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

Abstract

a b s t r a c tWe 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 μm 2 to 4.5 μm 2 ; a fraction of the craters was locally concentrated with a density of 0.1 μm 2 . 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 10 2 to 10 4 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.

Iron whiskers on asteroid Itokawa indicate sulfide destruction by space weathering

“…This corresponds to~1 × 10 −18 nm/yr, which is an extremely low rate. Submicron-sized impact craters on troilite in Itokawa particles show no evidence for whisker growth 51 . Moreover, examined troilite surfaces with iron whiskers do not correspond to crater floors.…”

Comparison of space weathering features in three particles from Itokawa

“…Compared to the most studied lunar soils, the samples from Itokawa are highly immature, with some grains showing almost no signs of surface modification at the nanoscale (Noguchi et al 2014a(Noguchi et al , 2014b. While microscale craters and melt splashes associated with micrometeoroid impacts have been observed on a number of grains (Harries et al 2016;Matsumoto et al 2018), most of the rim features seen on Itokawa particles are linked to solar wind irradiation and sputtering (Keller and Berger 2014;Noguchi et al 2014b; Thompson et al 2014).…”

“…2016; Matsumoto et al. 2018), most of the rim features seen on Itokawa particles are linked to solar wind irradiation and sputtering (Keller and Berger 2014; Noguchi et al. 2014b; Thompson et al.…”

Q-type asteroids: Possibility of non-fresh weathered surfaces

“…• Based on the concentration of the Neon ion and the number density of the solar flare tracks of Itokawa particles, it was estimated that the particle size remained within few centimeters for a duration of 150-550 yr (Nagao et al 2011) and approximately thousands to tens of thousands of years (Noguchi et al 2014;Keller & Berger 2014), respectively. • Matsumoto et al (2018) demonstrated that the dominant cratering process on the Itokawa particles in the submicrometer range was caused by the secondary ejecta created by primary impacts on the Itokawa surface. If it is assumed that the secondary crater formation rate is equivalent to the lunar impact flux, the measured age based on the number of submicrometer craters can be estimated to be approximately thousands of years.…”