO spectra are compatible with irradiation doses accumulated over ~ 4.5 Gyrs provided that early in the body's history their surfaces where enriched in methanol compared to cometary and ISM abundances, possibly through segregation processes at the surface.

### CRediT authorship contribution statement

E. Hénault: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. D. Baklouti: Writing – review
## & editing, Validation, Supervision, Investigation, Conceptualization.
**R. Brunetto:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Investigation, Funding acquisition, Conceptualization.
**Z. Djouadi:** Writing – review & editing, Validation, Supervision, Investigation, Conceptualization.
**R.G. Urso:** Writing – review & editing, Validation, Investigation, Conceptualization.
**P. Benoit-Lamaitrie:** Resources, Methodology.
**J. Bourçois:** Resources.
**O. Mivumbi:** Resources.
**C.M. Dalle Ore:** Validation, Conceptualization.
**A. Ricca:** Writing – review & editing, Validation, Project administration, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors thank N. Pinilla-Alonso, E. Quirico and C. Lisse for fruitful discussions and two anonymous referees for their helpful suggestions. We thank C. Bachelet and A. Gentils for access to SIDONIE and we thank S. Hervé for technical support. We thank Z. Dionnet for data preparation for SSHADE. The irradiations were performed using the INGMAR setup, a joint IAS-IJCLab (Orsay, France) facility funded by the P2IO LabEx (ANR-10-LABX-0038) in the framework Investissements d'Avenir (ANR-11 IDEX-0003-01). This research is supported by the CNES (JWST mission).

## Data availability

Experimental data are available on the SSHADE infrastructure within the DAYSY database. Reflectance spectra are directly accessible using this link: https://www.sshade.eu/data/EXPERIMENT_ZeD_20250609_01. Transmittance spectra are directly accessible using this link: https://www.sshade.eu/data/EXPERIMENT_ZeD_20250228_01.

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