Using in situ Heating in the Transmission Electron Microscope to Probe the Retention of Implanted Solar Wind Gases Trapped in Bubbles in Dust Grains Exposed at the Surface of the Moon

The solar wind, composed of ~96% hydrogen, ~4% helium, and <1% of other elements, implants high-energy (~1 keV/amu) ions into the surface of the Moon and other planetary bodies that lack atmospheres. This solar wind irradiation is a constituent process of space weathering, which over time, alters the chemical, microstructural, and spectral properties of the Moon and other airless bodies. At the sub-micron scale, the cumulative effects of this solar wind irradiation on grains exposed at the surface of the Moon and airless bodies for thousands to millions of years includes the sputtering of atoms, the progressive disruption of the original crystalline structure, the development of complex defects such as vesicles (akin to bubbles), and the implantation of solar wind gases. Advances in scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) capabilities have enabled the direct identification of solar wind-derived gases such as hydrogen, water, helium, and neon trapped in vesicles in space weathered lunar grains.

The retention of these solar wind-derived volatiles is of critical importance to understanding the mechanisms and timescales of the cycling of these species on the lunar surface. Micrometeoroid impacts, hypervelocity dust particles that constantly bombard the Moon, and the other dominant space weathering process, are proposed to be a major limit on the retention and lifetime of solar wind-derived species on the lunar surface. Here, we simulate the short-duration, high-temperature thermal conditions of a micrometeoroid impact on lunar dust grains within the transmission electron microscope and examine changes to the grains’ microstructure, chemistry, and solar wind volatile contents.