Moon under bombardment

The Moon’s surface is continuously bombarded by the solar wind – a stream of electrically charged particles ejected by the Sun. These high-energy ions can knock atoms out of the Moon’s uppermost layer of rock, forming an extremely thin envelope of gas around the Moon known as the exosphere. But how exactly this exosphere forms, has remained a major open question.

A research team at TU Wien, in collaboration with international partners, has now demonstrated that one of the key processes – solar wind–driven sputtering – has been significantly overestimated in previous models. The reason: earlier calculations neglected the rough and porous nature of real lunar regolith. For the first time, high-precision experiments using original samples from NASA’s Apollo 16 mission, combined with state-of-the-art 3D modeling, have allowed the team to determine realistic sputtering rates. The results have now been published in the journal Communications Earth & Environment (Nature Portfolio).

A Thin Atmosphere – But Where Does It Come From? 

“The Moon has no dense atmosphere like Earth – but it does have a tenuous exosphere, made up of individual atoms and molecules,” explains Prof. Friedrich Aumayr from the Institute of Applied Physics at TU Wien. “Understanding the origin of these particles remains one of the key questions in lunar science.”

Two mechanisms have been considered as main contributors: either particles are ejected by high-velocity micrometeorite impacts, or they are released via interaction with the solar wind – the constant stream of protons, helium ions, and other charged particles emitted by the Sun. Until now, however, reliable experimental data on actual solar wind sputtering from lunar material has been lacking.

First Experiments with Real Lunar Rock

For the first time, precision experiments have now been performed at TU Wien using original Moon rock from NASA’s Apollo 16 mission. “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,” explains Johannes Brötzner, PhD student at TU Wien and lead author of the new publication. “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.”

The result: the real erosion rate caused by the solar wind has been drastically overestimated. The actual yield is up to an order of magnitude lower than previously assumed. This is primarily due to the structure of the regolith – a porous, loosely bound layer of dust covering the Moon’s surface. When incoming ions strike the regolith, they often lose their energy in multiple collisions inside microscopic cavities, rather than immediately ejecting surface atoms. As a result, the sputtering efficiency is significantly reduced compared to a smooth, dense surface.

Micrometeorites Outweigh the Solar Wind 

“Our study provides the first realistic, experimentally validated sputtering yields for actual lunar rock,” says Friedrich Aumayr. “Not only does this explain why earlier models overestimated solar wind erosion – it also helps resolve a previously unresolved scientific discrepancy: 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. Our new experimental data independently confirms this conclusion from an entirely different perspective.“

Key Insights for Lunar and Mercury Missions

These results are especially timely: NASA’s Artemis program is advancing in a new era of lunar exploration, and ESA’s and JAXA’s BepiColombo mission is set to deliver the first in-situ measurements of Mercury’s exosphere in the coming years. Interpreting these 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.