Secondary submicrometer impact cratering on the surface of asteroid 25143 Itokawa

Dennis Harries, Shogo Yakame, Yuzuru Karouji, Masayuki Uesugi, Falko Langenhorst

Earth and Planetary Science Letters, Volume 450, 15 September 2016, Pages 337-345
Available online 12 July 2016



• Submicrometer impact craters on RA-QD02-0265 are due to secondary projectiles.
• Exceptionally high crater densities do not necessarily indicate long exposure times.
• Secondary cratering potentially modifies the spectral properties of the regolith.
• Young regolith grains support dynamic overturn or loss of the regolith on Itokawa.”

“Particle RA-QD02-0265 returned by the Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa displayed a unique abundance of submicrometer-sized (≤500 nm) impact craters, which are rarely observed among the Hayabusa samples. The particle consists of intensely twinned diopside that was subjected to a large-scale shock event before exposure to the space environment on the surface of 25143 Itokawa. Intense (sub-)micrometer-scale impact cratering may suggest a long surface exposure and, hence, a long residence time of regolith material on the surface of small asteroids, bearing implications for the dynamical evolution of these bodies. However, our combined FE-SEM and FIB/TEM study shows that the degree of solar wind-induced space weathering and the accumulation of solar flare tracks are not exceptionally different from other Hayabusa particles with surface exposure ages estimated to be less than 1 ka. A 500 nm wide crater on the surface of RA-QD02-0265 exhibits microstructural damage to a depth of 400 nm below its floor and contains residues of Fe–Ni metal, excluding a formation by space craft exhausts or curatorial handling. The geometrical clustering among the 15 craters is unlikely random, and we conclude that the craters have formed through the impacts of secondary projectiles (at least partially Fe–Ni metal) created in a nearby (micro-)impact event. Besides structural damage by the solar wind and deposition of impact-generated melts and vapors, secondary impact cratering on the submicrometer-scale is another potential mechanism to modify the spectral properties of individual regolith grains. The lack of extensively exposed regolith grains supports a dynamic regolith on the surface of 25143 Itokawa.”