SPACE/COSMOS
Moon under bombardment
Where does the Moon’s exosphere come from? A TU Wien study using real lunar rock reveals that the erosive effect of solar wind ions on the Moon has been vastly overestimated.
image:
Moon rock in a vacuum chamber at TU Wien
view moreCredit: TU Wien
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.
The author team from TU Wien: Richard A. Wilhelm, Gyula Nagy, Johannes Brötzner (first author of the study), Martina Fellinger, Friedrich Aumayr (left to right)
Credit
David Rath, TU Wien
Journal
Communications Earth & Environment
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Solar wind erosion of lunar regolith is suppressed by surface morphology and regolith properties
NASA, JAXA XRISM satellite X-rays Milky Way’s sulfur'
NASA/Goddard Space Flight Center
image:
This composite shows a section of the interstellar medium scientists X-rayed for sulfur using the Japan-led XRISM (X-ray Imaging and Spectroscopy Mission). X-ray binary GX 340+0 is the blue dot in the center. The composite contains a blend of imagery in X-rays (represented in deep blue), infrared, and light.
view moreCredit: DSS/DECaPS/eRosita/NASA’s Goddard Space Flight Center
An international team of scientists have provided an unprecedented tally of elemental sulfur spread between the stars using data from the Japan-led XRISM (X-ray Imaging and Spectroscopy Mission) spacecraft.
Astronomers used X-rays from two binary star systems to detect sulfur in the interstellar medium, the gas and dust found in the space between stars. It’s the first direct measurement of both sulfur’s gas and solid phases, a unique capability of X-ray spectroscopy, XRISM’s (pronounced “crism”) primary method of studying the cosmos.
“Sulfur is important for how cells function in our bodies here on Earth, but we still have a lot of questions about where it’s found out in the universe,” said Lía Corrales, an assistant professor of astronomy at the University of Michigan in Ann Arbor. “Sulfur can easily change from a gas to a solid and back again. The XRISM spacecraft provides the resolution and sensitivity we need to find it in both forms and learn more about where it might be hiding.”
A paper about these results, led by Corrales, published June 27 in the Publications of the Astronomical Society of Japan.
Using ultraviolet light, researchers have found gaseous sulfur in the space between stars. In denser parts of the interstellar medium, such as the molecular clouds where stars and planets are born, this form of sulfur quickly disappears.
Scientists assume the sulfur condenses into a solid, either by combining with ice or mixing with other elements.
When a doctor performs an X-ray here on Earth, they place the patient between an X-ray source and a detector. Bone and tissue absorb different amounts of the light as it travels through the patient's body, creating contrast in the detector.
To study sulfur, Corrales and her team did something similar.
They picked a portion of the interstellar medium with the right density — not so thin that all the X-rays would pass through unchanged, but also not so dense that they would all be absorbed.
Then the team selected a bright X-ray source behind that section of the medium, a binary star system called GX 340+0 located over 35,000 light-years away in the southern constellation Scorpius.
Using the Resolve instrument on XRISM, the scientists were able to measure the energy of GX 340+0’s X-rays and determined that sulfur was present not only as a gas, but also as a solid, possibly mixed with iron.
“Chemistry in environments like the interstellar medium is very different from anything we can do on Earth, but we modeled sulfur combined with iron, and it seems to match what we’re seeing with XRISM,” said co-author Elisa Costantini, a senior astronomer at the Space Research Organization Netherlands and the University of Amsterdam. “Our lab has created models for different elements to compare with astronomical data for years. The campaign is ongoing, and soon we’ll have new sulfur measurements to compare with the XRISM data to learn even more.”
Iron-sulfur compounds are often found in meteorites, so scientists have long thought they might be one way sulfur solidifies out of molecular clouds to travel through the universe.
In their paper, Corrales and her team propose a few compounds that would match XRISM’s observations — pyrrhotite, troilite, and pyrite, which is sometimes called fool’s gold.
The researchers were also able to use measurements from a second X-ray binary called 4U 1630-472 that helped confirm their findings.
“NASA’s Chandra X-ray Observatory has previously studied sulfur, but XRISM’s measurements are the most detailed yet,” said Brian Williams, the XRISM project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Since GX 340+0 is on the other side of the galaxy from us, XRISM’s X-ray observations are a unique probe of sulfur in a large section of the Milky Way. There’s still so much to learn about the galaxy we call home.”
XRISM is led by JAXA (Japan Aerospace Exploration Agency) in collaboration with NASA, along with contributions from ESA (European Space Agency). NASA and JAXA developed Resolve, the mission’s microcalorimeter spectrometer.
Journal
Publications of the Astronomical Society of Japan
Article Title
XRISM insights for interstellar sulfur
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