Seiji Sugita

Small planetary bodies record the processes that shaped the Solar System, but their surfaces and interiors often evolve in unexpected ways. Deimos, the potato-shaped, 12-km-diameter outer satellite of Mars, has a strikingly smooth surface covered by fine regolith, in contrast to its heavily cratered and grooved sibling, 22-km-diameter Phobos. Moreover, Deimos’s topography is dominated by a single large south-polar depression, whose formation we simulate to constrain the moon’s interior and surface geomechanical properties and to explain the origin and distribution of its regolith. Here we find that an oblique impact by a 320-m-diameter projectile can quantitatively account for this depression, the global regolith layer and surface features such as bright streaks and crater infill. Our results show that Deimos’s upper layers are extremely weak and that its interior is highly porous and dissipative, resembling the recently visited rubble-pile asteroids rather than lunar regolith. This suggests that small dark moons and asteroids, despite their specific histories, may converge on similar physical states.

Rubble‐pile asteroids, characterized by loose aggregates of debris held together by gravity, represent both a significant planetary hazard and a key to understanding planetesimal formation. Geologic mapping of these bodies provides essential insights into their origins, evolution, and surface processes. This knowledge is crucial for advancing space mission capabilities, from precise navigation and safe sampling to assessing the feasibility of future asteroid mining. Here, we present the most comprehensive geologic map of Ryugu in comparison to previous early‐stage and single‐feature maps, integrating Optical Navigation Camera (ONC) mosaics, LIDAR‐derived topography, Mobile Asteroid Surface Scout (MASCOT) descent images, and spectral slope data. The analysis includes reclassification of previously identified features and newly identified features from global to local scales. The geologic mapping technique followed United States Geological Survey (USGS) planetary cartography standards using the ArcGIS Pro platform. This global framework provides an essential geologic context for interpreting returned samples and constraining rubble‐pile evolution. We identified geomorphological, structural, and surface texture units on Ryugu's unconsolidated rubble‐pile surface using 0.2–0.7 m/pixel image mosaics. Elongated troughs at the poles and mirrored, oblique lineaments near the equatorial ridge are consistent with rotational strain. Compared with another rubble‐pile asteroid Bennu, Ryugu shows a more uniform boulder distribution, and its immensely prominent equatorial ridge indicates a more intense period of rotational deformation. However, the western bulge deviates from this trend; its fewer boulders, underdeveloped ridges, and older degraded craters suggest selective resurfacing. These features show evidence of catastrophic disruption, rotational reshaping, and ongoing surface movement.

Introduction: Comet Interceptor [1] is a joint space mission of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). Its primary goals are to provide the first-ever in-situ characterisation of a Long Period Comet (LPC), which could be a dynamically-new comet or an interstellar object, and to perform the first simultaneous multi-point exploration of a cometary coma and nucleus.Comet Interceptor is the first rapid response mission [2]. The mission will be launched in piggy back mode between the second semester of 2028 and the first semester of 2029 on Ariane 6, towards the Sun-Earth Lagrange point L2, where it will wait for its target comet passage.Comet Interceptor consists of one main spacecraft (S/C A), and two deployable probes, named Probe B1 and Probe B2, which are provided by JAXA and ESA, respectively, allowing unique and simultaneous observations of the target from different geometries. The S/C A is devoted to the physical characterisation of the cometary nucleus and coma via remote sensing and in situ observations during the selected target fly-by.Science Objectives: The mission will investigate the processes of planetesimal formation and disentangle primordial versus evolutionary processes by comparing the composition and physical properties of a pristine LPC with those of the Short Period Comets (SPCs) previously investigated by space missions, in particular by the Rosetta mission which explored comet 67P for more than two years. Specifically, the objectives of Comet Interceptor are:1) Comet Nucleus Science: What is the surface composition, shape, morphology, and structure of the target object?2) Comet Environment Science: What is the composition of the coma, its connection to the nucleus (activity) and the nature of its interaction with the solar wind?Science Activities: The search for the target comet is underway, and preparations are being made for the scientific exploitation of the data from the mission’s three spacecraft.The selection and scientific investigations of the target comet, as well as the development of the mission instruments and science operation areas, are supported by Working Groups (WGs). These are the Science Operation WG, Target Identification WG, and Comet Environment WG. The latter comprises three sub-WGs, covering the Comet Nucleus, Near-Environment (inner dust and gas coma), and Far-Environment (outer dust and gas coma and tails) topics. The specific tasks of the Comet Environment WG are to develop and provide results from scientific models addressing questions relevant to the implementation and operation of the mission and to achieve its scientific objectives.Here, we provide a brief overview of the mission, and we present and describe the aims and activities of the working groups over the past year. One of the recent activities of all the Comet Environment sub-working groups is the preparation of a large repository of numerical models to favour the successful planning for scientific operations and data interpretation. Other activities include the analysis of available data on comets to better understand the environment expected during the encounter and establish base line parameters for target selection.A list of virtual LPC targets has been established and these targets are under evaluation. Additional LPCs are expected to be detected in the near future thanks to ground-based survey, notably from the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) [3]. These are going to be considered as proxy to inquire on the:1) Nucleus science: (i) measure the size, shape, and rotation rate of the target comet nucleus; (ii) return resolved images of the surface that reveal its morphology: (iii) constrain the nucleus composition directly via remote sensing observations (imaging, spectroscopy, and polarimetry).2) Environment science: (i) map the bulk neutral composition of the coma and determine any local structure and connection to the nucleus ices distributions and active areas; (ii) derive coma isotopic composition by mapping the distribution of neutral gasses and their relation to nucleus inhomogeneity; (iii) characterise the structure of the coma dust environment and determine any connection to the nucleus; (iv) characterise coma dust properties, and determine dust fluxes; (v) analyse motion and evolution of ion rays and other coma and tail features (i.e., dust and gas); (vi) characterise the plasma environment around the target, determine any resulting boundaries and assess energy, mass, and momentum transfer. References: [1] Jones, G.H. et al. (2024) Space Sci. Rev. 220, 9. [2] Snodgrass, C. & Jones, G.H. (2019) Nat. Comm. 10, 5418. [3] Inno, L., et al. (2025) Icarus 429, 116443.

The ESA Hera mission [1] is the European contribution to the first asteroid deflection test with the NASA DART mission. The DART probe successfully impacted Dimorphos, the small moon of the binary asteroid Didymos, on September 26, 2022. The orbital period modification of Dimorphos around Didymos, measured by ground-based observatories, was reduced by 33 minutes from its original value of 11h 55 min. The Italian LICIACube obtained some images of Dimorphos within the first few minutes following impact. However, we still do not know Dimorphos' final properties, modified by this impact, because no other spacecraft remained long enough in close proximity to collect such observations. Is there a crater on Dimorphos' surface, and if so, what are its properties? Or, was Dimorphos entirely reshaped? Answers to these questions are crucial for numerical simulations of the DART impact, required to reproduce these final properties to be validated as well as for our general understanding of the impact response of such objects in low-gravity regimes.Hera will, therefore, perform the investigation starting in November 2026 for at least 6 months. Although we already have images of Dimorphos before impact thanks to DART, we are confident that the images that Hera will take of the same body will have little similarity with those already obtained. This is because an impact with likely global effects took place. Furthermore, Didymos, the primary of the binary system may also look very different compared to pre-DART impact images. It is expected that some fraction of the ejecta from Dimorphos impacted Didymos, which may have modified its morphology and surface properties. Hera will, therefore, reveal an entirely new world, transformed by an event aimed at protecting us from an incoming asteroid, allowing us to assess the efficiency of the kinetic impactor technique. Thanks to its main spacecraft and its two cubesats, we will have detailed knowledge of the system, from the surface to the interior of the two components, including the composition and thermal properties through a spectrometer and a JAXA-contributed thermal infrared imager.So far, all newly obtained data of asteroids from previous missions have revealed some surprises [2]. Two months before arrival, we can expect Hera to also surprise us and challenge our understanding of these small worlds.Acknowledgments: The authors acknowledge support from ESA, ASI, CNES, DLR, JAXA, NASA.ReferencesMichel P., et al., Planetary Science Journal 2022, 3, 160-180.Michel P., et al., Space Science Reviews 2025, 221, 270.

Enceladus may host a subsurface ocean with biologically relevant chemistry. Plumes released from this ocean preserve information on its chemical state, and previous analyses suggest weakly to strongly alkaline pH. Constraining the ocean’s pH is critical for assessing its habitability, as pH governs the chemical energy available for potential life. This requires identifying pH-sensitive minerals in plume deposits. Raman spectrometers, which have recently been incorporated into flight instruments, offer a potential approach for mineral identification on icy moons. However, their applicability to pH estimation from plume-derived minerals has not been investigated. We evaluate whether Raman measurements of plume particles deposited on the surface of Enceladus can distinguish between weakly and strongly alkaline ocean models. Fluids with pH values of 9 and 11 were frozen under vacuum conditions analogous to those on Enceladus. The resulting salt deposits were then analyzed using a Raman system designed to simulate the SuperCam instrument for Mars, featuring a pulsed laser, standoff geometry, and a spectral resolution coarser than 10 cm ⁻¹ . The obtained Raman spectra showed diagnostic peaks for pH-dependent carbonate precipitation: at pH 9, signals consistent with NaHCO ₃ (673, 1045, and 1267 cm ⁻¹ ) and Na ₂ CO ₃ (1081 cm ⁻¹ ) were detected, whereas only Na ₂ CO ₃ was observed at pH 11. These findings demonstrate that flight-like Raman spectrometers can distinguish these key mineral phases without high-resolution laboratory optics. This result validates that in situ Raman spectroscopy is a practical approach to constrain the pH of the subsurface ocean, a key parameter for assessing its chemical evolution and potential habitability.

Observations of exoplanet transits by small satellites have gained increasing attention for reducing biases in the detection of long-period planets. However, no unambiguous detection of an exoplanet has yet been demonstrated using optics with apertures smaller than 60 mm. Here, we investigated the detectability of exoplanet transits using the telescopic Optical Navigation Camera (ONC-T) on board the Hayabusa2 spacecraft, which has an effective aperture of only 15 mm. We conducted transit observations of the hot Jupiters WASP-189 b and MASCARA-1 b, collecting data for 10 and four events, respectively. The transit signal was detected with a signal-to-noise ratio (SNR) of 13 for WASP-189 b and 8 for MASCARA-1 b for each event. Stacking all events improved the SNR to 40 and 16, respectively. The transit midtimes of each event were measured with a precision of 6 minutes and were consistent with Transiting Exoplanet Survey Satellite (TESS) data to within 2 minutes. The planet-to-star radius ratio was determined with an absolute precision of 0.004 (6% relative) and agreed with TESS results to within 0.002 (3% relative). The recent ONC-T and TESS data enabled an update to the planetary ephemerides. We report a 4 σ discrepancy between the updated orbital period of MASCARA-1 b and previously reported values. ONC-T sets a new record for the smallest-aperture instrument to detect an exoplanet transit from space, advancing the frontier of exoplanet science with miniature instrumentation. Our results suggest that optics as small as ONC-T may be capable of detecting transiting long-period Jupiters: a population that remains underrepresented in current surveys.

Particles collected from the asteroid Ryugu by the Hayabusa2 spacecraft offer a unique opportunity to investigate the magnetic record of the primitive solar system, as any terrestrial magnetic contamination is minimal and can be accounted for. In previous studies, stepwise alternating field demagnetization (AFD) measurements of natural remanent magnetization (NRM) records have been conducted on seven Ryugu particles. However, due to the limited number of samples, there is no consensus regarding the interpretation of the results of these measurements. To address this problem, we performed stepwise AFD measurements of the NRM on 28 Ryugu particles. Twenty‐three of the particles exhibited one or two stable NRM components, whereas the remaining five did not. Isothermal remanent magnetization‐based paleointensity values derived from stable NRM components varied by more than one order of magnitude. These NRM characteristics were consistent with those observed in previous studies. Therefore, as a reflection of the original nature of the NRM record, some Ryugu particles exhibited stable NRM components, whereas others did not. The Ryugu particles investigated in this study and those from a previous study exhibited spatially inhomogeneous NRM directions within individual particles, constraining the NRM acquisition time to before the final solidification of the current Ryugu particles. A mechanism of remanence acquisition that can explain the observed NRM characteristics is a chemical remanent magnetization associated with the growth of framboidal magnetite during aqueous alteration in Ryugu's parent body.

Accurate age determination is fundamental to advancing our understanding of planetary evolution. While sample-return missions provide the most accurate age measurements, in situ methods via one-way missions offer a significantly lower-cost alternative, enabling more frequent mission opportunities. The K–Ar isochron method, which combines spot-by-spot analysis using laser-induced breakdown spectroscopy (LIBS) for K and mass spectrometry (MS) for Ar, shows promise for in situ age dating since LIBS and MS have been used in planetary missions. Previous studies analyzed K-rich terrestrial rocks and chondrites using this LIBS–MS method. Martian meteorites, the only available samples of Martian rocks, are crucial for validating this technique for Mars missions. However, their low K concentrations and young ages have precluded measurements using this method. To overcome these challenges, we updated a LIBS–MS instrument for in situ K–Ar dating and applied it to three Martian meteorites: NWA 817, Zagami, and NWA 1068. We obtained isochron ages of 1351 ± 337 Ma, 165 ± 74 Ma, and 122 ± 163 Ma (2 σ ), respectively, in agreement with previously reported crystallization ages. The amounts of nonradiogenic ⁴⁰ Ar estimated using the y -intercept on the isochron plot are also consistent with those of previous studies. Our results suggest that this instrument is sufficiently precise to calibrate Martian crater age models and determine the absolute ages of young volcanic activity on Mars in the Amazonian period.

The compositions of planetary atmospheres provide key constraints on the origin and evolution of the planets. Mars has a thin atmosphere dominated by CO2, with Ne detected by the Viking mission, although its abundance and isotopic composition is not well defined. A major technical challenge for in situ Ne measurements on Mars is to separate Ne from Ar before mass spectrometry, because 40Ar2+ interferes with 20Ne+. Previous studies have demonstrated that Ne–Ar separation can be achieved using polyimide membranes. In order to characterize the temperature dependence of permeation relevant to Martian mission, we investigated gas permeation through a 100-µm-thick polyimide sheet at 6 °C, −18 °C, and −38 °C, and measured the permeated He, Ne, and Ar amounts, as well as 20Ne/22Ne ratio. The amounts of permeated He and Ne decrease with decreasing temperature. At −38 °C, the permeated 4He and 20Ne amounts are lower than the room temperature values by factors of ~6 and ~7, respectively. No permeated 40Ar above background level was detected. The 20Ne/22Ne ratios corrected for mass-dependent fractionation agree with the terrestrial atmospheric ratio within analytical uncertainty, except during initial non-steady-state permeation at −38 °C. Temperature dependency of permeated amounts indicates that the permeation fluxes through a 100-µm-thick polyimide at 20 °C give 2.2 × 10−11 and 2.7 × 10−12 cm3STP/sec/cm2/Pa for 4He and 20Ne, respectively. Under Martian atmospheric pressure, permeation through a 100-µm-thick polyimide for 40–60 min at temperature between ~10 °C and room temperature provides a Ne amount of 1–2 × 10−9 cm3STP/100 cm2 with effective Ne-Ar separation.

Analyzing primitive extraterrestrial samples from asteroids is key to understanding the evolution of the early solar system. The OSIRIS‐REx mission returned samples from the B‐type asteroid Bennu, providing a valuable opportunity to compare them with the Ryugu samples collected by the Hayabusa2 mission. This study examines the representativeness of a fraction of the Bennu samples, which was allocated from NASA to JAXA, by nondestructive characterization of their physical and spectral properties without atmospheric exposure. The reflectance and observed spectral features in the visible‐to‐infrared range of the Bennu sample resemble those from the spectroscopic analysis of different fractions. Additionally, we found differences in the slope of the visible range and band‐center of ~2.7 μm band between the samples and the asteroid surface, which could be explained by the degree of space weathering. A comparative analysis of the Bennu and Ryugu samples revealed spectral similarities, including absorption features indicative of Mg‐rich phyllosilicates, organics, and carbonates, without any evidence of sampling bias or terrestrial alteration. This finding can be used as a benchmark for subsequent Ryugu–Bennu comparative studies.