Abstract
A projectile accelerated by the Hayabusa2 Small Carry-on Impactor successfully produced an artificial impact crater with a final apparent diameter of 14.5 ± 0.8 m on the surface of the near-Earth asteroid 162173 Ryugu on April 5, 2019. At the time of cratering, Deployable Camera 3 took clear time-lapse images of the ejecta curtain, an assemblage of ejected particles forming a curtain-like structure emerging from the crater. Focusing on the optical depth of the ejecta curtain and comparing it with a theoretical model, we infer the size of the ejecta particles. As a result, the typical size of the ejecta particles is estimated to be several centimeters to decimeters, although it slightly depends on the assumed size distribution. Since the ejecta particles are expected to come from a depth down to ~1 m, our result suggests that the subsurface layer of Ryugu is composed of relatively small particles compared to the uppermost layer on which we observe many meter-sized boulders. Our result also suggests a deficit of particles of less than ~1 mm in the subsurface layer. These findings will play a key role in revealing the formation and surface evolution process of Ryugu and other small Solar System bodies.
Highlights
Particle size distributions on and inside small Solar System bodies reflect the evolution of their surfaces and the physical processes on the bodies
We focus on the part of the ejecta curtain consisting of particles ejected with sufficient velocities so as to be insignificantly affected by the late excavation stage effects of gravity or target strength
The Deployable Camera 3 (DCAM3)-D images were taken at 1 frame per second at maximum for more than 1 h including the time of Small Carry-on Impactor (SCI) impact
Summary
Particle size distributions on and inside small Solar System bodies reflect the evolution of their surfaces and the physical processes on the bodies. Where ve is the ejection velocity of individual ejecta particles at the horizontal distance x from the impact point, g is the gravitational acceleration on the surface of Ryugu (∼1.2 × 10−4 m s−2 , Watanabe et al 2019), and C2 and μ are given by 0.64 and 0.41, respectively, for sand targets. This scaling law holds for the particles ejected from a region sufficiently far from the impact point and the crater rim, where neither gravity nor material strength affects the ejection velocity (Housen et al 1983).
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