Water Formation During Micrometeoroid Impact on the Lunar Surface: A Molecular Dynamics Study

Introduction: The detection of water on the lunar surface by spacecraft flybys and ground observations has raised questions about its origins. While solar wind is likely an important source of H, leading to H₂O and -OH formation, other external processes such as interplanetary dust implantation, micrometeoroid impacts, and dielectric breakdown may also be important in the formation of water molecules. Whilst laboratory observations and studies are informative, they only report end result data, without providing a complete insight on the evolution of the surface composition. In addition, macroscopic simulation models using laboratory equipment are unable to simultaneously resolve chemical reactions at the atomic scale during collisions. In order to better understand water formation and retention, studies at the atomistic level need to be conducted. Molecular dynamics (MD) simulations can capture atomic bonds and molecule formation during different processes, allowing for studies of the atomistic processes underlying water production.

Huang et al. [1], used MD to study water production and retention after a micrometeoroid impact for near surface hydrogen implantation. Their simulations used a reactive force field (ReaxFF) in order to capture bond breaking and formation of H₂O molecules while testing different impact velocities for a 6nm impactor. Their results showed that the optimum velocity water production was 16 km/s while they also noted that even at the nanoscale molecules can still reach the Moons exosphere. Their work was limited to hydrogen at the surface only, focusing on water production during normal micrometeoroid impacts. However, the majority of solar wind protons are distributed throughout the substrate rather than concentrated at the near surface according to the site location on the Moon. Research is needed on the role of implantation depth and incidence angle on the water formation behavior during micrometeoroid impacts. Thus, in this study, we consider water formation and retention during micrometeoroid impacts for different realistic hydrogen implantation depth cases in a silica substrate. Using atomistic modelling we also examine the effect of different micrometeoroid sizes at normal and 30^o incident angles on water production and retention.

 

Methodology: We conducted MD simulations of micrometeoroid impacts on the lunar surface to better understand the underlying physics of water formation, retention. We used a reactive force field (ReaxFF) potential, originally parameterized to describe a silica-water system and previously used in micrometeoroid and H diffusion simulations in amorphous SiO₂ [1]-[4].

First, a 433 × 433 × 290 ų substrate with a free surface is created, representing a part of the outer surface of a lunar grain. Hydrogen atoms were then placed according to two different depth distribution cases. A near surface distribution is created by placing ~30000 H atom at the surface only and then impacting. A distribution with depth was created by running separate binary collision approximations following best practices for SW impacts [5] and then depositing H in the MD substrate according to these results. Micrometeoroids of 6nm and 9nm in diameter were simulated at velocities ranging from 12-20 km/s and at normal and 30^o degrees angles of incidence.

 

Results: MD simulations showed that water generation during a micrometeoroid impact is highly dependent on impact velocity, incident angle, and initial depth distribution of H. In the majority of cases there was an increase in H₂O generation with impactor velocity. Interestingly, when the hydrogen is distributed throughout the depth (as opposed to at the near surface) there is a difference in both H₂O production rate and retention. For example, for a 9 nm impactor at 30 degrees there was in increase in retained water by 20% when H was distributed according to SW impacts. In contrast, for this same impactor there was a distinct drop in retained water by 24% when H was deposited at the surface only. We also observe more H₂O ejecta when compared to the distributed H cases. Therefore, whether micrometeorites lead to the formation or loss of H₂O strongly depends on the initial deposition profile of the implanted H. In addition, other ejected atoms and molecules such as H, H₂ and -OH leave the substrate during impact, these ejecta can potentially contribute to the lunar exosphere or be redistributed at nearby sites.

Results also indicate a dependence on the impact energy in the normal direction for the different cases simulated. For example, a 6nm micrometeoroid at 20 km/s and normal impact angle exhibits similar distribution behavior as a 9nm micrometeoroid at 12 km/s and normal impact angle. This indicates a dependence in impact energy as the two cases have close energies of 5 x 10^-14 J and 5.8 x 10^-14 J respectively. In addition, a decrease from initial concentration was observed at the near surface of the substrate, 0 - 40 Å in depth. This is likely due to the high impact energy experienced at the surface that raises the local temperature, breaking the chemical bonds of the surface water. Results point to the importance of considering both the impactor behavior (velocity/size/angle) along with the initial hydrogen deposition when studying water production. Those areas on the Moon which are directly exposed to SW and thus have H deposited at depth are more likely to retain produced water.

References:

[1] Huang, Z. et al. (2021), Geophysical Research 142 Letters, 48(15)

[2] Fogarty, J. C. et al., (2010) Journal of Chemical Physics Vol 132, Issue 17, p. 174704

[3] Morrissey, L. S. et al., (2022) Icarus Vol 379, p. 114979

[4] Sheikholeslam, S. A. et al., (2016) J Mater Chem C Mater Vol 4, Issue 34, pp. 8104–8110

[5] Morrissey L. S. et al. (2023) Planet. Sci. J. 4 67