Comparison of space weathering features in three particles from Itokawa

Abstract

The return of samples from S-type asteroid 25143 Itokawa have enabled significant improvements in our understanding of space weathering on asteroids and the link between S-type asteroids and ordinary chondrites. We report on three new particles, providing details on space weathering of adjacent grains within a particle and several different phases. The features we observe are consistent with formation via irradiation from the solar wind, as opposed to micrometeoroid bombardment. We also see differences in the degree of weathering for grains collected from the two different touchdown locations on the asteroid. Continued analysis of new grains from Itokawa allow us to draw a more complete picture of the variety of space weathering features and the processes that lead to their formation on Itokawa and other airless bodies.

Space weathering signatures in sulfide and silicate minerals from asteroid Itokawa

“…In pentlandite, this S depletion is accompanied by Ni loss, as discussed below. The depth of sulfur depletion is similar to results reported for a pyrrhotite grain by Keller and Berger ( 2014 ) and in troilite by Burgess and Stroud ( 2021 ). Furthermore, these measurements are supported by ion irradiation experiments that have identified possible mechanisms that could limit sulfur depletion in Fe-sulfide rims to the upper few nanometers.…”

“…As in the pentlandite grain, we identified O in the nanocrystalline rim. Oxidized rims on sulfides from asteroid Itokawa have been reported previously by Burgess and Stroud ( 2021 ). The oxidation of sulfides was most likely produced by the exposure of the regolith particle to Earth’s atmosphere during sample preparation.…”

“…However, most laboratory simulations and studies of returned samples have focused on understanding the effects of space weathering processes on silicate minerals which dominate lunar soils and ordinary chondrites (Keller and McKay 1993 , 1997 ; Dukes et al 1999 ; Sasaki et al 2001 ; Kohout et al 2014 ; Loeffler et al 2016 ). Meanwhile, the situation is less constrained for minerals such as Fe- and Fe–Ni-sulfides that are found on other airless planetary bodies including ordinary chondritic asteroids like Itokawa (Keller and Berger 2014 ; Matsumoto et al 2020 ; Burgess and Stroud 2021 ), in carbonaceous materials like those returned from asteroid Ryugu (Han et al 2022 ; Matsumoto et al 2022 ; Viennet et al 2022 ) and those expected from asteroid Bennu (Hamilton et al 2019 ), and in CB chondrites the closest meteoritic counterpart for Psyche (Weisberg et al 2001 ; Elkins-Tanton et al 2020 ).…”

“…Similar to investigations of lunar soils, analysis of the space weathering characteristics of Itokawa grains has predominantly focused on silicate minerals, including pyroxene, olivine, and plagioclase. Those analyses have revealed completely and partially amorphous rims, multilayer rims that are chemically distinct from the underlying grain, vesiculated textures, and Fe- and Fe-sulfide nanoparticles (npFeS) (Noguchi et al 2011 , 2014a , b ; Keller and Berger 2014 ; Thompson et al 2014 ; Burgess and Stroud 2021 ). In particular, the presence of npFeS in Itokawa samples was novel, as the nanoparticles in space-weathered lunar samples are predominantly composed of Fe 0 (Keller and McKay 1993 ; Thompson et al 2016 ; Burgess and Stroud 2018 ).…”

“…Laboratory simulations and analysis of returned samples are also crucial to the interpretation of remote sensing observations. Sulfur depletion has been measured in sulfide-bearing Itokawa regolith particles (Keller and Berger 2014 ; Burgess and Stroud 2021 ), but was not identified in the X-ray fluorescence spectrometry (XRF) analyses of the surface of asteroid Itokawa (Arai et al 2008 ). However, the XRF data show a global variation of S that could result from localized space weathering processes (Arai et al 2008 ).…”