Analysis of Returned Samples from Asteroid Itokawa Shows that Sulfides and Silicates Respond Differently to Space Weathering

EDS chemical mapping (e) and line scan (f) of pentlandite (Fe-Ni-sulfide), indicating depletion of S and Ni and enrichment of Fe in the crystalline rim of RC-MD01-0025 Itokawa regolith particle. Credit: Chaves et al., 2022.

In 2010, the Japanese Aerospace Exploration Agency’s (JAXA’s) Hayabusa mission returned samples from S-type asteroid 25143 Itokawa that revolutionized our understanding of the links between asteroids and meteorites. The Itokawa samples were found to have oxygen isotope compositions matching those of ordinary chondrites (OC), which confirmed a hypothesized link between OC and S-type asteroids. However, there are differences in reflectance spectra between OC and S-type asteroids. This has been attributed to chemical and microstructural changes in regolith particles on the surfaces of airless rocky bodies caused by space weathering, while meteorites may come predominantly from interior material. The nature of these changes has been investigated with lunar soil and Itokawa grains, as well as space weathering experiments. However, most of these studies have focused on silicate minerals (e.g., olivine) rather than sulfides (e.g., troilite and pentlandite), which may respond differently.

A new study by Laura Chaves and Michelle Thompson from Purdue University focused on space-weathered rims of troilite, pentlandite, and olivine in an Itokawa particle. Using high-resolution transmission electron microscopy, they observed depletions of sulfur (S) and nickel (Ni) in the rims of troilite and pentlandite. The sulfides also had crystalline rims, whereas the olivines had crystalline and amorphous rims. This suggests that sulfides are more resistant to amorphization by solar wind irradiation. For the Ni and S depletion, the authors suggest that Ni-S molecules, rather than individual atoms, were removed because, although individual Ni and iron (Fe) binding energies are much higher than S in the crystalline lattice, the Ni-S bond is stronger than the Fe-S bond in pentlandite. Understanding the chemical and structural characteristics of space-weathered pentlandite will be important in identifying space weathering signatures in Hayabusa2 samples recently returned from asteroid Ryugu, in which pentlandite has been identified, and potentially in OSIRIS-REx samples en route from asteroid Bennu. READ MORE