長谷川 直

The lifetime of weak porous boulders on main belt asteroids was experimentally investigated using their impact strength <mml:math altimg="si4.svg"><mml:msup><mml:msub><mml:mi>Q</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:msub><mml:mo>∗</mml:mo></mml:msup></mml:math>. We conducted impact disruption experiments on weak porous targets to simulate the conditions of boulders found on C-type asteroids such as Ryugu and Bennu. Projectiles made of polycarbonate and nylon were impacted on spherical targets with the diameter from 30 to 120 mm at velocities ranging from 0.6 to 6.1 kms<SUP loc="post">‑1. The impact angle was normal to the target surface. We varied the target's tensile strength, denoted as <mml:math altimg="si7.svg"><mml:msub><mml:mi>Y</mml:mi><mml:mi mathvariant="normal">t</mml:mi></mml:msub></mml:math>, and the mass of the target by more than one order of magnitude. Our findings revealed that <mml:math altimg="si4.svg"><mml:msup><mml:msub><mml:mi>Q</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:msub><mml:mo>∗</mml:mo></mml:msup></mml:math> increased with increasing the tensile strength, and it slightly depended on the boulders' sizes. Additionally, we observed that <mml:math altimg="si4.svg"><mml:msup><mml:msub><mml:mi>Q</mml:mi><mml:mi mathvariant="normal">S</mml:mi></mml:msub><mml:mo>∗</mml:mo></mml:msup></mml:math> depended on impact velocity according to the scaling theory for catastrophic disruption, expressed as <mml:math altimg="si152.svg"><mml:msubsup><mml:mi>Q</mml:mi><mml:mi mathvariant="normal">S</mml:mi><mml:mo>∗</mml:mo></mml:msubsup><mml:mo>=</mml:mo><mml:mn>0.15</mml:mn><mml:msup><mml:mi>D</mml:mi><mml:mrow><mml:mo>‑</mml:mo><mml:mn>0.25</mml:mn></mml:mrow></mml:msup><mml:msubsup><mml:mi>v</mml:mi><mml:mi mathvariant="normal">i</mml:mi><mml:mn>0.53</mml:mn></mml:msubsup><mml:msup><mml:mfenced><mml:mfrac><mml:msub><mml:mi>Y</mml:mi><mml:mi mathvariant="normal">t</mml:mi></mml:msub><mml:mi>ρ</mml:mi></mml:mfrac></mml:mfenced><mml:mn>0.74</mml:mn></mml:msup></mml:math>, where v<SUB loc="post">i, D and ρ represent impact velocity, target diameter and target density, respectively. We also investigated the momentum transfer efficiency, denoted as <mml:math altimg="si102.svg"><mml:mi>β</mml:mi></mml:math>, for monolithic asteroids with weak strength. <mml:math altimg="si102.svg"><mml:mi>β</mml:mi></mml:math> was determined for cratered targets with different tensile strengths and was found to be well scaled by a characteristic velocity, denoted as v*=<mml:math altimg="si155.svg"><mml:msqrt><mml:mrow><mml:msub><mml:mi>Y</mml:mi><mml:mi mathvariant="normal">t</mml:mi></mml:msub><mml:mo>/</mml:mo><mml:mi>ρ</mml:mi></mml:mrow></mml:msqrt></mml:math>, for the targets with a strength smaller than 283 kPa. The relationship obtained was <mml:math altimg="si156.svg"><mml:mi>β</mml:mi><mml:mo>‑</mml:mo><mml:mn>1</mml:mn><mml:mo>=</mml:mo><mml:msup><mml:mn>10</mml:mn><mml:mrow><mml:mo>‑</mml:mo><mml:mn>3.3</mml:mn></mml:mrow></mml:msup><mml:msup><mml:mfenced close=")" open="("><mml:mrow><mml:msub><mml:mi>v</mml:mi><mml:mi mathvariant="normal">i</mml:mi></mml:msub><mml:mo>/</mml:mo><mml:msup><mml:mi>v</mml:mi><mml:mo>∗</mml:mo></mml:msup></mml:mrow></mml:mfenced><mml:mn>1.29</mml:mn></mml:msup></mml:math>. However, the β value for the strongest target with 731 kPa did not conform to this equation. Based on these results, we estimated the lifetime of boulders on main belt asteroids with various strength and sizes. Boulders with the strength between 200 kPa and 1.7 MPa, estimated for asteroids Ryugu and Bennu, are unlikely to be disrupted in less than 10 Ma for sizes larger than 4 m. However, boulders smaller than several 10 cm may not survive longer than 10 Ma, a duration almost corresponding to the surface age of Ryugu. The boulder with the size of 160 m on asteroid Ryugu could potentially survive longer than 90–183 Ma at the main belt if the strength exceeds 200 kPa....

Context. Primitive asteroids consisting of mainly phyllosilicates and opaque minerals have great variation at near-ultraviolet (NUV) wavelengths (0.35–0.5 μm). The absorption in NUV could be indicative of phyllosilicates that reflect their formation environments such as the distribution of water, temperature, and pressure. The asteroid collisional families are the fragments of large primordial bodies that record the early Solar System environments. Aims. Our objective is to investigate the reflectance spectrophotometry of primitive asteroid families in NUV to visible (VIS) wavelengths to constrain the internal structure and formation of primordial bodies. Methods. The NUV-VIS reflectance spectrophotometry of 38 primitive asteroid families was investigated using two spectrophotometric surveys, the Eight Color Asteroid Survey (ECAS) and the Sloan Digital Sky Survey (SDSS). We classified the members of the primitive asteroid families based on Tholen's taxonomy. After grouping these families into eight overarching types, we discussed the compositions of primitive asteroid families based on the NUV, 0.7 μm, 3 μm absorptions, and the near-infrared (NIR) spectral slopes. Results. We have found a correlation between the 0.7 μm absorption band and the NUV absorption among the asteroid families, suggesting that both features are caused by the charge transfer of interlayer iron in phyllosilicates. This implies that NUV absorption can be a valuable indicator of Fe-rich phyllosilicate abundance. Furthermore, we have revealed correlations between the NUV absorption, VIS slope, albedo, and the NIR slope. Primitive asteroid families with strong NUV absorption exhibit a high albedo and a low NIR slope (1.25–2.14 μm). The Pallas family deviates from this general trend due to its exceptionally high albedo. This anomaly, combined with the Pallas family's unique density and the deep and sharp 3 μm absorption, suggests that the Pallas family could be a potential source of CR chondrites. Overall, our results demonstrate that NUV absorption aligns well with established indicators of phyllosilicate presence (0.7 μm and 3 μm absorptions). The largest bodies in the high-NUV absorption families show a sharp 3-μm feature, while the red-dominant families show a w-shaped 3-μm feature. Notably, two young endmember families (Theobalda, F-dominant; Veritas, G-dominant) highlight that NUV absorption is not solely linked to aging or space weathering, but likely reflects inherent compositional differences. The Polana–Eulalia complex family and the Theobalda family, dominated by F types (&gt;80%), exhibit minimal to non-NUV absorption, suggesting that their primordial bodies contained little Fe-rich phyllosilicates, such as CI drated carbonaceous chondrites. Conversely, the Veritas family, with over 80% of C and G types, displays stron. indicative of an Fe-rich phyllosilicate-rich parent body, such as CM chondrites....

Context. Ch-type asteroids are distinctive among other dark asteroids in that they exhibit deep negative polarization branches (NPBs). Nevertheless, the physical and compositional properties that cause their polarimetric distinctiveness are less investigated. Aims. We aim to investigate the polarimetric uniqueness of Ch-type asteroids by making databases of various observational quantities (i.e., spectroscopic and photometric properties as well as polarimetric ones) of dark asteroids. Methods. We conducted an intensive polarimetric survey of 52 dark asteroids (including 31 Ch-type asteroids) in the R_C band to increase the size of polarimetric samples. The observed data were compiled with previous polarimetric, spectroscopic, and photometric archival data to find their correlations. Results. We find remarkable correlations between these observed quantities, particularly the depth of NPBs and their spectroscopic features associated with the hydrated minerals. The amplitude of the opposition effect in photometric properties also shows correlations with polarimetric and spectral properties. However, these observed quantities do not show noticeable correlations with the geometric albedo, thermal inertia, and diameter of asteroids. Conclusions. Based on the observational evidence, we arrive at our conclusion that the submicrometer-sized structures (fibrous or flaky puff pastry-like structures in phyllosilicates) in the regolith particles could contribute to the distinctive NPBs of hydrated asteroids.