Thermal-induced Alterations in Lunar Soil Grains Revealed via In Situ TEM Heating

Abstract

Impact-induced thermal modifications occur frequently in the solar system, and the microstructures of extraterrestrial samples can reveal the thermal-induced alterations that have occurred to their parent bodies. The characterization and reproduction of these sub-micron features are challenging and require new analytical techniques. The combination of a dual-beam system and transmission electron microscopy (TEM) techniques, with in situ TEM heating, is a promising method. This method can be used to systematically understand the chemical and microstructural changes in extraterrestrial samples resulting from thermal-induced alterations. To better reproduce the microstructures observed in the Chang’E-5 soil grains, a comprehensive series of TEM experiments was conducted by varying the in situ heating conditions and starting materials, including Martian meteorites and Earth samples. The experimental results showed that the nanophase iron (np-Fe0) particles, which appeared in both Fe-rich pigeonite and Fe-poor olivine, as well as the np-Fe0 content and morphology, varied with increasing heating time (0 – 90 min) at 800 °C. These results indicated that the formation mechanism of np-Fe0 particles varied in different matrices. Therefore, our established approach can reveal the thermal-induced alterations in celestial bodies and can be applied effectively for studying the formation of microscopic features in extraterrestrial bodies.