Three-Dimensional Structure of Hayabusa Samples: Origin and Evolution of Itokawa Regolith

Abstract

Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles' size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily form much of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.

Population characteristics of submicrometer-sized craters on regolith particles from asteroid Itokawa

“…In addition, microscopic meteoroid impacts contribute to changes in the optical properties, chemical composition, and structures of regolith surface material ( Noguchi et al, 2011;Harries and Langenhorst, 2014;Keller and Berger, 2014;Noguchi et al, 2014;Thompson et al, 2014;Bonal et al, 2015;Matsumoto et al, 2015;Harries et al, 2016;Matsumoto et al, 2016 ). In addition, regolith activity on Itokawaprobably driven by impact processes-has been identified based on grain motion ( Nagao et al, 2011;Matsumoto et al, 2016 ), fracturing ( Tsuchiyama et al, 2011;Langenhorst et al, 2014 ;Matsumoto et al, 2016 ), and abrasion ( Tsuchiyama et al, 2011 ). These previous studies have shown that Itokawa particles contain a record of the collective processes of regolith evolution on this small asteroid.…”

“…The crater abundance on the small Itokawa particles from room A is comparable to or slightly lower than that expected from the model for larger particles ( > 80 μm), indicating that the size dependence of the crater abundance is not distinct for room A particles. The slightly lower abundance of craters on small Itokawa particles ( < 80 μm) could have been caused by regolith fragmentation from larger to smaller particles through continuous meteorite bombardments ( Tsuchiyama et al, 2011 ) or cyclic thermal fatigue ( Delbo et al, 2014 ). The regolith fragmentation could have produced smaller particles with fresh surfaces during the time they were present on Itokawa, because the timescale of regolith fragmentation could have been shorter than that of blister formation ( < 10 3 years; Matsumoto et al, 2016 ).…”

Size Sorting on the Rubble-Pile Asteroid Itokawa

“…1(a), is about 540 m along its longest axis, and is believed to be a socalled "rubble pile," formed through gravitational accretion followed by collisional and weathering processes 7 . Raised areas 2 on Itokawa are populated by boulders ranging in diameter 3,4 from 5 to 40 m, while depressions are filled with smooth seas of smaller particles 5 ranging from fine dust to centimeter-sized pebbles 6 . It is not known how this segregation came about, and understanding this may shed light on the processes that asteroids 7,8,9 -and perhaps other bodies 10,11 -undergo during formation and development.…”

“…This implies that there must be overwhelmingly more small particles, by number, than large, and so most collisions that made up the asteroid must have been from small particles. As an estimate, if we take pebbles on Itokawa to be of order 5,6,28 1 cm in diameter and boulders to be of order 3 10 m, the diameters would differ by factor of a thousand. If there were equal volumes of pebbles and boulders, then there would be a billion times more pebbles than boulders.…”

Granular cohesion and fast rotators in the NEA population

“…Instead, the pellet did not fire during contact between the sample horn and the asteroid surface, although the spacecraft did collect a minute amount of material from the surface of the asteroid. This was in essence a non-destructive sampling method and there-fore, the collected sample represents the naturally occurring grain sizes on the asteroid surface [14].…”