Surface Compositions of Trojan Asteroids

Red (S > 10%/0.1 μm) spectral slopes are common among Centaurs and trans-Neptunian objects (TNOs) in the outer solar system. Interior to and co-orbital with Jupiter, the red (S ∼ 10%/0.1 μm) slopes of D-type main-belt and Jupiter Trojan asteroids are thought to reflect their hypothesized shared origin with TNOs beyond the orbit of Jupiter. In order to quantify the abundance of red-sloped asteroids within the main belt, we conducted a survey using the NASA Infrared Telescope Facility and the Lowell Discovery Telescope. We followed up on 32 candidate red objects identified via spectrophotometry from the Sloan Digital Sky Survey’s Moving Object Catalog to confirm their steep spectral slopes and determine their taxonomic classifications. We find that our criteria for identifying candidate red objects from the Moving Object Catalog result in a ∼50% confirmation rate for steeply red-sloped asteroids. We also compare our observations of main-belt asteroids to existing literature spectra of the Jupiter Trojans and steeply red-sloped main-belt asteroids. We show that some red-sloped asteroids have linearly increasing reflectance with increasing wavelength, while other red-sloped asteroids show a flattening in slope at longer near-infrared wavelengths, indicating a diversity among the population of spectrally red main-belt asteroids suggestive of a variety of origins among the population of steep-sloped asteroids.

Primitive asteroids with low albedos and red slopes in the visible and near-infrared (VNIR) are found in both the main belt and the Jupiter Trojan clouds. In order to determine whether the VNIR spectral similarities of primitive main-belt asteroids and Jupiter Trojans are reflective of a true compositional similarity, we compare the mid-infrared silicate emission features of main-belt and Jupiter Trojan asteroids. Using archival data from the Spitzer Space Telescope’s Infrared Spectrograph and observations from the Stratospheric Observatory for Infrared Astronomy’s FORCAST instrument, we analyze the 5–40 μm spectra of 13 primitive main-belt asteroids and compare them to those of Jupiter Trojans in the literature. We find that while many primitive asteroids in the main belt resemble their Trojan counterparts with strong spectral signatures of olivine-rich high-porosity silicate regoliths, we identify (368) Haidea as a spectrally distinctive asteroid that lacks strong evidence of olivine in its mid-IR spectrum. Differences in silicate compositions among D-type asteroids imply a diversity of origins for primitive asteroids.

The optical properties of silicate-rich asteroidal surfaces (namely, S-complex asteroids) evolve over time under the influence of several processes, known as "space weathering" (SW), and are amply analyzed in laboratory experiments. A first estimate of the spectral reddening rate, due to SW, on family S-type main-belt asteroids (MBAs; Jedicke et al.) has been confirmed and generalized, using a different approach, which also provided us with clues about the relative relevance of the various SW processes (Marchi et al.). However, in the main belt, the S complex accounts for about 40% of the listed bodies, and even less in terms of mass. Most of the remaining main-belt population is composed of spectrally featureless asteroids (i.e., with nonsilicate-dominated spectra) and can be divided into two main groups: the C complex and the X complex; the three complexes comprise up to more than 90% of the MBAs. The question as to whether SW has general observable consequences for the whole main belt is still open. In this work we show, on the basis of a wide statistical analysis, that the spectral trends due to SW are similar for the three major complexes. We have also been able to identify some underlying relevant physical processes. Our observational findings, and the related clues, are also supported by new experimental results, which are also summarized here. The resulting scenario has several important implications for the formation of evolutionary models of the main asteroid belt as well as for the existing taxonomical classification.

Asteroid Orbit Determination Using Bayesian Probabilities

Reflectance spectroscopy remains a cornerstone of remote mineralogical analysis of airless bodies. However, traditional spectral unmixing techniques face limitations when dealing with nonlinear mixing, instrument-specific noise, or spectral coverage constraints. To overcome these, we have developed a suite of robust convolutional neural network (CNN) models trained to derive modal and chemical compositions of olivine–pyroxene-dominated surfaces, along with asteroid taxonomy classification. This abstract presents: (1) a concise overview of the model and its accessibility through a public web tool, (2) applications of the models to data from Itokawa, measured by the Hayabusa spacecraft, and (3) the inferred surface composition, space weathering state, and their correlation with topographic context. Forthcoming results will also include observations from the HyperScout-H imager, which is currently undergoing calibration. Model Overview and RobustnessOur neural network models were trained on laboratory reflectance spectra of olivine and pyroxene-rich samples, along with a combined dataset from DeMeo et al. (2009) and Binzel et al. (2019). The models predict:Relative volumetric mineral abundances (olivine, orthopyroxene, clinopyroxene), Compositional endmembers (e.g., fayalite and ferrosilite contents), Taxonomic class probabilities using a confidence-based scheme. To ensure robustness across instruments and spectral resolutions, the training dataset and models were systematically validated using spectra with varying resolution, wavelength coverage, transmission functions, and noise levels. The models achieved better than 10 percentage point precision in modal and chemical compositions (Korda et al., 2023a) and over 90% confidence in taxonomy classification (Korda et al., 2023b) when applied to high-quality input spectra (see Fig. 1). Comparable performance on realistic spectra with added noise and reduced spectral resolution (Korda and Kohout, 2024) confirms the models’ reliability under observational constraints and supports their use for rapid compositional and taxonomical classifications. Figure 1: Model performance: composition predictions for various wavelength grids (left) and taxonomy classification as a confusion matrix (right). Web Tool for Public Use To support broader adoption of our spectral analysis models, we offer an online tool featuring a user-friendly web interface (Korda and Kohout, 2024), accessible at sirrah.pythonanywhere.com (see Fig. 2). Users can upload one or more reflectance spectra and apply optional preprocessing steps such as outlier removal, denoising, wavelength trimming, or the application of an instrument’s spectral transmission function. Both raw and preprocessed spectra can be visualised and downloaded. Subsequently, users can apply the available machine-learning models for taxonomic classification and mineralogical composition—including models optimised for specific instruments such as HyperScout-H and ASPECT. All predictions are returned in downloadable CSV format. For data privacy, all uploaded input files are automatically deleted after processing. Figure 2: Web interface for uploading and analysing asteroid reflectance spectra. Users can apply preprocessing, visualise data, and run machine-learning models for taxonomy and composition. Correlations with Space Weathering and TopographyA key strength of the model is its ability to capture subtle spectral variations linked to space weathering processes. Unlike traditional methods that rely only on discrete spectral features, the model interprets the full spectral shape, including nonlinear mixing effects and weathering trends. In Korda et al. (2023b), we applied the models to spectra of Itokawa and observed a notable decrease in the olivine-to-pyroxene ratio in spectrally mature (weathered) regions. This aligns with olivine’s lower resistance to space weathering: exposure to micrometeorite impacts and solar wind alters its crystalline structure, weakening its diagnostic spectral features. As a result, weathered surfaces appear relatively pyroxene-rich. Notably, our compositional predictions for Itokawa are in close agreement with laboratory analyses of returned samples, validating the model’s physical accuracy and further strengthening confidence in its predictions. Surface composition and space weathering on Itokawa show a clear relationship with topography. Topographic highs, such as ridge crests and fresh crater walls, exhibit fresher material—likely due to recent exposure or regolith loss. In contrast, low-lying regions like valleys and depressions host older, spectrally mature regolith that has accumulated over time (see Fig. 3). These spatial patterns support the interpretation that downslope transport and regolith gardening are key processes shaping spectral evolution. The observed smooth transitions in spectral properties support a scenario of gradual weathering and ageing of surface materials. Taxonomically, we observe a smooth transition along the Q–S spectral sequence across Itokawa’s surface. This sequence, widely recognised as a proxy for surface maturation, allows our model to infer the relative ages of different terrains. While absolute dating is beyond the scope of our models, it can be supplemented by the machine-learning model of Palamakumbure et al. (202X), which predicts absolute surface ages from reflectance spectra. Together, these models offer a comprehensive view of both compositional and temporal evolution. Figure 3: Predicted match score of the S-type asteroids on the surface of Itokawa. The numbers designate fresh and mature areas. Forthcoming Application to Mask and DeimosWe are currently preparing reflectance spectra from ESA’s Hera spacecraft and its onboard HyperScout-H instrument, which observed both Mars and its moon Deimos. These datasets will soon be analysed using our neural network models, marking the first application of our method to Martian system targets. The analysis will investigate surface composition, space weathering effects, and geological diversity on both bodies, offering new insights into the origin and evolution of Deimos and the mineralogical context of the Martian surface. Preliminary results will be presented at the conference. ConclusionOur neural network-based framework offers a powerful and adaptable tool for interpreting reflectance spectra of airless bodies. It delivers detailed insights into surface mineralogy, chemical composition, and taxonomy, and is robust to variations in instrument characteristics. When applied to spatially resolved targets, the resulting taxonomy and composition maps can serve as proxies for geological processes, highlighting correlations with surface topography and weathering state. These capabilities support inferences about regolith dynamics, including downslope transport and resurfacing. The accompanying web interface ensures accessibility for the planetary science community and facilitates spectral interpretation for current and future missions. ReferencesBinzel et al. (2019): DOI 10.1016/j.icarus.2018.12.035DeMeo et al. (2009): DOI 10.1016/j.icarus.2009.02.005Korda et al. (2023a): DOI 10.1051/0004-6361/202243886Korda et al. (2023b): DOI 10.1051/0004-6361/202346290Korda and Kohout (2024): DOI 10.3847/PSJ/ad2685Palamakumbure et al. (202X): Under revision

The surface composition of Trojan asteroids: constraints set by scattering theory

To date, reflectance spectra of Jupiter Trojan asteroids have revealed no distinctive absorption features. For this reason, the surface composition of these objects remains a subject of speculation. Spectra have revealed, however, that the Jupiter Trojan asteroids consist of two distinct sub-populations that differ in the optical to near-infrared colors. The origins and compositional differences between the two sub-populations remain unclear. Here, we report the results from a 2.2–3.8 μm spectral survey of a collection of 16 Jupiter Trojan asteroids, divided equally between the two sub-populations. We find clear spectral absorption features centered around 3.1 μm in the less-red population. Additional absorption consistent with that expected from organic materials might also be present. No such features are see in the red population. A strong correlation exists between the strength of the 3.1 μm absorption feature and the optical to near-infrared color of the objects. While, traditionally, absorptions such as these in dark asteroids are modeled as being due to fine-grain water frost, we find it physically implausible that the special circumstances required to create such fine-grained frost would exist on a substantial fraction of the Jupiter Trojan asteroids. We suggest, instead, that the 3.1 μm absorption on Trojans and other dark asteroids could be due to N–H stretch features. Additionally, we point out that reflectivities derived from WISE observations show a strong absorption beyond 4 μm for both populations. The continuum of 3.1 μm features and the common absorption beyond 4 μm might suggest that both sub-populations of Jupiter Trojan asteroids formed in the same general region of the early solar system.

Space weathering and the color–color diagram of Plutinos and Jupiter Trojans

We conducted a two-band imaging survey observation using the Subaru Telescope and its wide-field camera, Suprime-Cam, to study the visible colors and size distribution of Jupiter’s Trojan asteroids. The survey covered an area around Jupiter’s L4 Lagrange point totaling 9.2 square degrees. We detected 120 Trojan asteroids in this survey. From these Trojan asteroids, we extracted 44 unbiased samples with absolute magnitudes in the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> band ranging from 12.9 to 16.9 mag (corresponding to diameter ranges of approximately ∼3–16 km assuming an albedo of 0.05) and analyzed their <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>−</mml:mo> <mml:mi>i</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> color and size distributions. Large Jupiter Trojan asteroids are known to be classified into two color groups, “red” and “less red.” We found that such bimodality in the color distribution is absent for small Jupiter Trojan asteroids, which is consistent with previous studies. Previous studies have also shown that these two groups have different slopes in the magnitude distributions from each other, which was explained by conversion of red objects to less-red fragments through catastrophic disruptions. In contrast, we found that the size frequency distributions of our two sample groups divided by the color of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>g</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>−</mml:mo> <mml:mi>i</mml:mi> <mml:mo accent="false">′</mml:mo> </mml:math> = 0.7 (in AB magnitude) are quite similar. Our results can provide new insights into collisional evolution of color and size distribution of small Jupiter Trojans.

Spectral Properties and Heterogeneity of Phobos from Measurements byPhobos 2

Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission☆

Introduction: Meteorites seem to come from a small number of primary parent bodies [1]. B-, C-, Cb-, Cg-, P- and D-types, representing not less than 66% of the mass of the main belt have no analogues clearly identified in the meteorite collections [2]. However, meteorites are not the only cosmomaterials found on Earth since no less than 30&amp;#160;000 tons of interplanetary dust particles (IDPs) enter the Earth&amp;#8217;s atmosphere each year [3]. IDPs originate from different parent bodies throughout the solar system [4, 5, 6]. The link between IDPs and asteroids can be investigated thanks to Vis-NIR spectroscopy commonly used for the classification of asteroids (0.4 - 2.5 &amp;#181;m). The reflectance measurements in the visible range (0.4 - 0.8 &amp;#956;m) performed on IDPs in the 90s&amp;#160;[7] and the simulated visible near infrared (Vis-NIR) spectra of IDPs with comparison of mid infrared (Mid-IR) spectra [2] have shown that IDPs may be good analogues to some asteroids and in particular to the classes not sampled by meteorites. But Vis-NIR reflectance measurements of IDPs is challenging and we must understand how the measurement on an isolated micro-metric particle can be affected by physical parameters of the sample such as size, composition, and roughness.We report here the requirements, the abilities as well as the limitations of the technique and the results obtained on 15 IDPs particles ranging 7-31 &amp;#181;m in size in the Vis-NIR range (0.45 - 1.0 &amp;#181;m).Experiments: Our setup, installed in a clean room, consists of a Vis-NIR spectrometer (Maya2000 Pro from Ocean Optics) coupled to a macroscope (Leica Z16 APO). A Vis-NIR optical fiber (100 or 50 &amp;#956;m in diameter) is used to collect the light diffused by the sample which is unilaterally illuminated by a halogen source through a 1000 &amp;#956;m diameter fiber (phase angle of ~ 45&amp;#176;). By changing the magnification and/or the diameter of the collection fiber it is possible to adapt the collection spot to the grain size down to 7 &amp;#956;m size.Results and discussion: To obtain a reliable reflectance spectrum of a micro-metric grain with this setup, we show that it is necessary to average spectra taken at different azimuth angles, by rotating the particle several times in the observation plane with respect to the incident light.Based on the study of spectral slopes we found that for particles with sizes below ~ 17 &amp;#181;m the spectral slope increases linearly with decreasing particle sizes. This behavior is due to a bias encountered in the reflectance measurement in this size range, inducing thus a loss of the chemical information. For particle sizes larger than &amp;#8764; 17 &amp;#181;m the spectral slopes seem randomly distributed between &amp;#8764; -0.3 and 0.4 &amp;#181;m&amp;#8722;1, and the spectra must therefore carry chemical information of the particles.We found that the visible reflectance levels of the IDPs show a multimodal distribution. There is a lack of IDPs with reflectance level ~ 5 and ~ 8%. In addition, the majority of IDPs have rather low reflectance levels (&lt; 10%). Some particles have reflectance levels that may be influenced by the presence of magnetite, which is sometimes found in extraterrestrial materials and could form upon atmospheric entry.Among the studied particles we identified an IDP (L2079C18) exhibiting a feature at 0.66 &amp;#181;m which is similar to the one observed by remote sensing at the surface of hydrated asteroids. This is the first detection of a hydration band in the reflectance spectrum of an IDP which could indicate a possible link between hydrated IDPs with hydrated asteroid surfaces.Acknowledgments: We are grateful to the CAPTEM NASA for providing the IDPs. This work is supported by the Programme National de Plan&amp;#233;tologie (PNP) of CNRS/INSU, co-funded by CNES. The authors also thank the ANR RAHIIA SSOM and the P2IO LabEx (ANR-10-LABX0038) in the framework Investissements d&amp;#8217;Avenir (ANR11-IDEX-0003-01) for their supports. We thank O. Mivumbi and Y. Longval for their help and technical support for the development of the device.&amp;#160;

Due to their strong resonances with their host planet, Trojan asteroids can remain in stable orbits for billions of years. As a result, they are powerful probes for constraining the dynamical and chemical history of the solar system. Although we have detected thousands of Jupiter Trojans and dozens of Neptune Trojans, there are currently no known long-term stable Earth Trojans (ETs). Dynamical simulations show that the parameter space for stable ETs is substantial, so their apparent absence poses a mystery. This work uses a large ensemble of N-body simulations to explore how the Trojan population dynamically responds if Earth suffers large collisions, such as those thought to have occurred to form the Moon and/or to have given Earth its late veneer. We show that such collisions can be highly disruptive to the primordial Trojan population, and could have eliminated it altogether. More specifically, if Earth acquired the final 1% of its mass through collisions, then only ∼1% of the previously bound Trojan population would remain.

The NASA Lucy mission was designed to provide the first reconnaissance of Jupiter Trojan asteroids. These primitive bodies hold important clues to the origin and evolution of the Solar System. The Lucy spacecraft is currently en route to its encounters with Trojan asteroids in the L4 swarm in 2027 and 2028, and the L5 swarm in 2033 [1]. On its way through the Main Belt, Lucy flew past the asteroid (52246) Donaldjohanson on April 20, 2025. The main purpose of the flyby was to support mission preparation for the primary science targets, the Trojan asteroids. The Linear Etalon Imaging Spectral Array (LEISA) mapping spectrometer [2,3], part of the L’Ralph instrument, collected multiple spectral datasets during the encounter with Donaldjohanson. LEISA covers the spectral range 0.97 – 3.95 μm with a variable spectral resolving power ranging from ~50 to 160 (Δλ

The unique composition and space environment of Mercury provides an opportunity to test models of space weathering and can potentially provide constraints on the relative importance of physical and chemical processes involved in space weathering. In this work we apply a space weathering model to multispectral images of Mercury and incorporate the geologic context of spectra into the interpretation of the model results. We find that albedo variations on Mercury's surface cannot be explained by the addition of an opaque mineral alone. Instead, differential accumulation of space weathering products, related to composition, contributes significantly to albedo variations on Mercury's surface. Even relatively immature crater ejecta on Mercury have several times more space weathering derived iron than mature lunar soils, complicating interpretations of Mercury's surface composition.