Atom probe tomography of space-weathered lunar ilmenite grain surfaces

摘要

The surfaces of airless bodies, such as the Moon and asteroids, are subject to space weathering, which alters the mineralogy of the upper tens of nanometers of grain surfaces. Atom probe tomography (APT) has the appropriate 3-D spatial resolution and analytical sensitivity to investigate such features at the nanometer scale. Here, we demonstrate that APT can be successfully used to characterize the composition and texture of space weathering products in ilmenite from Apollo 17 sample 71501 at near-atomic resolution. Two of the studied nanotips sampled the top surface of the space-weathered grain, while another nanotip sampled the ilmenite at about 50 nm below the surface. These nanotips contain small nanophase Fe particles (3 to 10 nm diameter), with these particles becoming less frequent with depth. One of the nanotips contains a sequence of space weathering products, compositional zoning, and a void space (15 nm in diameter) which we interpret as a vesicle generated by solar wind irradiation. No noble gases were detected in this vesicle, although there is evidence for He-4 elsewhere in the nanotip. This lunar soil grain exhibits the same space weathering features that have been well documented in transmission electron microscope studies of lunar and Itokawa asteroidal regolith grains.