The surfaces of the Moon and many asteroids are constantly exposed to streams of charged particles emitted by the Sun, known as the solar wind. Over time, these particles can gradually change the structure and composition of minerals on airless planetary bodies through a process called space weathering.
One mineral of particular interest is troilite (FeS), a common iron–sulfur mineral found in meteorites and lunar rocks. Observations of lunar and asteroid samples have shown that troilite often contains less sulfur on its surface than expected. However, the mechanism behind this sulfur loss has not been fully understood.
In this study, scientists simulated solar wind exposure by bombarding troilite with hydrogen ions in laboratory experiments. The experiment used high-energy hydrogen particles similar to those found in the solar wind to irradiate a polished troilite sample.
Microscopic analysis revealed that the irradiation created a damaged surface layer about 80 nanometers thick. Within this layer, the crystal structure of troilite changed from a single crystal to a polycrystalline structure, and tiny dome-shaped surface features appeared. Most importantly, the chemical composition of the mineral changed dramatically. The sulfur content decreased from approximately 37% in the unaltered crystal to less than 5% in the irradiated surface layer. The researchers estimate that solar wind irradiation could remove sulfur from troilite at a rate of roughly 0.1% per year on the Moon. Over long periods, this process could result in significant sulfur loss from planetary surfaces.
These findings offer important experimental evidence showing how solar wind radiation can modify sulfur-bearing minerals. Understanding this process helps scientists explain the chemical evolution of lunar soil and enhances our knowledge of how the space environment gradually reshapes the surfaces of the Moon, asteroids, and other airless worlds.
Reference
C. Sun et al., “Revealing the loss of sulfur on troilite under simulated solar wind H⁺ irradiation,” Astronomy & Astrophysics, vol. 702, A81, 2025, doi: 10.1051/0004-6361/202555234
