The discovery of silicon oxide nanoparticles in space-weathered of Apollo 15 lunar soil grains

Space weathering is an important process on the Moon and other airless celestial bodies. The most common space weathering effects are amorphization of the top surface of soil grains and formation of nanophase iron particles (npFe) within the partially amorphous rims. Hence, space weathering significantly affects optical properties of the surface of the Moon and other airless celestial bodies. Transmission electron microscope (TEM) analysis of Apollo 15 soil grains displays npFe (<5 nm in size) embedded in the space-weathered rim (similar to 60 nm in thickness) of a pyroxene grain, consistent with previous studies. In contrast, submicron-sized fragments that adhere to the pyroxene grain show distinct space weathering features. Silicon oxide nanoparticles (npSiO(x) ) were observed with npFe in a submicron-sized Mg-Fe silicate fragment. This is the first discovery of npSiO(x) as a product of space weathering. The npSiO(x) and the coexisting npFe are similar to 10-25nm in size, significantly larger than the typical npFe in the space weathered rim of the pyroxene grain. The coexisting npSiO(x) and npFe were probably formed directly in micrometeorite shock-induced melt, instead of in a solar-wind generated vapor deposit or irradiated rim. This new observation will shed light on space weathering processes on the Moon and airless celestial bodies. (C) 2017 Elsevier Inc. All rights reserved.