Simulation of solar wind space weathering in orthopyroxene

• We simulated space weathering via solar wind using Plasma Source Ion Implantation.
• We examined the compositional changes in the outer 50 nm of implanted orthopyroxene.
• We used scanning transmission electron microscopy with electron energy loss spectroscopy.
• Preferential sputtering was observed at the surface of the orthopyroxene.
• Nucleation of nanophase iron was observed in the outer 30 nm of the helium implanted samples.

We have simulated solar wind-based space weathering on airless bodies in our Solar System by implanting hydrogen and helium into orthopyroxene at solar wind energies (~1 keV/amu). Here we present the results of the first scanning transmission electron microscope (STEM) study of one of these simulants. It has been demonstrated that the visible/near infrared (VNIR) reflectance spectra of airless bodies are dependent on the size and abundance of nanophase iron (npFe0) particles in the outer rims of regolith grains. However, the mechanism of formation of npFe0 in the patina on lunar regolith grains and in lunar agglutinates remains debated. As the lattice is disrupted by hydrogen and helium implantation, broken bonds are created. These dangling bonds are free to react with hydrogen, creating OH and/or H2O molecules within the grain. These molecules may diffuse out through the damaged lattice and migrate toward the cold traps identified at the lunar poles. This mechanism would leave the iron in a reduced state and able to form npFe0. This work illustrates that npFe0 can be nucleated in orthopyroxene under implantation of solar wind hydrogen and helium. Our data suggest that the solar wind provides a mechanism by which iron is reduced in the grain and npFe0 is nucleated in the outer surfaces of regolith grains. This formation mechanism should also operate on other airless bodies in the Solar System.

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