Scientists from the Vienna University of Technology (TU Wien) studying the origin mystery of the Moon’s thin, gaseous exosphere have found that a key mechanism previously credited with much of its formation, called solar wind-driven sputtering, has likely been significantly overestimated in previous studies.
The authors of the new study caution that they did not remove solar wind-driven sputtering as a cause of the exosphere’s formation. However, the Tu Wien team did ultimately conclude that the calculations based on computer models simulating the moon’s surface material had neglected the rough and porous nature of real lunar regolith, resulting in an overestimation of its exosphere impact.
Solar Wind and the Lunar Exosphere
In a statement announcing the team’s research, Prof. Friedrich Aumayr from the Institute of Applied Physics at TU Wien explained that, unlike Earth, the Moon lacks a dense atmosphere of particles and gases. However, the Moon does have a thin envelope of individual atoms and molecules surrounding the planet called an exosphere. The professor said understanding the origin of this envelope of particles “remains one of the key questions in lunar science.”
The two most likely mechanisms identified by previous studies involve either particles being ejected by high-velocity micrometeorite impacts or the surface material’s interaction with the constant stream of protons, helium ions, and other highly charged particles emitted by the Sun, known as solar wind-driven sputtering. However, the team notes that very reliable experimental data on how much of the exosphere could be caused by these solar wind particles colliding with the moon’s surface “is lacking.”
Hoping to improve the sputtering models of previous efforts, the TU Wien team gained the access needed to perform high-precision experiments with real lunar regolith samples collected in the 1970s by NASA’s Apollo 16 mission. Johannes Brötzner, PhD student at TU Wien and lead author of the study, explained how access to this material, combined with state-of-the-art 3D modelling, was crucial in determining how much sputtering impacts the development of the moon’s exosphere.
“Using a specially developed quartz crystal microbalance, we were able to measure the mass loss of lunar material due to ion bombardment with extremely high accuracy,” Brötzner explained. “In parallel, we conducted large-scale 3D computer simulations on the Vienna Scientific Cluster, allowing us to incorporate the actual surface geometry and porosity of lunar regolith into our calculations.”
After comparing their results to previous estimations of the effects of solar wind-driven sputtering on lunar exosphere creation, the team found that the data from real regolith was markedly different from that used in models simulating regolith. According to the statement, the true yield of exospheric particles from this process “is up to an order of magnitude lower” than previously estimated.
A further analysis of the data revealed the likeliest cause for the differences between real and simulated regolith was the material’s surface structure. Unlike smoother models, the team found that the “porous, loosely bound layer of dust” that makes up the moon’s surface regolith dramatically mitigated the impact of the solar wind’s bombardment. Specifically, their real lunar regolith-based models showed that incoming ions often lose significant energy when bouncing around inside microscopic cavities in the porous surface rather than using that energy to eject atoms.
Findings Could Help Solve Another Lunar Mystery
Aumayr said that along with providing the first “realistic, experimentally validated sputtering yields for actual lunar rock,” the team’s findings could help solve another lunar mystery.
“A recent Science Advances study based on isotope analysis of Apollo samples concluded that, over geological timescales, micrometeorite impacts – not the solar wind – are the dominant source of the lunar exosphere,” he explained. “Our new experimental data independently confirms this conclusion from an entirely different perspective.”
Because NASA’s Artemis program involves a series of missions to the moon with an ultimate goal of establishing a permanent base, the study authors suggest that findings like this could help mission planners determine how to best utilize the moon’s natural processes to their advantage. The researchers also note that with both Artemis and the ESA and JAXA BepiColombo missions, the latter of which is preparing to deliver the first “in-situ” measurements of Mercury’s exosphere in the coming years, interpreting these types of data “will require a detailed understanding of the underlying surface erosion mechanisms – and that is precisely where TU Wien’s research makes a crucial contribution.”
The study “Solar wind erosion of lunar regolith is suppressed by surface morphology and regolith properties” was published in Communications Earth & Environment.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.