Surface Of Ryugu Research Articles

A comprehensive study of apatite grains in Ryugu rock fragments

AbstractApatite is present as an accessory phase in many meteorites and is often formed as a secondary product of aqueous alteration. Its propensity to incorporate rare earth elements (REE) results in apatite usually being the main REE‐bearing phase in hydrously altered meteorites. Asteroid Ryugu is thought to have experienced pervasive aqueous alteration and material collected from the surface of Ryugu is expected to provide insight into asteroidal aqueous alteration processes without influence by terrestrial weathering. Morphologies and mineral associations of apatite grains from five rock fragments collected from the asteroid Ryugu by the Hayabusa2 spacecraft were examined and their REE concentrations were measured by synchrotron X‐ray fluorescence (SXRF) spectroscopy. The main minerals associated with apatite are dolomite, magnetite, and pyrrhotite. Grain boundary corrosion of the interfaces between apatite assemblages and the surrounding matrix suggest that paragenetic formation on the asteroid was followed by a later episode of hydrous alteration. Light REE (LREE) concentration levels recorded at 20–150 times those of bulk CI levels together with a steady increase from LREE toward enrichment of medium REE (MREE, up to Er) at 50–400 times bulk CI levels may suggest postgenetic removal of LREE from Ryugu apatite grains by late‐stage circulation of a hydrothermal fluid.

Microstructural and chemical features of impact melts on Ryugu particle surfaces: Records of interplanetary dust hit on asteroid Ryugu.

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

Textural Study of Vesicles in Tagish Lake (C2-ung) Meteorite Fusion Crust: Constraints on Vesicle Formation during Their Entry into the Earth’s Atmosphere

Vesicles are characteristic structures within the outer layer of many stony meteorites’ fusion crusts. Although these features are well-developed in hydrated carbonaceous chondrites and some micrometeorites, their formation mechanism remains poorly understood. This study provides new insights into the understanding of physical vesiculation processes by presenting the results of vesicle size distribution (VSD)—i.e., a quantitative method for vesicle analysis—applied to the study of the Tagish Lake (C2-ung) meteorite fusion crust. Tagish Lake was chosen because it shows a scoriaceous texture and a significant number of vesicles (about 24,000 vesicles/mm2), thus allowing statistical analysis. Vesicles range from being spherical to irregular-shaped and from a few μm to ~70 μm (equivalent diameter) in size. Vesicle size distribution and cumulative number density analyses show a high nucleation event and a fractal distribution of the vesicle population, respectively. We suggest these features are due to disequilibrium degassing processes, which simultaneously produce continuous/accelerating vesicle nucleation and growth. Finally, possible analogies between the scoriaceous Tagish Lake fusion crust and the space-weathered “frothy layer” on the surface of Ryugu’s grains could be found in terms of vesicularity.

Space weathering acts strongly on the uppermost surface of Ryugu

Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu’s surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu’s global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa’s spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids.

Evidence of global space weathering by solar wind on asteroid 162173 Ryugu

Reflectance spectra (0.3–4 μm) of powder samples of the third largest stone C0002 returned from the asteroid Ryugu by JAXA Hayabusa2 mission were measured and compared with proximity observation data taken by the Optical Navigation Camera - Telescopic (ONC-T) and the Near-Infrared Spectrometer (NIRS3) onboard the Hayabusa2 spacecraft. Although the characteristics of the 2.7 μm absorption band due to structural hydroxyl of the Ryugu powder samples are consistent with CI1 chondrite meteorites, NIRS3 spectra of the global surface of Ryugu exhibit a wavelength shift of +6 nm in the 2.7 μm absorption band center in comparison with those of both the powder samples and an artificial crater on the asteroid formed by the Small Carry-on Impactor (SCI). This is consistent with effects of solar wind-induced space weathering being present across the global surface of asteroid Ryugu. The darkness of the Ryugu surface in comparison with the returned samples and an upturn in reflectance observed in the average ONC-T spectrum at ultraviolet wavelengths may also be due to space weathering.

MASCOT’s in situ analysis of asteroid Ryugu in the context of regolith samples and remote sensing data returned by Hayabusa2

The Hayabusa2 mission provided a unique data set of asteroid Ryugu that covers a wide range of spatial scale from the orbiter remote sensing instruments to the returned samples. The MASCOT lander that was delivered onto the surface of Ryugu aimed to provide context for these data sets by producing in situ data collected by a camera (MasCam), a radiometer (MARA), a magnetometer (MasMag) and a spectrometer (MicrOmega). In this work, we evaluate the success of MASCOT as an integrated lander to bridge the gap between orbiter and returned sample analysis. We find that MASCOT’s measurements and derivatives thereof, including the rock morphology, colour in the visible wavelengths, possible meteorite analogue, density, and porosity of the rock at the landing site are in good agreement with those of the orbiter and the returned samples. However, it also provides information on the spatial scale (sub-millimetres to centimetres) at which some physical properties such as the thermal inertia and reflectance undergo scale-dependent changes. Some of the in situ observations such as the presence of clast/inclusions in rocks and the absence of fine particles at the landing site was uniquely identified by MASCOT. Thus, we conclude that the delivery of an in situ instrument like MASCOT provides a valuable data set that complements and provides context for remote sensing and returned sample analyses.Graphical

Soluble organic molecules in samples of the carbonaceous asteroid (162173) Ryugu.

The Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu's parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.

Measurement of Microscopic Thermal Diffusivity Distribution for Ryugu Sample by Infrared Lock-in Periodic Heating Method

The thermophysical properties of small Solar System bodies are essential to be determined, on which the thermal evolution of small bodies largely depends. The carbonaceous asteroid Ryugu is one of the small undifferentiated bodies formed in the early Solar System. Hayabusa2 explored the asteroid Ryugu and returned the surface samples in 2020 for detailed on-ground investigation, including measurements of thermal properties. Because the available sample amount was limited, this study developed a novel method to measure the thermal diffusivity of small and irregularly shaped samples of about 1 mm in diameter by combining lock-in thermography and periodic heating methods on the microscale. This method enables us to measure the thermal diffusivity of both flat-plate and granular shape samples by selecting the suitable detecting direction of the temperature response. Especially, when the sample has a flat-plate shape, the anisotropic distribution of the in-plane thermal diffusivity can be evaluated. This method was applied to six Ryugu samples, and the detailed anisotropic distribution of the thermal diffusivity was obtained. The measurement results showed that the samples show local thermal anisotropy caused by cracks and voids. The average thermal diffusivity among all samples was (2.8 to 5.8) × 10−7 m2·s. Based on the density and specific heat of the samples obtained independently, the thermal effusivity was estimated to be 791 J·(s1/2·m2·K) to 1253 J·(s1/2·m2·K), which is defined as the resistance of surface temperature to the change of thermal input. The determined thermal effusivity, often called thermal inertia in planetary science, is larger than the observed value of 225 ± 45 J· (s1/2·m2·K) of the asteroid Ryugu's surface, obtained from the diurnal temperature change of the rotating asteroid by a thermal infrared camera onboard Hayabuas2. This difference is likely to be attributed to the difference in the analytical scale between the sample and the surface boulders compared with the thermal diffusion length. Consequently, it was found that the present result is more representative of the thermal diffusivity and thermal inertia of local part of individual Ryugu particles.

The noble gas and nitrogen relationship between Ryugu and carbonaceous chondrites

Carbonaceous chondrites are considered to have originated from C-type asteroids and represent some of the most primitive material in our solar system. Furthermore, since carbonaceous chondrites can contain significant quantities of volatile elements, they may have played a crucial role in supplying volatiles and organic material to Earth and other inner solar system bodies. However, a major challenge of unravelling the volatile composition of chondritic meteorites is distinguishing between which features were inherited from the parent body, and what may be a secondary feature attributable to terrestrial weathering. In December 2020, the Hayabusa2 mission of the Japan Aerospace Exploration Agency (JAXA) successfully returned surface material from the C-type asteroid (162173) Ryugu to Earth. This material has now been classified as closely resembling CI-type chondrites, which are the most chemically pristine meteorites. The analysis of material from the surface of Ryugu therefore provides a unique opportunity to analyse the volatile composition of material that originated from a CI-type asteroid without the complications arising from terrestrial contamination. Given their highly volatile nature, the noble gas and nitrogen inventories of chondrites are highly sensitive to different alteration processes on the asteroid parent body, and to terrestrial contamination. Here, we investigate the nitrogen and noble gas signature of two pelletized grains collected from the first and second touchdown sites (Okazaki et al., 2022a), to provide an insight into the formation and alteration history of Ryugu. The concentration of trapped noble gas in the Ryugu samples is greater than the average composition of previously measured CI chondrites and are primarily derived from phase Q, although a significant contribution of presolar nanodiamond Xe-HL is noted. The large noble gas concentrations coupled with a significant contribution of presolar nanodiamonds suggests that the Ryugu samples may represent some of the most primitive unprocessed material from the early solar system. In contrast to the noble gases, the abundance of nitrogen and δ15N composition of the two Ryugu pellets are lower than the average CI chondrite value. We attribute the lower nitrogen abundances and δ15N measured in this study to the preferential loss of a 15N-rich phase from our samples during aqueous alteration on the parent planetesimal. The analyses of other grains returned from Ryugu have shown large variations in nitrogen concentrations and δ15N indicating that alteration fluids heterogeneously interacted with material now present on the surface of Ryugu. Finally, the ratio of trapped noble gases to nitrogen is higher than CI chondrites, and is closer to refractory phase Q and nanodiamonds. This indicates that Ryugu experienced aqueous alteration that led to the significant and variable loss of nitrogen, likely from soluble organic matter, without modification of the noble gas budget, which is primarily hosted in insoluble organic matter and presolar diamonds and is therefore more resistant to aqueous alteration.

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

Derivation of 1.064 μm normal albedos on the C-type asteroid Ryugu from laser pulse intensity measurement of the Hayabusa2 LIDAR

Japanese asteroid explorer Hayabusa2 arrived at C-type asteroid 162,173 Ryugu in June 2018. The laser altimeter (LIDAR) onboard Hayabusa2 measured its own transmitted laser and returned pulse intensities from a Ryugu surface until November 2019. Because the Ryugu surface is extremely rough, topography dominates over the material properties in the conventional derivation of normal albedo. Thus, we developed a method to retrieve the normal albedo from the rough surface of a C-type asteroid at a LIDAR laser wavelength of 1.064 μm. The albedo map covering an equatorial band between – 40° and + 20° in latitude was created with 3˚-by-3˚ resolution using the intensity data obtained before the conjunction of the spacecraft with the Sun. The average of the normal albedo is 0.0405 ±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document} 0.0027, whereas approximately half of the 3°-by-3° grids are between 0.04 and 0.045. The low and uniform normal albedo feature is common to other remote-sensing observations of Ryugu by visible and near-infrared cameras onboard Hayabusa2.Graphical

Particle size distributions inside and around the artificial crater produced by the Hayabusa2 impact experiment on Ryugu

Japanese Hayabusa2 spacecraft has successfully carried out an impact experiment using a small carry-on impactor (SCI) on an asteroid (162173) Ryugu. We examine the size distribution of particles inside and outside an artificial impact crater (the SCI crater) based on the images taken by the optical navigation camera onboard the Hayabusa2 spacecraft. The circumferential variation in particle size distribution inside the SCI crater is recognized and we interpret that major circumferential variation is caused by the large boulders inside the SCI crater that existed prior to the impact. The size distribution inside the SCI crater also shows that the subsurface layer beneath the SCI impact site had a large number of particles with a characteristic size of – 9 cm, which is consistent with the previous evaluations. On the other hand, the size distribution outside the SCI crater exhibits the radial variation, implying that the deposition of ejecta from the SCI crater is involved. The slope of the size distribution outside the crater at small sizes differs from the slope of the size distribution on the surface of Ryugu by approximately 1 or slightly less. This is consistent with the claim that some particles are buried in fine particles of the subsurface origin included in ejecta from the SCI crater. Thus, the particle size distributions inside and outside the SCI crater reveal that the subsurface layer beneath the SCI impact site is rich in fine particles with – 9 cm in size while the particles on the surface have a size distribution of a power-law form with shallower slopes at small sizes due to the deposition of fine ejecta from the subsurface layer. Finally, we discuss a process responsible for this difference in particle size distribution between the surface and the subsurface layers. The occurrence of segregation in the gravitational flow of particles on the surface of Ryugu is plausible.Graphical

Characterization of the MASCOT landing area by Hayabusa2

Context. After landing on C-type asteroid Ryugu, MASCOT imaged brightly colored, submillimeter-sized inclusions in a small rock. Hayabusa2 successfully returned a sample of small particles from the surface of Ryugu, but none of these appear to harbor such inclusions. The samples are considered representative of Ryugu. Aims. To understand the apparent discrepancy between MASCOT observations and Ryugu samples, we assess whether the MASCOT landing site, and the rock by implication, is perhaps atypical for Ryugu. Methods. We analyzed observations of the MASCOT landing area acquired by three instruments on board Hayabusa2: a camera (ONC), a near-infrared spectrometer (NIRS3), and a thermal infrared imager. We compared the landing area properties thus retrieved with those of the average Ryugu surface. Results. We selected several areas and landforms in the landing area for analysis: a small crater, a collection of smooth rocks, and the landing site itself. The crater is relatively blue and the rocks are relatively red. The spectral and thermophysical properties of the landing site are very close to those of the average Ryugu surface. The spectral properties of the MASCOT rock are probably close to average, but its thermal inertia may be somewhat higher. Conclusions. The MASCOT rock can also be considered representative of Ryugu. Some of the submillimeter-sized particles in the returned samples stand out because of their atypical spectral properties. Such particles may be present as inclusions in the MASCOT rock.

Clustering analysis of high spatial resolution spectra of asteroid (162173) Ryugu from Hayabusa2/NIRS3

Asteroids, and more globally small bodies, are keys to unravel the origin and the evolution of our Solar system. Remnants of the material which formed planets early on, they captured the composition and the conditions of formation of the latter at that time, since they did not undergo too harsh internal alteration during their lifetimes. The Hayabusa2 spacecraft explored the Cb-type asteroid (162173) Ryugu between June 2018 and November 2019, notably collecting data with its Near-InfraRed Spectrometer (NIRS3) in the 1.8–3.2 ​μm range. We selected five NIRS3 datasets among the ones having the highest spatial resolution on which we applied the G-mode multivariate statistical analysis, in order to spot small heterogeneities at the surface of Ryugu. Both global scale and local scale cases were investigated. With a confidence level of 3σ, we obtained two classes at global and local scales, while decreasing to 2σ results in the identification of more classes (five at global scale, four at local scale). Overall, our results are consistent among them and with previous studies. In particular, terrains spectrally redder as well as terrains spectrally bluer than the average surface are newly identified by our analysis. The floors of all main craters at the surface of Ryugu appear redder than their surroundings, and are associated with a probable smaller grain size material. A large depression of the Northern hemisphere may contain bluer heterogeneities likely due to a more packed or/and fresher, subsequently deposited material. Finally, a comparison with data from the Hayabusa2 optical camera ONC-T shows no evident correlation between the spectral properties in the near-infrared and in the visible.

MicrOmega/MASCOT first results

MASCOT has been selected to be integrated on board the JAXA Hayabusa2 mission, with a suite of 4 instruments: a camera MASCam, a radiometer MARA, a magnetometer MASMag and MicrOmega, a hyperspectral near infrared microscope, designed to characterize the composition of Ryugu surface materials down their grain scale (25 ​μm large). A joined article (Ho et al., this issue) describes and gives references to MASCOT design, goals, development and operation, along its cruise, landing and on asteroid phase.MicrOmega has been activated and operated as planned, while demonstrating high instrumental performances. However, its configuration when MASCOT went at rest on Ruygu located MicrOmega line of sight over a deep depression, with Ryugu surface much too far to be illuminated and imaged, precluding a direct characterization.In this paper, we present and discuss MicrOmega operations and outcomes.

Hayabusa2 operation for MASCOT delivery to Ryugu surface

The asteroid explorer Hayabusa2 was launched by Japan Aerospace Exploration Agency (JAXA) on December 3rd, 2014. The primary mission of the spacecraft is to sample pieces of the asteroid and return it to Earth for more advanced scientific analysis on the Earth. After three-year cruise phase, Hayabusa2 finally arrived at the asteroid Ryugu on June 28, 2018, and mission operations started. Hayabusa2 carries multiple rovers, separates them to land on the asteroid surface. One of these rovers, called MASCOT, was developed under the international cooperation between Deutsches Zentrum für Luft-und Raumfahrt (DLR) and Center National d'études Spatiales (CNES). This rover was planned to be separated to land on the surface of the asteroid and planned to perform several missions on the asteroid surface. In order to support these missions, the mother ship Hayabusa2 was requested to separate this rover at a very low altitude about 50 ​m, and to hover about 3 ​km after separation to achieve a reliable communication link with MASCOT. On October 2–5, 2018, we performed the operation for MASCOT release. In this paper, we introduce the flight results of the entire operation for the MASCOT release.

Thermal analysis and constraints for the MASCOT landing site selection on the Asteroid Ryugu

In June 2018, after 4 years of cruise, the Japanese space probe Hayabusa2 [1-Watanabe S. et al.: Hayabusa2 Mission Overview. (2017)] reached the Near-Earth Asteroid (162173) Ryugu. Hayabusa2 carried a small Lander named MASCOT (Mobile Asteroid Surface Scout) [2-Ho T. M. et al.: MASCOT-The Mobile Asteroid Surface Scout on-board the Hayabusa2 mission. (2017)], jointly developed by the German Aerospace Center (DLR) and the French Space Agency (CNES), to investigate Ryugu's surface structure, composition and physical properties including its thermal behaviour and magnetization in-situ. The Microgravity User Support Centre (DLR-MUSC) in Cologne was in charge of providing all thermal conditions and constraints necessary for the selection of the final landing site and for the final operations of the Lander MASCOT on the surface of the asteroid Ryugu.This article provides a comprehensive assessment of these thermal conditions and constraints, based on predictions performed with the Thermal Mathematical Model (TMM) of MASCOT using different asteroid surface thermal models, ephemeris data for approach as well as descent and hopping trajectories, the related operation sequences and scenarios and the possible environmental conditions driven by the Hayabusa2 spacecraft.A comparison with the real telemetry data confirms the analysis and provides further information about the asteroid characteristics.

High-resolution observations of bright boulders on asteroid Ryugu: 2. Spectral properties

Many small boulders with reflectance values higher than 1.5 times the average reflectance have been found on the near-Earth asteroid 162,173 Ryugu. Based on their visible wavelength spectral differences, Tatsumi et al. (2021, Nature Astronomy, 5, doi:10.1038/s41550-020-1179-z) defined two bright boulder classes: C-type and S-type. These two classifications of bright boulders have different size distributions and spectral trends. In this study, we measured the spectra of 79 bright boulders and investigated their detailed spectral properties. Analyses obtained a number of important results. First, S-type bright boulders on Ryugu have spectra that are similar to those found for two different ordinary chondrites with different initial spectra that have been experimentally space weathered the same way. This suggests that there may be two populations of S-type bright boulders on Ryugu, perhaps originating from two different impactors that hit Ryugu's parent body. Second, the model space-weathering ages of meter-size S-type bright boulders, based on spectral change rates derived in previous experimentally irradiated ordinary chondrites, are 105–106 years, which is consistent with the crater retention age (<106 years) of the ~1-m deep surface layer on Ryugu. This agreement strongly suggests that Ryugu's surface is extremely young, implying that the samples acquired from Ryugu's surface should be fresh. Third, the lack of a serpentine absorption in the S-type clast embedded in one of the large brecciated boulders indicates that fragmentation and cementation that created the breccias occurred after the termination of aqueous alteration. Fourth, C-type bright boulders exhibit a continuous spectral trend similar to the heating track of low-albedo carbonaceous chondrites, such as CM and CI. Other processes, such as space weathering and grain size effects, cannot primarily account for their spectral variation. Furthermore, the distribution of the spectra of general dark boulders, which constitute >99.9% of Ryugu's volume, is located along the trend line in slope/UV-index diagram that is occupied by C-type bright boulders. These results indicate that thermal metamorphism might be the dominant cause for the spectral variety among the C-type bright boulders on Ryugu and that general boulders on Ryugu may have experienced thermal metamorphism under a much narrower range of conditions than the C-type bright boulders. This supports the hypothesis that Ryugu's parent body experienced uniform heating due to radiogenic energy rather than impact heating.

Geometric correction for thermographic images of asteroid 162173 Ryugu by TIR (thermal infrared imager) onboard Hayabusa2

The thermal infrared imager (TIR) onboard the Hayabusa2 spacecraft performed thermographic observations of the asteroid 162173 Ryugu (1999 JU_3) from June 2018 to November 2019. Our previous reports revealed that the surface of Ryugu was globally filled with porous materials and had high surface roughness. These results were derived from making the observed temperature maps of TIR using a projection method onto the shape model of Ryugu as geometric corrections. The pointing directions of TIR were calculated using an interpolation of data from the SPICE kernels (NASA/NAIF) during the periods when the optical navigation camera (ONC) and the light detection and ranging (LIDAR) observations were performed. However, the mapping accuracy of the observed TIR images was degraded when the ONC and LIDAR were not performed with TIR. Also, the orbital and attitudinal fluctuations of Hayabusa2 increased the error of the temperature maps. In this paper, to solve the temperature image mapping problems, we improved the correction method by fitting all of the observed TIR images with the surface coordinate addressed on the high-definition shape model of Ryugu (SFM 800k v20180804). This correction adjusted the pointing direction of TIR by rotating the TIR frame relative to the Hayabusa2 frame using a least squares fit. As a result, the temperature maps spatially spreading areas were converged within high-resolved 0.5^circ by 0.5^circ maps. The estimated thermal inertia, for instance, was approximately 300sim350 Jm^{-2}s^{-0.5}K^{-1} at the hot area of the Ejima Saxum. This estimation was succeeded in case that the surface topographic features were larger than the pixel scale of TIR. However, the thermal inertia estimation of smooth terrains, such as the Urashima crater, was difficult because of surface roughness effects, where roughness was probably much smaller than the pixel scale of TIR.

Development of image texture analysis technique for boulder distribution measurements: Applications to asteroids Ryugu and Itokawa

Recent asteroid missions have revealed that many sub-kilometer asteroids are rubble piles. Large parts of their surfaces are covered with boulders larger than tens of centimeters. An evaluation of the abundance and size distribution of boulders provides clues to understand surface processes on boulder-covered asteroids. Here we report a new method that automatically measures the abundance of small boulders (sub-pixel to a few pixels), whose boundaries cannot be recognized with visual inspection, by quantifying the surface radiance variation that occurs during the spinning of the asteroid. After validating our approach with previous boulder counting data, we apply this method to images of the asteroids Ryugu and Itokawa, which were visited by JAXA's Hayabusa and Hayabusa2, and obtain a global distribution of the boulders larger than 0.75–3 ​m, which corresponds to 1.5–6 pixels. We find that the boulder number density of this size range is smaller (1) on the western bulge than on the eastern hemisphere and (2) on the equatorial ridge than on the higher latitudes, both of which exceed the number density of boulders ​> ​5 ​m by an order of magnitude. The boulder size distribution at 1.25–20 ​m shows that the boulders smaller than 1 ​m are more abundant at the equator than at mid-latitudes, while those larger than 1 ​m in diameter are more abundant at mid-latitudes than at the equator. This contrast suggests size-dependent migration of boulders in the latitudinal direction. We also find that the typical boulder size (the size reaching the cumulative areal coverage of 50%) is 1.9 ​m at the equatorial region (10°S-10°N) while it is 2.6 ​m at mid-latitudes (40°S-50°S, 40°N-50°N). The typical boulder size is also smaller in the western bulge (2.0–2.2 ​m). We construct global maps of the power-law index of the size frequency distribution of boulders and find minor variations over the entire surface of Ryugu (−2.53 ​± ​0.03) for boulders larger than 1.25 ​m. This small variation suggests homogeneous size sorting processes on Ryugu. Surface roughness does not show a significant correlation with the v-band albedo but shows a high anti-correlation (R ​= ​−0.73) with the current geological slope on the eastern hemisphere. Our method is useful enhancement of smooth area detection and boulder distribution characterization that will be applicable to other planetary explorations in the future, including those of Phobos and other asteroids.