Abstract
We investigated impact crater structures on regolith particles from asteroid Itokawa using scanning electron microscopy. We observed the surfaces of 51 Itokawa particles, ranging from 15 µm to 240 µm in size. Craters with average diameters ranging from 10 nm to 2.8 µm were identified on 13 Itokawa particles larger than 80 µm. We examined the abundance, spatial distribution, and morphology of approximately 900 craters on six Itokawa particles. Craters with sizes in excess of 200 nm are widely dispersed, with spatial densities from 2.6 µm2 to 4.5 µm2; a fraction of the craters was locally concentrated with a density of 0.1 µm2. The fractal dimension of the cumulative crater diameters ranges from 1.3 to 2.3. Craters of several tens of nanometers in diameter exhibit pit and surrounding rim structures. Craters of more than 100 nm in diameter commonly have melted residue at their bottom. These morphologies are similar to those of submicrometer-sized craters on lunar regolith. We estimated the impactor flux on Itokawa regolith-forming craters, assuming that the craters were accumulated during direct exposure to the space environment for 102 to 104 yr. The range of impactor flux onto Itokawa particles is estimated to be at least one order of magnitude higher than the interplanetary dust flux and comparable to the secondary impact flux on the Moon. This indicates that secondary ejecta impacts are probably the dominant cratering process in the submicrometer range on Itokawa regolith particles, as well as on the lunar surface. We demonstrate that secondary submicrometer craters can be produced anywhere in centimeter- to meter-sized depressions on Itokawa's surface through primary interplanetary dust impacts. If the surface unevenness on centimeter to meter scales is a significant factor determining the abundance of submicrometer secondary cratering, the secondary impact flux could be independent of the overall shapes or sizes of celestial bodies, and the secondary impact flux could have similar values on Itokawa and the Moon.
Highlights
Solar system objects without atmospheres are continuously exposed to hypervelocity impacts
Considering that T = 102 to 103 yr is most likely the direct-exposure age according to the noble-gas analysis (Nagao et al, 2011), we conclude that secondary ejecta impacts are probably the dominant cratering process in the submicrometersize range on Itokawa regolith particles, as well as on the lunar surface
The estimated range of flux for craters on Itokawa particles is higher than the interplanetary dust flux and comparable to that for the submicrometer craters on lunar rocks
Summary
Solar system objects without atmospheres are continuously exposed to hypervelocity impacts. Impact processes are considered to be among the fundamental agents causing the modification of surface geological features of airless bodies. They include impact cratering (e.g., Melosh, 1989), regolith formation (e.g., Melosh, 1989), regolith mixing, and migration (e.g., Robinson et al, 2001; Veverka et al, 2001; Miyamoto et al, 2007). Surface evolution caused by hypervelocity impacts can offer important clues to understanding the history of airless bodies in the solar system.
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