Surface Binding Energies for Amorphous Plagioclase Feldspar Calculated using Molecular Dynamics

TL;DR

This study uses molecular dynamics to compare surface binding energies and sputtering yields of amorphous versus crystalline plagioclase feldspar, finding minimal differences due to similar elemental bonds despite structural disorder.

Contribution

It provides the first detailed molecular dynamics analysis of SBEs for amorphous feldspar, revealing that amorphous structure causes a wider SBE distribution but similar sputtering yields to crystalline forms.

Findings

Amorphous SBEs have a wider distribution than crystalline SBEs.

Sputtering yields are similar (<20% difference) for amorphous and crystalline surfaces.

Atomic bonds are consistent across structures, explaining similar sputtering behavior.

Abstract

Despite the well-established presence of amorphous compounds on planetary bodies such as the Moon and Mercury due to space weathering, the direct effect of atomic arrangement on the surface binding energies (SBEs) of elements on these bodies remains largely unexplored. Accurate SBE values are essential for reliably predicting sputtering yields and the energy distribution of ejecta. Here, we use molecular dynamics simulations to quantify SBEs for the different elements sputtered from amorphous atomic arrangements of the plagioclase feldspar end members, albite and anorthite, and compare to their crystalline counterparts. Results show that while the mean elemental SBEs from amorphous surfaces are not significantly different from their crystalline counterparts, the random orientation in amorphous structures gives rise to a spectrum of bonding configurations, resulting in a distribution of SBEs with a wider range. This contrasts with the clearly discretized set of SBE values associated with the ordered atomic structure of crystalline surfaces. We then consider sputtering by H, He, and a solar wind combination of 96% H and 4% He. For each of these cases, we demonstrate that there is minimal difference (<10% for albite and <20% for anorthite) between the sputtering yields of amorphous and crystalline surfaces. We attribute these results to the presence of the same elemental bonds across different atomic arrangements, which leads to similar mean SBEs and, consequently, comparable sputtering yields.