Context. The solar wind impinging on the lunar surface results in the emission of energetic neutral atoms. This particle population is one of the sources of the lunar exosphere. Aims. We present a semi-empirical model to describe the energy spectra of the neutral emitted atoms. Methods. We used data from the Advanced Small Analyzer for Neutrals (ASAN) on board the Yutu-2 rover of the Chang'E-4 mission to calculate high-resolution average energy spectra of the energetic neutral hydrogen flux from the surface. We then constructed a semi-empirical model to describe these spectra. Results. Excellent agreement between the model and the observed energetic neutral hydrogen data was achieved. The model is also suitable for describing heavier neutral species emitted from the surface. Conclusions. A semi-analytical model describing the energy spectrum of energetic neutral atoms emitted from the lunar surface has been developed and validated by data obtained from the lunar surface.

Using the near-infrared spectral reflectance data of the Chandrayaan-1 Moon Mineralogy Mapper (M-3) instrument, we report an unusually bright structure of 30 x 60 km(2) on the lunar equatorial farside near crater Dufay. At this location, the 3-mu m absorption band feature, which is commonly ascribed to hydroxyl (OH) and /or water (H2O), at local midday is significantly (similar to 30%) stronger than on the surrounding surface and, surprisingly, stronger than in the illuminated polar highlands. We did not find a similar area of excessively strong 3-mu m absorption anywhere else on the Moon. A possible explanation for this structure is the recent infall of meteoritic or cometary material of high OH /H2O content forming a thin layer detectable by its pronounced 3-mu m band, where a small amount of the OH /H2O is adsorbed by the surface material into binding states of relatively high activation energy. Detailed analysis of this structure with next-generation spacecraft instrumentation will provide further insight into the processes that lead to the accumulation of OH /H2O in the lunar regolith surface.

Lunar OH/H2O has been confirmed and mapped by analyzing the 3 mu m absorption band in spectra acquired by the Moon Mineralogy Mapper (M-3) instrument. Space weathering leads to accumulation of submicroscopic iron particles in the uppermost layer of the regolith which gradually changes the spectral signature of airless planetary bodies and thus may affect the detection of lunar OH/H2O. The contribution of this paper is twofold. (1) Our new technique combines Hapke reflectance modeling and ab initio Mie scattering calculations to model the scattering behavior of submicroscopic iron which governs the optical effects due to space weathering. (2) Thermally corrected M-3 spectra of mature and immature sample points in mare and highland regions are used to assess the performance of the simulation framework and are analyzed to understand maturity-related changes of the OH/H2O band depth. We find that the simulation method can convincingly reproduce the spectral changes of maturing lunar soil. It becomes clear that there is only a minor effect on the 3 mu m absorption feature. This finding makes the analysis of the lunar OH/H2O mapping largely invariant with respect to space weathering. In general, the absorption features around 1 and 2 mu m are more strongly obstructed than the feature around 3 mu m. Further, we discuss agglutination as the main cause for slight deviations found around the 2 mu m band and layering/clustering as a likely reason to explain predicted iron particle sizes that are larger than observed.

While the Earth and Moon are generally similar in composition, a notable difference between the two is the apparent depletion in moderately volatile elements in lunar samples. This is often attributed to the formation process of the Moon, and it demonstrates the importance of these elements as evolutionary tracers. Here we show that paleo space weather may have driven the loss of a significant portion of moderate volatiles, such as sodium and potassium, from the surface of the Moon. The remaining sodium and potassium in the regolith is dependent on the primordial rotation state of the Sun. Notably, given the joint constraints shown in the observed degree of depletion of sodium and potassium in lunar samples and the evolution of activity of solar analogs over time, the Sun is highly likely to have been a slow rotator. Because the young Sun's activity was important in affecting the evolution of planetary surfaces, atmospheres, and habitability in the early Solar System, this is an important constraint on the solar activity environment at that time. Finally, as solar activity was strongest in the first billion years of the Solar System, when the Moon was most heavily bombarded by impactors, evolution of the Sun's activity may also be recorded in lunar crust and would be an important well-preserved and relatively accessible record of past Solar System processes.

Molecular dynamics simulations are used to analyse the effects after 20 MeV sulfur ion impact into an ice mixture consisting of water, carbon dioxide, ammonia, and methanol. By using a so-called REAX, i.e., reactive, potential, the chemical processes occurring after the impact can be studied. Such impacts may occur in Jupiter's magnetosphere, where energetic S ions originate from Io's surface and irradiate ice surfaces of Jupiter's moons, of comets or ice dust particles entering the magnetosphere. By segmenting the ion trajectory to smaller pieces that fit into our simulation box, we can follow the ion from its impact point at the surface down to the depth where it is stopped. Electronic stopping is modelled by a thermal track model; it is necessary to use a sufficiently small track radius R in order to be able to include the hot-chemistry reactions occurring in the track volume. We find that the number of dissociations and ensuing reactions scales approximately linearly with the deposited energy density. In consequence, the total number of molecules produced is approximately proportional to the impact energy. In addition, the most complex molecules are formed at the highest energy densities. Smaller molecules such as formaldehyde and hydrogen peroxide, in contrast, are produced all along the ion track.