The surfaces of the Moon and Mercury: an experimental and numerical approach to ion sputtering

Because of their proximity to the Sun or their small size, Mercury and the Moon have not been able to retain their initially outgassed atmospheres; only thin collision-less exospheres surround these objects. In the tenuous exosphere of Mercury a number of different species have been detected up to now: H, He, O, Na, Ca, Mg, K. The total surface pressure of these species is about two orders of magnitude lower than the derived upper limit of the exospheric pressure of 10-10 mbar. Hence some additional yet unobserved volatile material probably populates Mercurys exosphere. The Lunar exosphere is very tenuous and consists mainly of volatile species with a contribution of refractory (rock-forming) elements. Most refractory particles in the exospheres of Moon and Mercury are released by energetic ions from the solar wind and from magnetospheric plasma precipitating onto the surface via ion implantation and sputtering processes. In our research project we will perform for the first time sputtering experiments in a laboratory with realistic analogue materials representative for the surface of the Moon and Mercury. These experiments will help us understand surface release processes and the origin of exospheres based on surface elements from airless bodies. The experiments and connected theoretical studies are carried out in preparation of the forthcoming BepiColombo (ESA) mission and Luna (Roskosmos) missions. The results of these sputtering experiments will lead to an improved instrument design for future space missions to the Moon. The results will also constrain and modify theoretical model input parameters related to the porosity and thermal state of soil analogues as well as binding energies of released minerals and stopping of energetic particles. Finally, the experiments will help us understand how planetary surfaces are eroded and modified as a result of ion impact, which also is relevant for the formation of Earth-like planets via small planetesimal building- blocks.

The FWF-SNSF funded project "The surfaces of the Moon and Mercury: an experimental and numerical approach to ion sputtering" aimed to understand how the surfaces of the Moon and Mercury are affected by space weathering caused by solar (ion) radiation (the so-called solar wind). These two celestial bodies lack protective atmospheres, so their surfaces are altered over time by radiation and the formation of thin atmospheres composed of emitted surface particles. The project involved experiments in which materials similar to lunar and Mercury minerals were exposed to ion radiation in a laboratory. The effects observed in these experiments were then replicated in computer simulations. The project was a collaboration between the TU Vienna and the University of Bern. The two main researchers were Prof. Dr. Friedrich Aumayr from the TU Wien and Priv.Doz. Dr. Andre Galli from the University of Bern. The project involved two PhD students, Herbert Biber from the TU Vienna and Noah Jäggi from the University of Bern. All the five main objectives: 1. Create mineral films resembling lunar and Mercury surfaces and measure their sputtering under ion irradiation. 2. Create mineral pellets and irradiate them with ions to measure sputtering yields. 3. Quantify the importance of potential sputtering. 4. Interpret the results and compare them with sputtering simulation software. 5. Put the experimental and simulation results in the context of space research. were met very successfully, with more than 10 papers already published in top astrophysical, planetary, and surface science journals, and a final paper on lunar regolith sputtering and its implications for the lunar exosphere in preparation. The COVID-19 pandemic affected laboratory access and international collaboration. Despite these challenges, the project achieved significant milestones, including the determination of experimental sputtering yields and angular distributions of sputtered particles for various minerals and ion types, and the development of new codes to predict these. Due to the successful demonstration of our unique measurement technique, NASA could be convinced to provide 2.4 g of precious "real" lunar regolith ("moon dust") from the Apollo 16 landing site on the Moon for our experiments. The results of this project have broad implications for understanding the surfaces of celestial bodies such as the Moon and Mercury, and thus contribute to improved modeling of planetary surfaces and exospheres.