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 Mercury’s 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.