Incipient Space Weathering Observed on the Surface of Itokawa Dust Particles

Abstract

The reflectance spectra of the most abundant meteorites, ordinary chondrites, are different from those of the abundant S-type (mnemonic for siliceous) asteroids. This discrepancy has been thought to be due to space weathering, which is an alteration of the surfaces of airless bodies exposed to the space environment. Here we report evidence of space weathering on particles returned from the S-type asteroid 25143 Itokawa by the Hayabusa spacecraft. Surface modification was found in 5 out of 10 particles, which varies depending on mineral species. Sulfur-bearing Fe-rich nanoparticles exist in a thin (5 to 15 nanometers) surface layer on olivine, low-Ca pyroxene, and plagioclase, which is suggestive of vapor deposition. Sulfur-free Fe-rich nanoparticles exist deeper inside (<60 nanometers) ferromagnesian silicates. Their texture suggests formation by metamictization and in situ reduction of Fe(2+).

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).…”

“…The particles also showed that the asteroid was originally larger than 20 km in diameter, its interior was heated up to 800°C, and the asteroid was destroyed and re-accreted to form the present rubble pile asteroid form of Itokawa . Striking features of the Itokawa particles were records of various asteroidal surface processes, such as solar wind irradiation and micro-meteoroid impact Noguchi et al, 2011;Nakamura et al, 2012). These records suggest that the interior of Itokawa had already been thermally inactive, but active surface processes such as spaceweathering, re-surfacing and particle escape presently occur.…”

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

“…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 ).…”

Dust in the Solar System: Properties and Origins

“…Despite this limitation, researchers have already confirmed that S type asteroids are likely parent bodies of ordinary chondrite meteorites commonly found on Earth [14]. Investigations of space weathering and radiation damage are another area of intense interest [22] as these are the first samples directly obtained from the surface of an asteroid.…”