The mineralogy and space weathering of a regolith grain from 25143 Itokawa and the possibility of annealed solar wind damage

Abstract

We report the results of detailed mineralogical investigations by analytical scanning and transmission electron microscopy of particle RA-QD02-0115 recovered from the surface of asteroid 25143 Itokawa. We divided the 65 μm × 50 μm small particle into eight individual subsample slices via the focused ion beam method. The particle dominantly consists of olivine and contains inclusions of merrillite, tetrataenite/taenite, troilite, chromite, kamacite, and Cl-bearing apatite (in approx. decreasing order of frequency). The composition of olivine (fayalite 29.8 ± 1.1 mol% and molar Fe/Mn ratio of 57 ± 2) as well as the Ni-rich metal assemblage indicates an LL-type affinity in accord with previous classifications. The particle shows effects of solar wind irradiation on one of its principal faces. Olivine developed an approximately 34 nm wide rim composed of low-angle misoriented, nanometer-sized crystallites accompanied by a small amount of amorphous material. Exposed troilite developed a 4 to 8 nm wide polycrystalline rim with large-angle misorientations of the iron sulfide nanocrystallites. Merrilite shows marginally discernable surface damage but was too unstable under the electron beam for a detailed study. Cl-bearing apatite was found fully crystalline with no discernable rim structure. We discuss the unusual polycrystalline nature of the olivine rim in terms of possible annealing and recrystallization effects, which may have occurred during periods of time when Itokawa’s surface temperature may have been warmer due to closer perihelion distances. Model calculations show that the dynamical orbital evolution of near-Earth asteroids could lead to complex space weathering processes, arising from the competing interplay between irradiation-induced damaging and thermally driven annealing.