Introduction: Over the last few years, the Hayabusa2 (JAXA) and OSIRIS-REx (NASA) missions brought back samples from the carbonaceous asteroids Ryugu and Bennu [1,2]. Thanks to an agreement between NASA and JAXA, a fraction of both sample collections were exchanged and 0.66 g of Bennu samples were received and analyzed at JAXA’s Extraterrestrial Samples Curation Center (EsCuC) in Sagamihara, Japan [3,4]. The access to both asteroids collection, kept under strictly contamination-controlled conditions, offers a unique opportunity to perform a comparative analysis and better understand primitive asteroid composition and alteration processes. Here we focus on phyllosilicate-related spectral features using near infrared (NIR) hyperspectral microscopy and middle infrared (MIR) micro-spectrometry.

Samples and methods: The Bennu samples are received at ESCuC as five aggregate samples, in August 2024 [5]. After an initial characterization of Bennu bulk material [5,6], on individual millimeter-sized grains were extracted and then analyzed, similarly to Ryugu samples. Such a characterization, performed inside the N₂ purged chamber includes the MicrOmega NIR hyperspectral microscope (0.99-3.65 µm, ~20 µm spatial resolution) developed at Institut d’Astrophysique Spatiale (Université Paris-Saclay/CNRS) [6, 7, 8]. Complementary measurements were also conducted with a µ-FTIR point spectrometer (2-12 µm, 100 µm spot size) and a Leica visible microscope [5]. In total, more than 100 Bennu grains (corresponding to a total mass of 134.98 mg) were used in this study to characterize the properties of the phyllosilicates present in the matrix. In particular, we investigated the -OH stretching band at ~2.7 µm (MicrOmega), and the Si-O stretching band near 10 µm (µ-FTIR). 715 Ryugu grains (~1.13 g from chamber A and ~680 mg from chamber C) obtained in similar conditions at EsCuC as well as with laboratory data obtained on Ryugu samples [9-11] and on relevant meteorite analogs [12].

Results: At the mm-scale, MicrOmega measurements reveal a heterogeneous distribution of the -OH absorption band in Bennu grains, both in depth and peak position, similar to Ryugu samples. The average -OH peak position is centered at ~2.716 µm, with nearly 90% of the grains falling within a narrow ±3 nm range (Figure 1). The remaining grains split into two subsets: ~4% are shifted toward longer wavelengths, and ~4% display shorter-wavelength peak positions combined with band depths below 10%.

The redshifted Bennu subset exhibits -OH and Si-O peak positions comparable to the space-weathered Ryugu chamber Aβ population (Figure 1) [11, 13-16]. Beyond this similarity, small but significant differences emerge between the two collections: Bennu grains show systematically shallower ~2.7 μm (OH) bands, lack a deep ~2.7 µm band cluster equivalent to Ryugu's chamber Aα population (Figure 1), and host a shallow, blueshifted ~2.7 µm band population absent from Ryugu. The Christiansen feature is also shifted toward shorter wavelengths in Bennu samples compared to Ryugu samples. Together, these features suggest a less mature or chemically distinct phyllosilicate formation pathway on Bennu's parent body.

A comparison of the (2.7 µm) Band depth vs. Peak position distributions confirms that the two populations are similar yet statistically distinct. This distinction is best explained either by compositional heterogeneity within a single shared parent body, or by derivation from two separate parent asteroids that underwent slightly different alteration histories.

Figure 1. Dispersion diagram showing the comparison between peak position and band depth of the -OH band between 101 Bennu grains (blue) and 715 Ryugu grains (green). Initial bulk samples of Bennu grains are in red [8]. Remote spectra are from [17,18].

References : [1] Lauretta D.S. et al. (2024) Met. And Planet. Sci. 59,9,2453-2486, [2] Yada T. et al. (2022) Nat. Astron. 6, 214-220, [3] Yada, T. et al. (2023), Earth, Planets and Space 75, p. 170. [4] Tahara, R. et al. (2026) Meteoritics & Planetary Science 61.1, p. 182-207. [5] Fukai, R., et al. (2025), Meteorit Planet Sci. [6] Pilorget, C., et al. (2025) [7] Bibring et al. (2017) Astrobiology 17, 621-626, [8] Pilorget C. et al. (2021) Nat. Astron. 6, 221-225, Nat Commun 16, 9532. [9] Dionnet, Z. et al. (2023) Meteorit Planet Sci, 59,10.1111/maps. 14068. [10] Amano, K. et al. (2023), Sci Adv. 9, eadi3789. [11] Onuma, H. et al. Planet. Sci. J. 7 11 (2026) [12] Lantz, C. et al. (2024) Planet. Sci. J. 5 201. [13] Le Pivert-Jolivet T. et al. (2023) Nat. Astron. 7, 1445-1453, [14] Furukawa S. et al. (2025) Sci Rep 15, 14613 [15] Noguchi, T., et al. (2022). Microscopy and Microanalysis, 28(S1), 2726–2728. [16] Nakato, A., Yada, T., Nishimura, M. et al. Earth Planets Space 75, 45 (2023). [17] Hamilton, V.E., et al. (2019). Nat Astron 3, 332–340. 2019 [18] Kitazato, K. et al. (2019), Science364,272-5.

How to cite: Nardelli, L., Brunetto, R., Pilorget, C., and Hatakeda, K. and the ISAS/IAS MicrOmega Curation Team: Spectral variations between Bennu and Ryugu grains: probing alteration processes through a NIR–MIR spectral survey at the ISAS/JAXA Curation Center, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-113, https://doi.org/10.5194/epsc2026-113, 2026.

Erg Chech 002 is known as the ‘oldest andesite of the Solar System’ thought to have formed in the primitive crust of an early accreted and differentiated planetesimal in the beginning of the Solar System history [1]. Because of its unique spectral features, among which the presence of a small band around 650 nm, it was not found to match any known asteroid spectral type know in the literature [1]. Exploiting the Gaia Data Release 3 (DR3) dataset of visible reflectance spectra of asteroids [2], we found 142 asteroids as potential analogues of laboratory and modeled space-weathered spectra of Erg Chech 002 in a previous study [3]. However, the search for potential analogues of EC 002 using visible-range spectral data is challenging because of the similarity between the spectrum of andesitic and basaltic bodies in this range. Thus, these asteroids remained to be confirmed as analogues of the meteorites by observations in the near-infrared wavelength range, as this range contains diagnostic spectral features representative of specific mineralogies and allowing to distinguish basaltic from andesitic bodies.

As a follow up, we acquired new near-infrared observations with NASA’s IRTF of 20 of the main belt asteroids found as potential analogues of the meteorite [4]. We merged their near-infrared spectra with their visible Gaia DR3 spectra, and we classified the asteroids as a first insight of their composition. Among the observed asteroids, we obtained 16 V-types, one S-complex object, and one A-type. To complete our study, we included the literature spectra of asteroids 10537, 14390 and 7472, that were found to show peculiar spectral signatures (among which a small band around 650 nm), and are hypothesized to be fragments of differentiated planetesimals [5,6,7]. We analyzed the band centers and band area ratios of the observed and literature asteroids, since these diagnostic spectral features are directly related to the mineral composition of the asteroids and were found to be insensitive to space weathering [8]. The comparison of these diagnostic spectral features with those of Erg Chech 002, displayed in Fig.1, showed that none of the observed asteroids constitutes a good analogue for the meteorite. Instead, asteroids 10537 and 14390 emerge as potential matches to Erg Chech 002, and asteroid 7472 may match with a more olivine-rich sample of the meteorite. These asteroids could therefore belong to a new spectral class of andesitic bodies, and objects exhibiting similar spectral features in the main belt should be further studied, in order to better constrain the differentiation processes that took place in the early Solar System.

Fig.1: Band I Centre (BIC) with respect to BII/BI band area ratio (BAR) of Erg Chech 002 (stars), observed asteroids (dots), taxonomic end-members of DeMeo et al. (2009) taxonomy (colored letters), and literature asteroids 10537 (diamond), 7472 and 14390 (triangles). Light grey: mineralogical subdivisions defined in [8].

[1] Barrat, J.-A., Chaussidon, M., Yamaguchi, A., Beck, P., Villeneuve, J., Byrne, D.J., Broadley, M.W., Marty, B., Proc. Natl. Acad. Sci. 2021, 118.
[2] Gaia Collaboration, Galluccio, L., Astronomy & Astrophysics 2023, 674.
[3] Galinier, M., Delbo, M., Avdellidou, C., Galluccio, L., Marrocchi, Y., Astronomy & Astrophysics 2023, 671.
[4] M. Galinier, C. Avdellidou, M. Delbo, L. Galluccio., Icarus 2026, 453.
[5] Duffard, R., Roig, F., 2009. Planetary and Space Science 57 (2), 229–234.
[6] Moskovitz, N.A., Lawrence, S., Jedicke, R., Willman, M., Haghighipour, N., Bus, S.J., Gaidos, E., The Astrophysical Journal Letters 2008, 682.
[7] Leith, T.B., Moskovitz, N.A., Mayne, R.G., DeMeo, F.E., Takir, D., Burt, B.J., Binzel, R.P., Pefkou, D., Icarus 2017, 295, 61–73.
[8] Gaffey, M.J., Mineralogy of asteroids, in: AIP Conf. Proc., Eds. Telles, E., Dupke, R., Lazzaro, D., AIP Publishing, 2011, Rio de Janeiro; pages: 129-169.

Acknowledgements: MG and MD acknowledge financial support from CNES and the Action Specifique Gaia. MD is Leverhulme Visiting Professor at the University of Leicester with financial support from the Leverhulme Trust (UK). MD, CA, and LG acknowledge financial support from the ANR ORIGINS (ANR-18-CE31-0014). MG, CA and MD were Visiting Astronomers at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. This work has made use of data from the European Space Agency (ESA) mission Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

How to cite: Galinier, M., Avdellidou, C., Delbo, M., and Galluccio, L.: Near-infrared spectroscopy search for analogues of andesitic meteorite Erg Chech 002 in the Main Belt, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-523, https://doi.org/10.5194/epsc2026-523, 2026.

Abstract

Samples returned from the C-type asteroid Ryugu by the Hayabusa2 mission offer a unique window into pristine carbonaceous material from the early Solar System. Here we report results from two Raman-based investigations of Ryugu fragments A0542 (0.5 mg) and A0552 (0.7 mg).

First, using Raman spectroscopy performed after applying organic extraction to the samples for mass spectrometry analysis, we were able to identify the mineralogy more clearly i.e. we identified not only dolomite, magnetite, and pyrrhotite, easily detected by Raman spectroscopy, but also phyllosilicate-related phases, hydroxyapatite Ca₅(PO₄)₃(OH) and eskolaite (Cr₂O₃), which have been identified for the first time by Raman spectroscopy in Ryugu samples. Most importantly, we have detected for the first time in Ryugu the sulfide mineral erdite (NaFeS₂·2H₂O), which provides new constraints on aqueous alteration and sulfur geochemistry in primitive carbonaceous bodies.

In addition, Principal Component Analysis (PCA) applied to a Raman spectral database including Ryugu and a suite of meteoriotes allowed us to explore the spectral and compositional relationships between Ryugu and other primitive Solar System materials.

Introduction

Samples returned from the C-type asteroid Ryugu by the Hayabusa2 mission provide a unique opportunity to investigate pristine carbonaceous material from the early Solar System [1]. Our team received two Ryugu fragments, A0542 (0.5 mg) and A0552 (0.7 mg), through JAXA’s 4th Ryugu sample announcement of opportunity. This study addresses two complementary scientific objectives: (i) detailed mineralogical characterisation of Ryugu by Raman spectroscopy under reduced-fluorescence conditions enabled by an organic extraction step; and (ii) application of multivariate data analysis to Raman spectral databases.

Raman spectroscopic analysis of carbonaceous chondrites is commonly hindered by intense fluorescence that can obscure diagnostic spectral features beyond the D and G bands of organic matter [2]. By exploiting the organic extraction step required for high-resolution mass spectrometry (HRMS), we significantly reduced fluorescence interference and expanded the detectable mineral inventory of Ryugu samples.

On the other hand, we used multivariate techniques applied to Raman spectroscopy analysis. Multivariate statistical techniques, and in particular Principal Component Analysis (PCA) [3], offer a powerful tool to reduce the dimensionality of datasets while preserving the variance that distinguishes samples of different origin or composition. Here we apply PCA and k-means clustering to a comprehensive Raman spectral database that includes Ryugu samples alongside carbonaceous chondrites, with the aim of identifying the spectral relationships between Ryugu and other primitive carbonaceous bodies, particularly the CM chondrites Aguas Zarcas and Kolang, and establishing a validated spectral classification framework for primitive carbonaceous materials.

Materials and Methods

Two Ryugu fragments were used for these analyses. The smaller fragment, A0542 (0.5 mg), was analyzed without any prior treatment: Raman spectra were acquired directly and used for the multivariate PCA database. The larger fragment, A0552 (0.7 mg), was subjected to organic extraction for high-resolution mass spectrometry (HRMS), and micro-Raman measurements were subsequently performed on the residual material after this extraction step.

Micro-Raman measurements were performed on Ryugu particles from sample A0542, measured directly in the JAXA capsule after removal of the sapphire window. All measurements were carried out using a Renishaw inVia Qontor system (532 nm, 1800 l/mm grating, 50× objective).

For multivariate analysis, 100 Raman spectra per sample were acquired under identical conditions from Ryugu, Aguas Zarcas (CM2), Kolang (CM1/2), NWA 14792 (CR2), Los Vientos 123 (CO3), El Médano 485 (CO3), Paposo 087 (L6) and Zagami martian meteorite. All spectra were pre-processed uniformly: clipping to 100–1800 cm⁻¹, cosmic-ray removal, baseline subtraction, and z-score normalization. PCA and k-means clustering were then applied to reduce spectral dimensionality and identify grouping patterns and compositional relationships among primitive carbonaceous materials, particularly between Ryugu asteroids and the meteorites.

Results and Discussion

The results of the microRaman analysis show, in addition to confirming previously reported phases such as dolomite, magnetite, and pyrrhotite, the identification of phyllosilicate-related phases, hydroxyapatite, and eskolaite (Cr₂O₃). Most significantly, we report the first identification by Raman spectroscopy of erdite (NaFeS₂·2H₂O) in Ryugu samples, whose presence provides new constraints on aqueous alteration processes and sulfur geochemistry in primitive carbonaceous bodies (Figure 1D). It is worth noting that neither eskolaite nor erdite were detected during previous SEM investigations in these samples [4], which demonstrates the high interest of this methodological approach.

Figure 1. Raman spectra corresponding to different mineral phases identified for the first time by Raman spectroscopy in Ryugu sample: (A) phyllosilicate-related phase (B) hydroxyapatite,, (C) eskolaite, (D) erdite.

PCA applied to the spectral database reveals clear compositional relationships among the primitive carbonaceous materials studied. Ryugu clusters most closely with the CM chondrites Aguas Zarcas and Kolang, consistent with their shared aqueous alteration histories and carbonaceous compositions. This chemometric approach provides a robust framework for characterising the spectral affinities of Ryugu with other primitive Solar System materials.

Conclusions

Performing Raman spectroscopy after organic extraction significantly enhances mineral detection in Ryugu samples by suppressing fluorescence, leading to the first identification of erdite in Ryugu and providing new constraints on aqueous alteration and sulfur chemistry on the parent body. PCA of a comprehensive Raman spectral database reveals clear compositional affinities between Ryugu and the CM carbonaceous chondrites Aguas Zarcas and Kolang, establishing Raman spectral signatures as a robust tool for characterising the relationships among primitive carbonaceous bodies in the Solar System.

References

[1] Watanabe, S. et al. (2019). Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu. Science, 364, 268–272.

[2] Bonal et al., 2024 — The thermal history of Ryugu based on Raman characterization of Hayabusa2 samples, Icarus 408:115826.

[3] Wold, S., Esbensen, K., & Geladi, P. (1987). Principal component analysis. Chemometrics and Intelligent Laboratory Systems, 2(1–3), 37–52.

[4] Sánchez-España et al. (2025) Sulfides and phyllosilicates of the Ryugu asteroid: mineral controls of S- and N-sp

How to cite: J. Bonales, L., Menor-Salván, C., Blanco, C., Prieto-Ballesteros, O., Cho, Y., Sánchez-España, J., G. Moral, A., de Dios-Cubillas, A., Fernández-Sampedro1, M., P. Canora, C., Benito-Parejo, M., and Rodriguez-Prieto, J. A.: Raman Spectroscopy of Ryugu: Mineralogy and Multivariate Analysis, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-785, https://doi.org/10.5194/epsc2026-785, 2026.

Introduction

Asteroid parent bodies likely formed as porous aggregates whose internal structure evolved through thermal processing and progressive consolidation. One process considered important for the evolution of many ordinary chondrites and highly thermally altered carbonaceous chondrites is sintering. This process leads to material transfer within the body and to the formation of competent necks between individual grains, enhancing macroscopic properties such as thermal conductivity and mechanical strength (Neumann et al., 2014; Gail et al., 2015; Henke et al., 2016; Sakurai et al., 2025).

In this context, we perform laboratory experiments on sintered sphere packings with varying sinter degree and volume filling factor, measuring their thermal conductivity and tensile strength. A comparison with a numerical model developed in parallel (Agrawal et al., EPSC 2026), as well as with various analytical approaches, is intended to improve our understanding of the relationship between microscopic grain contacts and macroscopic properties. In particular, this comparison will help to identify the capabilities and limitations of existing models.

Laboratory experiments

For this study, we used soda-lime glass beads to produce a total of 12 samples with systematically varied sinter degree (expressed as mean sinter neck radius), 8 samples with reduced volume filling factors, and one reference sample of poured glass beads without sintering.

For the samples with varied sinter degree, glass beads were poured into a crucible and sintered in a furnace for 24 hours at temperatures of 635 °C, 640 °C, and 650 °C. Increasing temperature not only promoted the growth of sinter necks, but also induced global sample shrinkage, leading to an increase in volume filling factor from slightly to strongly sintered states.

For the samples with reduced volume filling factor, glass beads were mixed with sodium carbonate salt grains in mass ratios of 3:1 and 5:1. After sintering, the salt was removed by washing under running water, leaving behind a more porous sintered glass bead network. Since the presence of salt strongly affects the sintering behaviour, the resulting sinter degree could not be controlled easily, and therefore both volume filling factor and sinter state vary between samples.

Thermal conductivity measurements were performed under vacuum conditions (pressure below 10^-4 mbar) at room temperature using a transient hot disk sensor. The sensor is placed between two sample disks and acts as both heater and thermometer. From the applied heat input and the resulting temperature evolution of the disk, the thermal conductivity of the samples is derived (Gustafsson, 1990; Gustavsson et al., 1994; Bohac et al., 2000).

Tensile strength measurements will be conducted by gluing the samples to a sample holder on opposite sides and applying a controlled tensile load until failure, while recording the applied force and the cross-sectional area at the fracture plane (Blum et al., 2006).

Microscopic parameters such as sinter neck size distributions, global volume filling factors, volume filling factor gradients, and coordination numbers are determined or will be determined using secondary electron microscopy (SEM) and micro-computed tomography (µCT).

The results of the thermal conductivity measurements (Fig. 1) show good agreement with the model by Arakawa et al. (2019), who propose a linear dependence on the sphere contact radius (mean neck radius in this case) and an approximately quadratic dependence on the volume filling factor. Despite the fact that the samples do not contain uniform sinter neck sizes, but rather a polydisperse distribution, and although the model was originally developed for weakly bound van der Waals contacts rather than strongly sintered sphere packings, it reproduces the experimental data reasonably well.

Figure 1: Thermal conductivity of sintered glass-bead samples, normalized to the glass material conductivity, as function of the mean relative sinter neck size and the global volume filling factor. The relative neck radius refers to the ratio of the neck radius to the sphere radius. The thermal model from Arakawa et al (2019) is plotted for various volume filling factors as a function of the mean sinter neck size for comparison. For this model, a coordination number of  is currently assumed, but will soon be verified using µCT scans of the samples.

Next steps

As a next step, the tensile strength of the sintered glass bead samples will be measured and correlated with their thermal conductivity and microstructure. Combined, these measurements should improve our understanding of how grain-scale properties control the thermal and mechanical behaviour of asteroid material and help to relate remote sensing observations to laboratory analyses of returned samples.

References

Arakawa et al. (2019): Thermal conductivity and coordination number of compressed dust aggregates. Icarus, Vol. 324 (2019). https://doi.org/10.1016/j.icarus.2019.01.022

Blum et al. (2006): The Physics of Protoplanetesimal Dust Agglomerates. I. Mechanical Properties and Relations to Primitive Bodies in the Solar System. The Astrophysical Journal, Vol. 652 (2006). https://doi.org/10.1086/508017

Bohac et al. (2000): Parameter estimations for measurements of thermal transport properties with the hot disk thermal constants analyzer. Rev. Sci. Instrum., Vol. 71 (2000). https://doi.org/10.1063/1.1150635

Gail et al. (2015): Thermal evolution and sintering of chondritic planetesimals II. Improved treatment of the compaction process*. A&A, Vol. 576 (2015).  https://doi.org/10.1051/0004-6361/201424278

Gustafsson (1990): Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials. Rev. Sci. Instrum., Vol. 62 (1990). https://doi.org/10.1063/1.1142087

Gustavsson et al. (1994): Thermal conductivity, thermal diffusivity, and specific heat of thin samples from transient measurements with hot disk sensors. Rev. Sci. Instrum., Vol. 65 (1994). https://doi.org/10.1063/1.1145178

Henke et al. (2016): Thermal evolution and sintering of chondritic planetesimals III. Modelling the heat conductivity of porous chondrite material. A&A, Vol. 589 (2016).
https://doi.org/10.1051/0004-6361/201527687

Neumann et al. (2014): Modelling of compaction in planetesimals*. A&A, Vol. 567 (2014). https://doi.org/10.1051/0004-6361/201423648

Sakurai et al. (2025): Experimental study on thermal and mechanical properties of sintered glass materials: Implication for physical properties of primordial porous materials in the solar system. Icarus, Vol. 441 (2025). https://doi.org/10.1016/j.icarus.2025.116729

How to cite: Goldmann, M., Agrawal, B., Jennings, L. A., Greshake, A., Biele, J., Blum, J., Güttler, C., Grott, M., Klemme, S., Knollenberg, J., Patzek, M., and Gundlach, B.: Laboratory experiments on thermal and mechanical properties of sintered glass beads, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-570, https://doi.org/10.5194/epsc2026-570, 2026.

Airless planetary bodies experience repeated diurnal temperature variations that generate cyclic thermal stresses in surface rocks. These stresses can drive thermal fatigue, a process considered important for rock breakdown and regolith production on asteroid surfaces [1,2]. However, the long-term effectiveness of thermal fatigue, and whether it remains efficient across different asteroid materials, remains poorly constrained. 

We experimentally investigate thermal fatigue in the ordinary chondrites L3 Aba Panu and LL5 Chelyabinsk by subjecting them to 100 thermal cycles with a temperature amplitude of ΔT = 190 K. Acoustic emission monitoring was used to detect real-time fracturing. Our results show that both ordinary chondrites produce very limited fracture activity, reaching early saturation plateaus, an indication of a strong stress-memory effect. Extending these observations to the meteorite dataset from previous studies [3], Figure 1 shows that the porous carbonaceous chondrites CM2 Aguas Zarcas and CV3 Allende, generate higher cumulative AE activity, whereas the ordinary chondrite H3-5 shows intermediate activity, and L3 and LL5 meteorites reach plateaus early during thermal cycling.   We therefore quantify the efficiency of thermal fatigue across lithologies and show that asteroid surface breakdown is material dependent.

We additionally use Rise Amplitude – Average Frequency (RA-AF) analysis to evaluate the source characteristics of thermally induced cracking. Our findings indicate no systematic difference between heating and cooling related acoustic emission events, suggesting that fracture behaviour is not controlled by the direction of temperature change. However, the RA-AF distributions show differences in fracture behaviour between meteorite types. We conclude that porous carbonaceous chondrites are more susceptible to thermally driven breakdown that the ordinary chondrites investigated here.  

Figure  1. Cumulative acoustic emission hits as a function of thermal cycle number for five meteorite samples subjected to repeated thermal cycling. Periods of rapid increase correspond to increased fracture activity. Plateaus indicate intervals with little or no detectable fracturing.

[1] Delbo M., Libourel G., Wilkerson J., Murdoch N., Michel P., Ramesh K.T., Ganino C., Verati C., Marchi S., Nature 2014, 508, 233–236.

[2] Molaro J.L., Byrne S., Langer S.A., Journal of Geophysical Research: Planets 2015, 120, 255–277.

[3] Latsia N., Kaufmann E., Tsirvoulis G., Suhonen H., Granvik M., Borg J., Hagermann A., Icarus 2026, 455, 117130.

How to cite: Latsia, N., Tsirvoulis, G., Kaufmann, E., Suhonen, H., Granvik, M., Borg, J., and Hagermann, A.: Asteroids under stress, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-371, https://doi.org/10.5194/epsc2026-371, 2026.

The Size Frequency Distribution (SFD) of Kuiper Belt Objects (KBOs) has long been used to infer the formative history of these objects. Telescopic surveys have revealed an SFD remarkably consistent in overall shape with predictions from models of formation through streaming-instability and pebble cloud collapse (SI+PC) [1,2,3]. Inference of the populations too small (too faint) for direct telescopic detection is possible from the crater SFD on Pluto-Charon as seen by New Horizons [4], and the SFD of the Jupiter Family comets (JFCs) [5] which originate from the excited KBO population. Inference of the SFD from each of these indirect observations presents additional challenges, notably the assumed impactor scaling law and the difficulty in debiasing the observed JFC population. The Jupiter Trojans provide additional inference, as they are predicted to share a common primordial origin with the dynamically excited KBOs [6]. In Figure 1. We compile KBO SFD data from observational surveys, including ground and space-based surveys, the Pluto-Charon cratering record, and the SFD inferred from the JFCs. We also include Jupiter Trojan SFD data [7,8]. All KBO SFD data are in strong agreement when scaled to a common albedo and offset for relative on-sky densities and production functions. Notable conclusions are:

• Both the dynamically excited and cold KBO SFDs are consistent with expectations of SI+PC over most of the observable range, turning over to a shallower slope only at small object sizes (D~2 km).
• The JFC and cratering SFDs prove remarkably consistent, showing that the turnover is an intrinsic property of the KBO population and not due to cometary mass loss or assumed crater scaling laws.
• As the cold classical population comprises much of the Pluto-Charon, the turnover appears to be a primordial feature common to both the dynamically cold and excited populations.
• The differences exhibited of the Trojans for D<10 km are likely collisionally driven.

Figure 1: The compiled cumulative KBO SFD, renormalized by radius squared, akin to an R-plot. Black dashed lines are data drawn from the Pluto-Charon cratering record seen from New Horizons. The blue data are telescopic data (CLASSY: dark, JFCs: light) and the OSSOS limits are traced in light black. The slope inferred from the JWST Pencilbeam is shown in red. Trojan data are shown in magenta. The yellow curve shows the best-fit exponential taper which matches expectations from SI+PC simulations. 

References

1. Li, Rixin et al., Demographics of Planetesimal Formation by the Streaming Instability 2019, ApJ 885, 69L.

2. Kavelaars, J.J. et al., OSSOS Finds and Exponential Cutoff in the Size Distribution of the Cold Classical Kuiper Belt, 2021, ApJ 920L, 28K.

3. Eduardo, M. et al. The Luminosity Function of Ultra-Faint Trans-Neptunian Objects Detected by JWST, 2025, epsc.conf.1132E

4. Singer et al., Impact Craters on Pluto and Charon indicate a deficit of small Kuiper belt objects, 2019, Science, 363, 955S.

5. Fernandez, C., Thermal properties, sizes, and size distribution of Jupiter-family cometary nuclei, 2013, Icarus 226, 1138F.

6. Nesvorny, D. et al., Capture of Trojans by Jumping Jupiter, 2013, ApJ 768, 45N.

7. Yoshida et al., Small Jupiter Trojans Survey with Subaru/HSC, 2017, AJ, 154, 71Y.

8. Wong, I. and Brown, M., The Differing Magnitude Distributions of the Two Jupiter Trojan Populations, 2014, AJ 148, 112W.

How to cite: Fraser, W., Trilling, D., Singer, K., and Eduardo, M.: The Compiled Size Frequency Distribution of Kuiper Belt Objects, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-636, https://doi.org/10.5194/epsc2026-636, 2026.

The internal structure of asteroids remains poorly understood and is based entirely on inferences drawn from remote-sensing observations of their surfaces and theoretical modelling. Electromagnetic waves from space-based radars represent the most advanced method for characterising the internal structure of asteroids and comets [Herique2018]. In particular, the ESA-HERA mission, launched in October 2024 and equipped with a radar system called JuRa, will encounter the binary S-type asteroid 65803 Didymos in the coming months [Michel2022]. The main body of Didymos has a diameter of approximately 800 m, and Dimorphos, the asteroid’s small moon, has a diameter of approximately 160 m. Given the center frequency of the JuRa radar, the diameter of the main body corresponds to 160 λ in free space, and that of the small moon to 32 λ in free space.  

Imaging the interiors of these asteroids requires specific advances in both measurement techniques and imaging algorithms. As far as imaging procedures are concerned, these difficulties are mainly due to the large size of these structures relative to the wavelength, as well as their significant contrast in terms of permittivity and their high level of electromagnetic scattering.

This work focuses on a method based on frequency-domain diffraction tomography aimed at obtaining an image of the interior of these asteroids exploiting the observation equation (Eq.(1)) which relates the scattered field to the induced current [Dufaure2023]. The procedure has been optimised to take full account of polarisation and to reconstruct all components of the induced current vector (Eq.(2)).

For each frequency, a 3D image of the target is obtained, and the images from all frequencies are then combined to produce the final image. This imaging technique enables structural imaging of the interior of asteroids with relatively low memory requirements, even if the target is large.

To test this imaging procedure, we used the microwave analogy, making use of the scale invariance rule in Maxwell’s equations, which involves reducing the size of the analogue and the wavelength by the same factor,  whilst keeping the permittivity constant [Vaillon2014]. We have designed and produced   3D-printed scale models of the asteroid 25143 Itokawa [SorsaMD202] and the moon Dimorphos [Pajola2026]. The microwave scattering properties of these analogues were measured in an anechoic chamber using the MIMOSA equipment at the Fresnel Institute [Geffrin2025] (Figure 1). The configuration was chosen as quasi-monostatic to be similar to those used by the JuRa radar on the HERA mission and by the radar to be carried on board the Ramses mission to (99942) Apophis.  Imaging results obtained from different analogues of asteroids will be presented and analyzed.

Figure 1. Measurement of the microwave scattering of an analogue with MIMOSA setup of the Institut Fresnel, Marseille

References

[1] A. Herique and al. Direct observations of asteroid interior and regolith structure: Science measurement requirements, Advances in Space Research, 2018.

[2] P. Michel and al. The esa hera mission: Detailed characterization of the dart impact outcome and of the binary asteroid(65803) didymos, The Planetary Science Journal, 2022.

[3] A. Dufaure, C. Eyraud, L.-I. Sorsa, Y. O. Yusuf, S. Pursiainen, and J.-M. Geffrin. Imaging of the internal structure of an asteroid analogue from quasi-monostatic microwave measurement data - i. the frequency domain approach, Astronomy and Astrophysics, 674(A72), 2023.

[4] R. Vaillon and J.-M. Geffrin. Recent advances in microwave analog to light scattering experiments,  Journal of Quantitative Spectroscopy and Radiative Transfer, 146, 2014.

[5] Liisa-Ida Sorsa, Christelle Eyraud, Alain H´ erique, Mika Takala, Sampsa Pursiainen, and Jean-Michel Geffrin, Complex-structured 3d-printed wireframes as asteroid analogues for tomographic microwave radar measurements, Materials and Design, 198, 2021.

[6] Topi Pajala, Christelle Eyraud, Alain H´ erique, Jean-Michel Geffrin, and Sampsa Pursiainen, Rubble pile asteroid radar analogue model for dimorphos — the asteroid moon of 65803 didymos, Acta Astronautica, 2026.

[7] J. M. Geffrin and al. Mimosa: a new setup for scattering and diffraction measurements, 2025 IEEE Conference on Antenna Measurements and Applications (CAMA) (pp. 1-3), 2025.

How to cite: Eyraud, C., Pajala, T., Berquin, Y., Henry, G., Pursiainen, S., Hérique, A., and Geffrin, J.-M.: Structural imaging of asteroid analogues from lab-Measurements considering the polarization, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-906, https://doi.org/10.5194/epsc2026-906, 2026.

Space weathering (SW) progressively alters the optical and spectral properties of airless bodies through solar wind irradiation and micrometeorite impacts, modifying surface reflectance (e.g., Pieters et al. 2000). As a time-dependent process, SW provides a useful proxy for constraining surface exposure age and resurfacing histories of asteroids, offering insights into regolith dynamics, collisional evolution, and orbital environment effects across the inner Solar System. This work develops and applies machine-learning-based approaches to quantitatively estimate model-based SW ages of chondritic asteroids at multiple scales, from asteroid families to individual near-Earth asteroids (NEAs).

This study introduces a novel ensemble machine-learning framework design to exploit visible and near-infrared reflectance spectra. The framework combines a convolutional neural network (CNN), optimized to capture subtle wavelength-dependent spectral features, with four tree-based regression models, namely, gradient boosting regressor (GBR), K-nearest neighbour (KNN) regressor, extra tree regressor (ETR), and random forest regressor (RFR). These models are trained on published laboratory reflectance spectra of space-weathered silicate material, including olivine, pyroxene, olivine-pyroxene mixtures, and ordinary chondritic meteorites. SW effects are simulated experimentally through H⁺ irradiation to reproduce solar wind irradiation and pulse laser irradiation to mimic micrometeorite impacts, allowing the model to learn time-dependent spectral evolution under controlled conditions. The ensemble approach improves robustness and generalization by integrating nonlinear feature extraction with physically interpretable regression trends.

This framework is applied to spacecraft-derived spectral datasets of the near-Earth asteroids (25143) Itokawa and (433) Eros. For Itokawa, reflectance spectra obtained by the Near-Infrared Spectrometer onboard the Hayabusa mission reveal a highly heterogeneous distribution of SW ages, ranging from ~1.9 kyr to 2.5 Gyr. These results indicate rapid solar wind-driven alteration combined with efficient regolith turnover, consistent with the asteroid’s small size, rubble-pile structure, and observed grain-scale mobility. Older SW ages correspond to relatively stable regions such as Arcoon, while younger surface reflects recent resurfacing or regolith disturbance. In contrast, spectra from the Near-Infrared Spectrometer onboard NEAR Shoemaker show that Eros exhibits more spatially uniform and generally older SW ages (~0.4-2 Gyr), dominated by micrometeorite, reflecting a more mature yet dynamically evolving surface. These results align with previous spectral studies and with laboratory analyses of returned Hayabusa samples, validating the machine-learning approach.

Furthermore, we extend the study to population-level analysis using Sloan Digital Sky Survey (SDSS) data. A supportive Vector Regression (SVR) model, independently trained on laboratory SW spectra, is applied to SDSS visible reflectance data to estimate SW ages of S-type and V-type asteroid families, including Flora, Massalia, Vesta, Eunomia, Maria, Merxia, and Koronis (Figure 1). Several families display median SW age broadly consistent with their reported dynamical ages, suggesting quasi-steady surface evolution following family-forming collisions. In contrast, other families exhibit younger SW ages systematically, pointing to ongoing resurfacing driven by impacts, YORP-induced spin evolution, and slower SW rates at larger heliocentric distances. These results highlight the complex interplay between SW, regolith reworking, and dynamical processes on the timescale of hundreds of millions of years.

Overall, this work demonstrates that machine-learning techniques calibrate with laboratory spectral measurements, enabling a quantitative interpretation of asteroid surface exposure histories, moving beyond the largely qualitative or semi-quantitative approaches used in SW studies (e.g., Willman & Jedicke 2011, Nesvorny et al. 2005). By linking high-resolution spacecraft observations and large asteroid photometric datasets, the work provides a unified framework for interpreting SW signatures across spatial scales (Hapke 2001, Brunetto et al. 2015). The derived SW ages, however, are dependent on model-derived irradiation and impact conditions of the interplanetary environment, as well as assumptions embedded in laboratory simulations of solar wind and micrometeorite processes. Despite these limitations, the results offer new quantitative constraints on the interplay between SW, regolith dynamics, and asteroid evolution, advancing our understanding of surface processes on airless bodies through the Solar System.

Figure 1: SW age of asteroid families and corresponding box plot showing the statistical distribution of each family.

Brunetto et al. 2006 DOI 10.1051/0004-6361/202243587

Hapke 2001 DOI 10.1029/2000JE001338

Nesvorny et al. 2005 DOI 10.1016/j.icarus.2004.07.026

Pieters et al. 2000 DOI 10.1111/j.1945-5100.2000.tb01496.x

Willman & Jedicke 2010 DOI 10.1016/j.icarus.2010.08.022

How to cite: Palamakumbure, L., Korda, D., and Kohout, T.: Aging in space: Space weathering age of chondritic asteroids, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-308, https://doi.org/10.5194/epsc2026-308, 2026.

Trans-Neptunian Objects (TNOs) hold valuable clues about planetary formation processes. Recent spectroscopic observations of their surfaces indicate that they host C–O–H species, including CO₂, H₂O, CH₄, and complex organic molecules, suggesting the possibility of substantial internal carbon reservoirs. JWST observations of TNOs up to 800 km in diameter show surface ices that include carbon-bearing species such as CO₂, CO, CH₃OH, and complex organic molecules (Pinilla-Alonso et al., 2024). Although surface compositions vary, no systematic trend with object size suggests that these variations are dominated by surface processes.

The surface compositions of larger TNOs display strong methane bands in addition to H₂O ice and CO₂ (Brown, 2012), and recent hydrogen and carbon isotopic measurements of CH₄ on Eris and Makemake suggest an internal origin for these species (Grundy et al., 2024). The bulk densities of icy moons and dwarf planets support the idea that their refractory cores contain a mixture of CI chondrite and carbonaceous material, reinforcing the idea that carbon-bearing molecules at their surfaces may originate from internal activity.

In this study, we use thermodynamic modeling with Perple_X (Connolly, 2005) to investigate carbon speciation and fluid–mineral equilibria within TNO interiors under hydrated conditions. Models were computed over a wide range of oxygen fugacity (logfO₂= −10 to −50) and P–T conditions representative of TNO interiors (300–1300 K, 1–7000 bar), assuming a CI elemental composition with carbon content varying from a saturated to a less carbon-enriched system. The internal temperature of TNOs is derived from a thermal model of mid- and large-sized TNOs, Charon and Pluto, assuming 23% leaching of radioactive elements at a differentiation time of 500 Myr. These results are coupled with those from a kinetic model of carbonaceous matter evolution KIMCAM-E (Delarue et al., 2026) to constrain the evolution of metamorphic fluid composition with temperature and size.

The results for the core composition of carbon-saturated systems reveal that at high fO₂ and low temperatures, carbonates and hydrated minerals are stable, whereas at lower fO₂ and higher temperatures, hydrated minerals are no longer stable and carbon is progressively reduced to graphite. Pressure does not significantly influence these transitions, whereas changes in oxygen fugacity and temperature strongly affect the gas species released from the mineral assemblage into metamorphic fluids. Specifically, the results for COH fluid composition reveal that at high temperatures and in carbon-saturated systems, reduced phases such as methane are stable, while at lower temperatures, oxidized species and CO₂ are favored, consistent with the kinetic model. If the carbon content is decreased, methane is replaced by hydrogen-rich fluids, while carbon dioxide is replaced by water-rich fluids.

The predicted metamorphic evolution of mineral assemblages shows that the internal composition is directly reflected in fluid composition, which may eventually reach the surface and form the ice observed on TNOs. By constraining the internal core temperature from thermal models, conditions for TNO interiors can be projected onto phase diagrams derived from the Perple_X thermodynamic model in order to derive their mineralogy, and the fluids generated (figure). The temperature of the core is a function of object size; the larger the TNO, the higher the temperature it can reach for a given composition. As shown by the line of evolution, as temperature—and thus core size—increases, the core composition and fluids become more reduced.

Small TNOs (typically <800 km in diameter) have cold cores and high oxygen fugacity, supporting the idea of oxidized interiors where water and CO₂ are stable. Mid-sized TNOs, such as Charon, Quaoar, and Haumea, reach core temperatures high enough to fall within the hydrated domain, where water is the major component of the fluid. In contrast, large TNOs such as Eris, Makemake, Triton, and Pluto, with higher core temperatures and/or lower oxygen fugacity, likely host more reducing phases, leading to the release of reduced fluid species such as methane.

Overall, these findings suggest that redox-driven transformations and metamorphism of carbonaceous matter have significantly shaped the interiors and volatile emissions of icy, carbon-rich bodies in the outer Solar System, influencing their potential habitability. Implications for interpretation of JWST and earlier spectroscopic observations will be discussed.

Figure: Core conditions for TNOs of varying sizes are reported on phase diagrams predicted in this study. Fluid composition evolves from oxidized (CO₂-rich) to reduced (CH₄-rich) when size and internal temperature increases. Mineralogy evolves from carbonated to hydrous then anhydrous minerals.

Acknowledgement

This work was supported by Institut National des Sciences de l'Univers through Programme National de Planétologie, by the Agence Nationale de la Recherche (ANR, project OSSO BUCO, ANR-23-CE49-0003) and by the European Union (ERC, PROMISES, project #101054470). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

References

Brown, M. E. (2012). https://doi.org/https://doi.org/10.1146/annurev-earth-042711-105352

Connolly, J. A. D. (2005). https://doi.org/10.1016/j.epsl.2005.04.033

Grundy, W. M. et al. (2024). https://doi.org/10.1016/j.icarus.2023.115923

Pinilla-Alonso, N. et al.  (2024). https://doi.org/10.1038/s41550-024-02433-2

How to cite: Confortini, G., Delarue, C., Reynard, B., and Sotin, C.: Thermodynamic, thermal and kinetic modeling of carbon fate in TNO interiors, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-111, https://doi.org/10.5194/epsc2026-111, 2026.

Near-Earth asteroid (469219) Kamo‘oalewa (provisional designation 2016 HO3) is the first target of the ongoing Tianwen-2 sample-return mission. Ground based observations measured its rotation period at only about 28 minutes [1], while the low thermal inertia value (Γ = 150 or 181 J m⁻² K⁻¹ s^−1/2) estimated from the Yarkovsky effect determination implies a thermal insulating layer on the surface [2]. The first goal of this work is to facilitate the sample collection of the Tianwen-2 mission with a prediction of the current status of the possible regolith layer on Kamo‘oalewa. The second goal is to build a framework of regolith evolution on small fast-rotating asteroids as a reference for future observations and studies.

Figure 1: Net outward normal acceleration from gravity, centrifugal force, and van der Waals forces. Under the choice of surface parameters, cleanliness S =0.1 and particle radius r = 1 mm, the van der Waals force dominates the surface dynamical environment.

On the basis of a shape model reconstructed from light curve data, we use numerical simulations to calculate the acceleration from gravity, fast rotation, and the van der Waals cohesive force. Figure 1 shows the net outward normal acceleration for an ellipsoidal shape model on the left and a reconstructed model on the right. We found that the poles have a higher chance of hosting regolith, and the regolith may also rest on the walls of craters that face toward the rotation axis, making them potential sample collection sites. On the surface of Kamo‘oalewa, the van der Waals force is strong enough to hold particles up to several centimeters against the centrifugal force [3].

Surface dynamic environment analysis alone is not sufficient to determine the existence of regolith on Kamo‘oalewa. We further compared the mass change rates of regolith due to thermal fatigue fragmentation, micro-impact ejecta escape and electrostatic dust lofting (illustrated in Fig. 2), which are the smae processes considered in [4] for sub-kilometer asteroids.

Figure 2: Physical processes that produce and remove regolith on Kamo‘oalewa.

We use finite element analysis software, Abaqus, to simulate the thermal stress field of Kamo‘oalewa under seasonal and diurnal temperature cycles, with different thicknesses (H) of the regolith layer. At each latitude, the stress excursion at the top of the bedrock as a function of H is incorporated into the ordinary differential equations (ODEs) for regolith total mass and grain size distribution evolutions. The negative feedback between H and thermal fragmentation rate is considered together with regolith removal processes of micro-impact ejecta escape and electrostatic dust lofting in the ODEs.

Figure 3: Equilibrium regolith thicknesses at different latitudes. Panels a and b show the results for models with obliquities γ = 99.3◦ and γ = 45◦, respectively. The equilibrium thickness ranges for ordinary chondrite material are shown by the shaded red areas, and the ones for norite material by the blue. The dashed lines represent scenarios with only diurnal thermal fragmentation, with the same choice of colors for the different materials. The left and right ordinates show dimensionless and dimensional thicknesses of the regolith layer, respectively. In panel a, the black line indicates that the highest H (φ) is allowed by a cohesive strength of 0.055 Pa.

The competition between the regolith production and removal processes results in an equilibrium regolith thickness. As shown in Fig.3, the equilibrium thickness increases with latitude because the seasonal thermal fatigue is more efficient. Under different choices of materials and obliquities and a range of possible fragmentation rates, the equilibrium thickness is estimated to be 0.4-71 mm [5], equivalent to 0.3-53.5 diurnal regolith thermal skin depth. Our prediction of the equilibrium H satisfies both constraints from thermal inertia observations [2] and cohesive strength estimations on airless bodies. We therefore suggest that a thin regolith layer is likely to be observed on Kamo‘oalewa by the Tianwen-2 mission.

Reference

[1] Warner, B. D., Harris, A. W., & Pravec, P. 2021, Asteroid Lightcurve Data Base (LCDB) Bundle V4.0

[2] Fenucci, M., Novakovi´c, B., Zhang, P., et al. 2025, A&A, 695, A196

[3] Ren, J., Wu, B., Hesse, M. A., et al. 2024, A&A, 692, A62

[4] Hsu, H. W., Wang, X., Carroll, A., Hood, N., & Horányi, M. 2022, Nat. Astron., 6, 1043

[5] Ren, J., Wu, B., Liu, W., et al. 2026, A&A, 708, A142

How to cite: Ren, J., Wu, B., Liu, W., Yu, T., and Li, H.: Thin Regolith Layer Anticipated on the Surface of the Tianwen-2 Target Asteroid (469219) Kamo‘oalewa, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-142, https://doi.org/10.5194/epsc2026-142, 2026.

With a rotation period of 3.915341 ± 0.000005 h [1], (136108) Haumea is the fastest‑spinning known body larger than 100 km in the Solar System [2]. Photometric observations reveal a large‑amplitude, nearly sinusoidal light curve, which strongly suggests that Haumea has a triaxial shape [3]. A multi-chord stellar occultation in 2017 provided an elliptical limb with semi-axes lengths of 852 km and 569 km [4], which has been shown to be coherent with a nearly ellipsoidal, hydrostatic body made of an ice shell on top of a hydrated silicate core [5]. A new stellar occultation by Haumea happened on May 4, 2026 [6], which may refine the observational constraints on its shape. This motivates a new study of the range of hydrostatic shapes that Haumea can possibly have and the investigation of whether Haumea’s hydrostatic equilibrium figure must be close to an ellipsoid, or more complex geometries are likely.

We model Haumea as a body made of two or three homogeneous layers, which is justified because self-compression is negligible under the body’s relatively low pressures (<1 GPa). To compute the equilibrium figures, we use the BALEINES code [7], which solves iteratively for the shape of the layers’ boundaries so that they are equipotential surfaces.

Figure 1: Projections of the hydrostatic shape of a critical rotator in the sky plane at the time of the 2017 occultation (left) and the 2026 occultation (right). The ellipsoids obtained in Refs. [4] (O17) and [5] (D19) are shown in white dotted line for comparison.

Our models show that hydrostatic solutions compatible with the 2017 occultation can depart greatly from an ellipsoid, with deviations up to 110 km. The most extreme configurations even exhibit a pinch at the tip of the equatorial major axis, which indicates a state of critical rotation, i.e. the centrifugal and gravitational forces balance exactly each other thereat. Projections of pinched hydrostatic figures onto the sky plane at the epochs of the 2017 and 2026 occultations (Figure 1) show that while the earlier event would mask the pinch, the equatorial major axis being aligned with the line of sight, the latter could reveal substantial departures from an ellipse. This suggests that the recent 2026 occultation has the potential to discriminate between nearly ellipsoidal and strongly non‑ellipsoidal hydrostatic solutions. The modelled internal structures are physically plausible and consistent with current understanding of Haumea’s composition and thermal evolution, with a silicate core and a shell, whose density (830-980 kg/m³) can be interpreted as pure ice or a porous mixture of ice and rock. A third layer can either be a small high-density inner core (~4500 kg/m³), which could correspond to a metal-rich silicate layer, or a partially differentiated region between the core and the shell.

Overall, our results show that Haumea could be significantly different from an ellipsoid and could even be a critical rotator. The 2026 occultation is therefore an important opportunity to constrain the internal structure of the dwarf planet from which formation and evolution scenarios can be derived, with implications for other dwarf planets.

Acknowledgments: This work has been supported by the French Agence Nationale de la Recherche, project Roche, number ANR-23- 240 CE49-0012

References: [1] Lellouch et al. 2010, A&A, 518, L147. [2] Rabinowitz et al. 2006, ApJ, 639, 1238. [3] Lockwood et al. 2014, EM&P, 111, 127. [4] Ortiz et al. 2017, Nature, 550, 219. [5] Dunham et al. 2019, ApJ, 877, 41. [6] Ortiz et al. 2026, MNRAS, 548, stag692. [7] Staelen & Huré 2026, A&A, 708, A316. 

How to cite: Staelen, C., Rambaux, N., Chambat, F., Castillo-Rogez, J., and Charnoz, S.: Non-ellipsoidal hydrostatic shape of Haumea, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-359, https://doi.org/10.5194/epsc2026-359, 2026.

The early differentiation of planetesimals is recorded by iron meteorites. Planetesimal thermal evolution models combined with constraints from Hf–W isotopic core formation ages and inferred core sizes, provide insights into the initial formation conditions of both non-carbonaceous (NC) and carbonaceous (CC) parent bodies (¹Kruijer2020), ²Spitzer2021). Most thermal evolution models have focused on ice-free planetesimals (³Kruijer2014, ⁴Neumann2012, ⁵Neumann2018, ⁶Kaminski2020), or have modelled the effect of water in an inconsistent manner (²Spitzer2021), leaving the physical mechanisms that enable ice-rich planetesimals to undergo differentiation poorly understood. However, the role of ice may significantly affect both heat transport and energy budgets. This leads us to investigate under which conditions ice-bearing planetesimals can undergo efficient heating and differentiation despite the presence of ice.

In this study, we investigate the thermal evolution, core formation times and core sizes of planetesimals using a one-dimensional numerical model. The model solves the heat conduction equation with an implicit finite-volume scheme that includes temperature and pressure-dependent thermal properties of ice, latent heat effects associated with ice sublimation, porosity evolution through cold compaction and sintering, and the onset of silicate convection that takes place at the rheological temperature transition (RTT), when the matrix disaggregate, allowing metal-silicate segregation. We did not take into account the eventual metal and silicate melt migration before the RTT. Because core growth is continuous, the modelled Hf-W core formation time represent an integrated, volume-weighted differentiation age rather than an instantaneous core formation event. We explore a range of initial ice mass fractions and accretion times, assuming instantaneous accretion of a 40 km radius body (⁷Morbidelli2022) and a chondritic composition (⁸Jarosewich1990).

Figure 1: Top panel: Time evolution of the internal temperature of a planetesimal with an initial radius of 40 km formed at 0.6 Myr after CAI with an initial ice mass fraction 31 wt%. Dashed lines represents isotherms. Bottom panel: Corresponding structural evolution of the body.

Fgure 2: Top panels: Modelled Hf-W core formation time as a function of accretion time and ice mass fraction. Bottom panels: Core size as a function of accretion time and ice mass fraction. Left panels: Simulations performed under conduction-only assumption. Right panels: Simulations performed accounting for silicate convection. Black dots represent individual simulations. The colormap represents the interpolated ice mass fraction. The red star represent the simulation result shown in figure 1. Red and blue shaded areas represent the mean ages of the volatile-rich NC and CC iron groups, respectively, as reported in ²Spitzer2021.

Our results show that ice mass fraction is the primary control on thermal evolution. Increasing the initial ice content reduces total content of rocky material that contain the radiogenic heat sources and simultaneously introduce an energy sink through ice sublimation. Ice also strongly affects thermal conductivity, which at low temperatures can exceed that of the rocky matrix by up to a factor of two, thereby enhancing conductive heat transport toward the surface. As a result, ice-rich bodies are more difficult to heat, narrowing the time-range allowing differentiation or even preventing core formation depending on initial conditions.

We also find that the radial structure of thermal conductivity has a major impact on thermal evolution. Since thermal conductivity is directly linked to porosity, the two-stage porosity evolution (cold compaction followed by sintering) plays a key role in regulating heat transport. Denser and sintered regions exhibit significantly higher conductivity, enhancing conductive heat transfer whereas low-conductivity layers act as insulating barriers that thermally decouple the interior from the surface.

The onset of silicate convection introduces a new heat transport regime within the planetesimal. Once triggered, convection rapidly redistribute internal heat and can lead to widespread melting. This convective regime is achieved once the rheological temperature transition is reached, thereby controlling the onset of convection and, consequently, the timing of core formation. Although this transition temperature depends on composition, it varies in a narrow range and therefore exerts only a limited influence on differentiation ages. The onset of convection results in the rapid formation of a metallic core throughout the body. In contrast, conduction-only models predict a gradual decrease in core size with increasing ice mass fraction and accretion time.

References:

¹ TS. Kruijer, T. Kleine and L.E. Borg. The great isotopic dichotomy of the early Solar System. Nature Astronomy. 2020

² F. Spitzer, C. Burkhardt,  F. Nimmo and T. Kleine. Nucleosynthetic Pt isotope anomalies and the Hf-W chronology of core formation in inner and outer solar system planetesimals. Earth and Planetary Science Letters. 2021

³ TS. Kruijer, M. Touboul, M. Fischer-Gödde, KR. Bermingham, RJ Walker and T. Kleine. Protracted core formation and rapid accretion of protoplanets. Science. 2014

⁴ W. Neumann, D. Breuer and T. Spohn. Differentiation and core formation in accreting planetesimals. Astronomy & Astrophysics. 2012

⁵ W. Neumann, TS. Kruijer, D. Breuer and T. Kleine. Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf-W Chronometry of Iron Meteorites. Journal of Geophysical Research: Planets. 2018.

⁶ E. Kaminski, A. Limare, B. Kenda and M. Chaussidon. Early accretion of planetesimals unraveled by the thermal evolution of the parent bodies of magmatic iron meteorites. Earth and Planetary Science Letters. 2020

⁷ A. Morbidelli, K. Baillie, K. Batygin, S. Charnoz, T. Guillot, D. Rubie and T. Kleine. Contemporary formation of early Solar System planetesimals at two distinct radial locations. Nature Astronomy. 2022.

⁸ E. Jarosewich. Chemical analyses of meteorites: A compilation of stony and iron meteorite analyses. Meteoritics. 1990.

How to cite: Mallet, L., Limare, A., and Morbidelli, A.: Coupled effects of ice, porosity, and convection on core formation of iron meteorites parent bodies, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-520, https://doi.org/10.5194/epsc2026-520, 2026.

Recently, the Juno spacecraft’s JADE and MAG instruments have revealed the thermal nature of Europa’s atmospheric source, long predicted for O₂ also at Ganymede. This implies the icy Galilean moon atmospheres are directly regulated by solar heating and coupled to their surface ice temperatures. However, the precise thermal desorption mechanism of O₂ and H₂ is not yet simulated numerically in exosphere general models to date, as a more detailed surface-atmosphere coupling is required to treat the thermally evolving and porous Galilean satellite surfaces. In this light, we couple a multi-layered implicit thermal solver MultIHeaTS to a volatile evolution model to predict the diurnal evolution of H₂O sublimation and O₂ thermal- desorption fluxes through the upper ∼meter of icy Galilean satellite regoliths. Radiolytic O₂ release is modelled with an Arrhenius law following recent thermal desorption analyses, constraining the activation energy for O₂-H from observations of trapped O₂ bubbles and variability inferred in the surface and atmosphere. Several depths are considered from ~ 1 mm to 1 m, with results estimating outgassing fluxes at depth and at the surface, as a lower limit. Porosity strongly affects the net outgassing flux and should therefore be included in new numerical investigations on the near-surface atmospheres of the Galilean satellites. The outgassing model serves as a benchmark for Saturnian satellites as well as more generally, icy bodies.

How to cite: Oza, A. and Mergny, C.: Thermal Outgassing and Sublimation from the Porous Regoliths of Europa, Ganymede, and Callisto at Depth, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-998, https://doi.org/10.5194/epsc2026-998, 2026.

Saturn’s icy moon Enceladus is one of the most compelling targets for future planetary exploration due to strong evidence for a global subsurface ocean, ongoing cryovolcanic activity, and the potential for habitable environments beneath its ice shell [1-3]. Future missions to Enceladus will require highly accurate spacecraft navigation to enable geophysical investigations, radar sounding, gravity-field recovery, and precise measurements of tidal deformation. Within the German Space Agency's Enceladus Explorer Initiative project RaTNOS (Radar Transponder based Navigation and Orbit determination for Satellites), we investigate advanced orbit determination strategies for spacecraft operating in Enceladus orbit using combinations of Earth-based radio tracking and local radar transponder measurements. 

An orbit determination framework was developed using open-source orbit estimation software Tudat(Py) (TU Delft Astrodynamics Toolbox in Python) [e.g., 4]. The framework estimates the spacecraft initial state vector together with additional dynamic and observational parameters. Simulations consider realistic dynamical perturbations, including higher-order gravity harmonics of Enceladus, perturbations from Saturn, and tracking noise from radar observations. Three tracking architectures are analysed: (1) classical communication between the spacecraft and Earth-based Deep Space Network (DSN) stations, (2) communication between the spacecraft and radar transponders deployed on the surface of Enceladus, and (3) a hybrid configuration combining DSN and transponder-based tracking. Our results demonstrate that local radar transponders improve orbit determination accuracy compared with DSN-only tracking. While a single transponder provides limited improvement because of short communication windows during orbital flyovers, configurations with multiple transponders yield significant gains. The analysis further shows that the number and spatial distribution of transponders exert a stronger influence on navigation performance than moderate variations in signal quality.

Precise orbit determination contributes to constraining geophysical properties of Enceladus. In particular, accurate measurement of tidal deformation through repeated orbital observations may provide critical insight into the internal structure and habitability of the moon. Our findings demonstrate that radar-assisted orbit determination architectures offer a promising pathway toward high-precision navigation for future Enceladus missions. Our methodology supports both mission design optimisation and the scientific interpretation of future geodetic and geophysical measurements around icy ocean worlds.

[1] Thomas et al. (2016) Enceladus’s measured physical libration requires a global subsurface ocean. Icarus, 264, 37-47.

[2] Hansen et al. (2011) The composition and structure of the Enceladus plume. Geophysical Research Letters, 38, 6.

[3] Xu et al. (2025) Enough Sulfur and Iron for Potential Life Make Enceladus’s Ocean Fully Habitable. The Astrophysical Journal Letters, 980, L10.

[4] Dirkx et al. (2022). The open-source astrodynamics tudatpy software-overview for planetary mission design and science analysis. EPSC2022, (EPSC2022-253).

How to cite: Neumann, W., Auer Wilkins, S., Oberst, J., Gambacorta, L., Benedikter, A., Marx, V., Stark, A., Hussmann, H., Wickhusen, K., and Vossiek, M.: Radar Transponder Based Orbit Determination Around Enceladus, Europlanet Science Congress 2026, The Hague, The Netherlands, 6–11 Sep 2026, EPSC2026-1351, https://doi.org/10.5194/epsc2026-1351, 2026.