Space weathered rims found on the surfaces of the Itokawa dust particles

Abstract

Abstract-On the basis of observations using Cs-corrected STEM, we identified three types of surface modification probably formed by space weathering on the surfaces of Itokawa particles. They are (1) redeposition rims (2-3 nm), (2) composite rims (30-60 nm), and (3) composite vesicular rims (60-80 nm). These rims are characterized by a combination of three zones. Zone I occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zone is composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of Zone I and directly attaches to the unchanged minerals (Zone III). Zone I of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two-layered structure: a combination of Zone I and Zone II, below which Zone III exists. Zone II is the partially amorphized zone. Zone II of ferromagnesian silicates contains abundant nanophase Fe. Radiation-induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in Zone II. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. Zone II of the composite vesicular rims contains vesicles. The vesicles in Zone II were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on Itokawa.

Depth profiling analysis of solar wind helium collected in diamond-like carbon film from <i>Genesis</i>

“…Surfaces of extraterrestrial natural samples directly irradiated by solar wind are covered by a deposition layer tens of nanometers in thickness (Keller and McKay, 1993;Noguchi et al, 2014). On the other hand, deposition layers have not been observed in Genesis samples (Calaway et al, 2009), although an organic film contamination layer ("brown stain") has been observed on some Genesis samples.…”

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

“…1 and 2 ) indicates that cratered surfaces exposed to the Sun are subject to solarwind accumulation. Previous investigations of the solar-flare track density showed that regolith particles spend up to of order 10 3 to 10 4 yr in the top few centimeters of Itokawa's surface ( Keller and Berger, 2014;Noguchi et al, 2014 ), which yields an upper limit to T . Measurements of solar-wind noble gases indicate that Itokawa particles were directly exposed to the space environment for 150-550 yr ( Nagao et al, 2011 ).…”

“…1 B and 2 B-F). The spotted structures are probably blisters, which are thought to have formed through accumulation of solar-wind-entrained hydrogen and helium implanted within 60-80 nm ( Noguchi et al, 2014;Matsumoto et al, 2015 ). The blisters might have developed through solar wind irradiation within the order of 10 3 years ( Noguchi et al, 2014 ).…”

“…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 ). 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 ).…”

Hayabusa2: Scientific importance of samples returned from C-type near-Earth asteroid (162173) 1999 JU3

“…Itokawa particles well preserve space-weathering and surface irradiation processes on their surfaces (e.g., Noguchi et al, 2011Noguchi et al, , 2014Busemann et al, 2014), and thus the surface structures and chemical and isotopic compositions of the returned particles from 1999 JU 3 will be carefully investigated to understand space-weathering and surface irradiation on the surface of C-type asteroids for the first time. Thermal metamorphism at the surface of this near-Earth asteroid will also be studied because it may have once had a perihelion smaller than 1 AU (Michel and Delbo, 2010), combined with temperature data obtained by TIR and MAS-COT, which could provide a constraint on the orbital evolution of 1999 JU 3 in the inner solar system (e.g., the maximum temperature and duration).…”