長谷川 直

Certain planetary surfaces may have been wet at the time of impact cratering, and wet surfaces can significantly affect the impact process modifying the crater size and morphology. We conducted crater formation experiments onto wet sand targets with different water contents and observed that both crater size and morphology systematically changed with the target water contents. The size of impact crater formed on wet sand target changes with water content of the target, because the cohesion and coefficient of friction of wet sand change with water content as we measure them by shear test. By varying impactor size (<mml:math><mml:mi>a</mml:mi></mml:math>) and velocity (<mml:math><mml:msub><mml:mi>v</mml:mi><mml:mi>i</mml:mi></mml:msub></mml:math>), we simulated changing normalized gravity defined as <mml:math><mml:mi>ga</mml:mi><mml:mo>/</mml:mo><mml:msup><mml:msub><mml:mi>v</mml:mi><mml:mi>i</mml:mi></mml:msub><mml:mn>2</mml:mn></mml:msup></mml:math> to establish a crater size scaling relationship for wet sand, where <mml:math><mml:mi>g</mml:mi></mml:math> is gravity. By unifying our results and previous studies, a crater size scaling relationship was established which can express the effect of water content. The results indicate that under low-gravity conditions crater size is primarily controlled by target cohesion, whereas under high-gravity conditions crater size is more strongly influenced by internal friction. Increasing water content in the target leads to an increase in target cohesion, resulting in a decrease in crater size under low-gravity conditions, whereas it decreases the coefficient of friction in the target, leading to an increase in crater size as water content increases under high-gravity conditions. The transition boundary between the regimes where crater size is dominated by cohesion and by friction changed with water content: The surface gravity at the transition boundary for a constant impact velocity and impactor size increases in three orders of magnitude from dry to wet sand. This improved scaling relationship may help to estimate crater formation mechanisms on planetary bodies with water-bearing surface or sublayers, which is important for understanding the target properties....

Carbonaceous asteroids such as Ryugu and Bennu have experienced the catastrophic disruption of their parent bodies and subsequent cratering impacts. Their internal structure resembles a rubble pile, and some of their boulders have large cracks on their surfaces. Indeed, regolith grains sampled and returned to Earth were found to have numerous cracks, some of which extended through the entire interior of the samples. In an impact, pre-existing large cracks are likely to affect subsequent disruption and fragmentation. Hence, pre-existing large cracks affect the size distribution and lifetime of these boulders, -- and thereby asteroids. However, it is unclear how exactly fracturing is influenced by pre-existing large cracks. The purpose of this study is to investigate how pre-existing large cracks influence the fragmentation of targets. We therefore conducted impact experiments on carbonaceous meteorites with and without internal cracks. A spherical alumina projectile with 1.0 mm and a glass projectile with 0.80 mm in diameter were fired into 1--2-cm-sized Allende (CV3), Murchison (CM2), and Aguas Zarcas (CM2) targets at nominal impact velocities of 2.0 and 4.0 km <inline-formula><tex-math>$\rm {s}^{-1}$</tex-math></inline-formula>, respectively. X-ray computed tomography imaging on the meteorite before impact and on the largest fragment after impact were performed. The experiments show that, for CV and CM meteorites, several of the newly exposed surfaces on the largest impact fragments were formed along pre-existing large cracks. Measurements of fragment size distributions from the disruption of CM meteorites revealed a deficiency of small fragments, potentially explaining the scarcity of fine particles on small carbonaceous asteroids....

The presence of water on the Moon and on asteroids that are thought to be poor in water, either because they formed inside the snow line or because they lost much of their water during differentiation, has been suggested by multiple studies; however, its form and origin remain unclear. In this study, we conducted hypervelocity impact experiments between serpentinite projectiles and steel targets. Serpentinite contains hydroxyl and simulates hydrated impactors such as primitive asteroids. The effects of impact velocity and angle on the survival and form of water delivered to the target surface were investigated using near-infrared reflectance spectroscopy and microscopic Raman spectroscopy. Reflectance spectra of projectile materials adhered to the crater surfaces suggested that, in all head-on impact experiments with velocities of 3─7 km s⁻¹, hydroxyl pre-existing in the projectile was almost completely lost. The spectra also showed olivine absorption features at shock pressures exceeding ∼80 GPa, and the olivine Raman peaks became narrower at higher impact velocities. Based on the comparisons with results from impact experiments with anhydrous projectiles, it is suggested that molecular water can be trapped in the melt at shock pressures below ∼100 GPa. In contrast, in the oblique impact experiments conducted in this study, decomposition of projectile material was suppressed, and hydroxyl was detected in crater samples. The current results, along with comparisons to impact velocities of asteroids in the main belt and on the Moon, suggest that molecular water derived from hydrated impactors can be detectable through spectroscopic observations....

Large main-belt asteroids with visible geometric albedos below 0.1 are predominantly classified within the C- and X-complex spectroscopic classes. C-type and dark X-type asteroids typically exhibit flat to slightly negative and positive spectral slopes, respectively. They are further distinguished by the presence (for C-types) or absence (for dark X-types) of a shallow absorption feature near 1.0-1.3 μm. We serendipitously discovered that the asteroids 1093 Freda and 1390 Abastumani display spectral characteristics intermediate between these two classes, combining a positive visible-to-near-infrared spectral slope with a shallow absorption band. A search in the literature reveals additional asteroids with similar properties. The existence of such objects, spanning a continuum of spectral shapes between C- and dark X-types, may point to a common genetic origin. Their spectral behavior could be explained by the presence of cronstedtite, an Fe-rich serpentine, on their surfaces....