Highly porous nature of a primitive asteroid revealed by thermal imaging.

Carbonaceous (C-type) asteroids1 are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites2,3 and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth's atmosphere. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid's boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m-2 s-0.5 K-1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.

On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective.

Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher δ18O, Δ17O, and ε54Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10's of km), <2.6 Myr after CAI formation. Subsequently, the Ryugu progenitor planetesimal was fragmented and evolved into the current asteroid Ryugu through sublimation.

Constraining surface properties of asteroid (162173) Ryugu from numerical simulations of Hayabusa2 mission impact experiment.

The Hayabusa2 mission impact experiment on asteroid Ryugu created an unexpectedly large crater. The associated regime of low-gravity, low-strength cratering remained largely unexplored so far, because these impact conditions cannot be re-created in laboratory experiments on Earth. Here we show that the target cohesion may be very low and the impact probably occurred in the transitional cratering regime, between strength and gravity. For such conditions, our numerical simulations are able to reproduce the outcome of the impact on Ryugu, including the effects of boulders originally located near the impact point. Consistent with most recent analysis of Ryugu and Bennu, cratering scaling-laws derived from our results suggest that surfaces of small asteroids must be very young. However, our results also show that the cratering efficiency can be strongly affected by the presence of a very small amount of cohesion. Consequently, the varying ages of different geological surface units on Ryugu may be due to the influence of cohesion.

Fragment velocity distribution in the impact disruption of thin glass plates.

We present the experimental results of the measurement of fragment velocity in an impact disruption. Cylindrical projectiles impact on a side (edge) of thin glass plates, and the dispersed fragments were observed using a high-speed camera. The fragment velocity did not depend on the mass but rather on the initial position of the fragment; the velocity component parallel to the projectile direction increased with the distance from the impacted side, while the component perpendicular to the projectile direction increased with the distance from the central axis parallel to the projectile direction. It appears that there are two mechanisms for fragment ejection: one is "spallation," where the fragment velocities depend on the particle velocity induced by shock waves, and the other is "elastic ejection," where the velocities are controlled by the strain energy stored in targets and are at most a few tens of meters per second. We performed a one-dimensional numerical simulation of elastic ejection with a discrete element method and obtained the velocity distribution as a function of the initial position. The numerical results are qualitatively consistent with the experimental ones.

Crack propagation in thin glass plates caused by high velocity impact.

Crack propagation within thin glass plates under high shock loading is directly observed using a high speed camera. The fractal dimension of cracks and the power-law exponents of the fragment area distributions are investigated as a function of time. Two models of the fragmentation process are proposed: in one case the cracks are net-like, while in the other the cracks are tree-like, and the relations between fractal dimension and power-law exponent are estimated and compared with the experimental results. It appears that at early stages of the fragmentation process the relation is described by the latter case, while at later stages it approaches that of the former case.