Simulating the Diffusion of Hydrogen in Amorphous Silicates: A ‘Jumping’ Migration Process and its Implications for Solar Wind Implanted Lunar Volatiles

We use molecular dynamics (MD) simulations to better explain the movement of atomic
hydrogen in amorphous silica and quantify the planetary science implications of these findings.
Previous MD simulations had a large range of predicted values and did not agree well with
experiment. Our simulations sample atomic motion for a longer duration and consider a wider
range of temperatures than previous simulations. In contrast to constant atomic motion, the
hydrogen atoms were shown to undergo random intermittent jumps from one oxygen atom to
another, the number of which increase with temperature. Predicted diffusion coefficients had a
better agreement to experimental values than previous MD simulations, suggesting the
importance of longer simulation durations for better statistics. The low activation energy and
jumps observed at lunar temperatures do not support the theory of diurnal variations in OH
content for an undamaged amorphous silica surface. Instead, we conclude that energetic solar
wind impacts can induce two competing atomic hydrogen motion processes in the exposed
surface: A prompt effect that induces jumps in the temperature spike volume, but also a long term
effect of damage in the structure that traps atomic hydrogen. We then use SDTrimSP to
quantify the damage created during exposure and MD to demonstrate the H retention and
trapping near these defects. Damage was shown to be dependent on impact energy, with
defects easily retaining implanted hydrogen. MD results like those presented herein on
unweathered surfaces are therefore most relevant to magnetic anomalies. As a result, we
demonstrate the importance of lunar volatile models to account for the damage state of the
substrate when modelling hydrogen diffusion, retention, and subsequent OH/water production.