The possibility of OH/H2O formation on the lunar surface has been proposed because of the interaction between protons from the solar wind and oxygen in the regolith. In this study, we examined olivine, pyroxene, plagioclase, and volcanic glass samples together irradiated with 7 keV H+ at a dose of 10(17) ions/cm(2) under the same experimental conditions to simulate the solar-wind proton implantation process on the Moon. By comparing the infrared spectral characteristics of these samples before and after H+ implantation through an infrared spectrometer, we confirm that OH forms in all minerals and glass after H+ implantation, with a remarkable amount of OH/H2O found in plagioclase. This indicates that plagioclase can capture more H+ than other silicate phases to form the OH/H2O. The absorption characteristics of OH/H2O formed by H+ implantation are distinct and associated with the mineral structure. The efficiency of OH/H2O formation by H+ implantation is affected by crystal structure. We conclude that OH/H2O formed by solar-wind implantation in the lunar soil is likely to be mainly preserved in plagioclase, and the estimated OH/H2O absorption strength from 0.7 to 3.6% at 3356 cm(-1) and from 0.9 to 4.8% at 3622 cm(-1) of plagioclase is consistent with those found by recent lunar spacecraft missions.

The solar wind implants protons into the top 20-30nm of lunar regolith grains, and the implanted hydrogen will diffuse out of the regolith but also interact with oxygen in the regolith oxides. We apply a statistical approach to estimate the diffusion of hydrogen in the regolith hindered by forming temporary bonds with regolith oxygen atoms. A Monte Carlo simulation was used to track the temporal evolution of bound OH surface content and the H-2 exosphere. The model results are consistent with the interpretation of the Chandrayaan-1 M-3 observations of infrared absorption spectra by surface hydroxyls as discussed in Li and Milliken (2017, ). The model reproduced the latitudinal concentration of OH by using a Gaussian energy distribution of f(U-0=0.5eV, U-W=0.078-0.1eV) to characterize the activation energy barrier to the diffusion of hydrogen in space weathered regolith. In addition, the model results of the exospheric content of H-2 are consistent with observations by the Lyman Alpha Mapping Project on the Lunar Reconnaissance Orbiter. Therefore, we provide support for hydroxyl formation by chemically trapped solar wind protons. Plain Language Summary Understanding the water content in the Moon's surface and its thin atmosphere is of interest for space missions. Water products have been detected in various forms (H2O and OH) on the Moon, which are not distinguishable in the observations. Herein, we examined the contribution of OH content. The solar wind implants protons (positively charged hydrogen atoms) into the top layers of the lunar soil. The implanted H atoms spread out in the grains interacting with other atoms like oxygen. We estimate the mobility of H atoms as they travel to the surface and escape into the Moon's exosphere (very thin atmosphere). The mobility of hydrogen is hindered because they can interact with other atoms or molecules as they travel in the soil. Some hydrogen will interact with oxygen and form OH. We used a Monte Carlo (probability) simulation to track the variation in the surficial amount of OH on the Moon's surface during day and night and hydrogen released in the exosphere (very thin atmosphere). It is found that considering the effect of a variety of trapping sites (interaction sites), hydrogen mobility is needed to reproduce the content of OH in the surface and hydrogen in the exosphere.

We have simulated solar wind-based space weathering on airless bodies in our Solar System by implanting hydrogen and helium into orthopyroxene at solar wind energies ( 1 keV/amu). Here we present the results of the first scanning transmission electron microscope (STEM) study of one of these simulants. It has been demonstrated that the visible/near infrared (VNIR) reflectance spectra of airless bodies are dependent on the size and abundance of nanophase iron (npFe(0)) particles in the outer rims of regolith grains. However, the mechanism of formation of npFe(0) in the patina on lunar regolith grains and in lunar agglutinates remains debated. As the lattice is disrupted by hydrogen and helium implantation, broken bonds are created. These dangling bonds are free to react with hydrogen, creating OH and/or H2O molecules within the grain. These molecules may diffuse out through the damaged lattice and migrate toward the cold traps identified at the lunar poles. This mechanism would leave the iron in a reduced state and able to form npFe(0). This work illustrates that npFe(0) can be nucleated in orthopyroxene under implantation of solar wind hydrogen and helium. Our data suggest that the solar wind provides a mechanism by which iron is reduced in the grain and npFe(0) is nucleated in the outer surfaces of regolith grains. This formation mechanism should also operate on other airless bodies in the Solar System. (C) 2015 Elsevier Ltd. All rights reserved.

After Vesta, the NASA Dawn spacecraft will visit the dwarf planet Ceres to carry out in-depth observations of its surface morphology and mineralogical composition in 2015. One of the important questions is whether Ceres has any outgassing activity that would lead to the formation of a thin atmosphere. The recent detection of water vapor emitted from localized source regions by Herschel (Kuppers et al., 2014) has only underscored this point. If the localized outgassing activity observed by Herschel is totally switched off, could a sizable surface-bounded exosphere still be maintained by other source mechanisms? Our preliminary assessment is that chemical sputtering via solar wind interaction and meteoroid impact are probably not adequate because of the large injection speed of the gas at production relative to the surface escape velocity of Ceres. One potential source is a low-level outgassing effect from its subsurface due to thermal sublimation with a production rate of the order of 10(24) molecules s(-1) as first considered by Fanale and Salvail (1989). If the water plumes are active, the fall-back of some of the water vapor onto Ceres' surface would provide an additional global source of water molecules on the surface with a production rate of about 10(25) molecules s(-1) In this work, different scenarios of building up a tenuous exosphere by ballistic transport and the eventual recycling of the water molecules to the polar cold trap are described. It turns out that a large fraction of the exospheric water could be transferred to the polar caps area as originally envisaged for the lunar polar ice storage. (C) 2014 Elsevier Ltd. All rights reserved.