[P-PS07] Formation and evolution of planetary materials in the Solar System

[PPS07-11] Nanostructures of iron sulfides in lunar soils and implications for the behavior of sulfur on airless bodies.

*Toru Matsumoto¹, Takaaki Noguchi¹, Yu Tobimatsu¹, Dennis Harries², Falko Langenhorst², Akira Miyake³, Hiroshi Hidaka⁴ (1.Kyushu University, 2.Friedrich Schiller University Jena, 3.Kyoto University, 4.Nagoya University)

Keywords:Moon, iron sulfides, space weathering

Alteration of materials on airless bodies with time is referred to as space weathering. Extensive studies for space weathering have focused on silicate minerals, whereas the behavior of other minerals in space-exposed environments remains poorly investigated. Recent analysis of Itokawa samples revealed unique features of space weathered rims on iron sulfides, such as selective sulfur loss and the formation of metallic iron whiskers [1]. In this study [2], we investigated surface features of iron sulfides in lunar mature soil grains for the general understanding of sulfide space weathering. We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for the observation of sulfides. SEM observations show that iron sulfides have vesicular textures and iron whiskers on their surfaces. Iron sulfides observed using TEM are composed of 2C troilite and NC-pyrrhotite. The space-weathered rim on the iron sulfides is characterized by crystallographic misorientations and the disappearance of superstructure reflections of 2C troilite in electron diffraction patterns. These crystallographic modifications are probably formed by solar wind irradiation. The rim contains opened vesicles that are aligned along the c-plane of the iron sulfides, as well as numerous tiny vesicles. The Fe/S ratio at the surface of the rim is higher than in non-altered regions, indicating that sulfur escaped from the sulfide surface. Iron whiskers protrude from the space weathered rim and consist of polycrystalline metallic iron. The sulfide rims and the iron whiskers are both coated with vapor-deposited materials rich in O and Si. The combined processes including the solar wind irradiation, heating during impact events, solar UV radiation, and the thermal cycling may cause vesicular textures, selective sulfur escape from the iron sulfides, and the formation of the iron whiskers. The structures of the space weathered rim support the notion that the enrichment of heavy sulfur isotopes in mature lunar soils is due to space weathering of iron sulfides [3]. The space weathered rims on lunar iron sulfides are similar to those observed in regolith particles from asteroid Itokawa. Therefore, alterations of iron sulfide might be common among airless bodies in the solar system.

[1]Matsumoto et al. 2020, Nature Communications 11, 1-8. [2] Matsumoto et al. 2021, Geochimica Cosmochimica Acta, accepted. [3] Kaplan and Smith 1970 Science 167, 541-543.