The Role of Solar Wind in the Formation of Hydroxyl on Airless Silicate Bodies in Space

Studies of the interaction of solar radiation with the surfaces of moons, asteroids and comets can provide fundamental information about the early history of our solar system and help elucidate the ways in which such systems evolve. Airless bodies in space such as the Moon, asteroids and interplanetary dust particles are subject to bombardment from energetic solar wind electrons and ions, ultraviolet photons, micrometeorites and cosmic rays. This process is known as space weathering and the cumulative effect of the radiation modifies the chemical and physical nature of the ices and minerals that make up such surfaces. The work presented here investigates the interaction of solar wind hydrogen and helium with analog minerals and lunar soil using infrared spectroscopy (FTIR), mass spectrometry (SIMS), and optical photometry. Experiments were performed under Ultra High Vacuum (UHV) pressures and over a temperature range of 15 K to 400 K to simulate lunar surface conditions, and radiation was performed using a mass analyzed ion accelerator that allowed samples to be irradiated with specific ions at energies in the range of the solar wind.
Radiation of surfaces in vacuum causes both formation and removal of atomic and molecular species. The sputtering rate of water from regolith surfaces was measured by first dosing un-compacted lunar soil with water and subsequently irradiating with 4 keV He+ ions to determine a sputtering cross section for molecules on the surface of grains. The amount of water present on the grains was determined by using X-ray Photoelectron Spectroscopy (XPS) to monitor the atomic percentage of oxygen on the surface in relation to silicon, and the composition of sputtered species and the ejection energy of surface ions were measured using SIMS and provide an estimate of the solar wind sputtering contribution to the lunar exosphere. Using the measured cross-section, the lifetime of a water molecule of the surface of a grain is 29 years at the lunar equator and decreases to an estimated ~12 years based on the additional contribution of hydrogen ion sputtering.
To simulate solar wind implantation on the lunar surface and investigate the effects of ion radiation on hydroxyl (deuteroxyl) concentration, thin sections of minerals and lunar rocks (100-200 μm thickness) were irradiated with 2-5 keV H+, D+ and He+ ions. Implanted H (D) ions were found to chemically bond with oxygen in the silicate samples, causing changes in the absorption spectra of the samples at 2.7 μm (3.1 μm) attributed to the OH (OD) stretch vibration, while He+ irradiation caused no changes in this band. The initial yield (OH formed per incident ion) was ~90% and the OH (OD) absorption band was found to saturate at implantation fluences of ~2x1017 H(D)/cm2. Irradiation also modified the Si-O stretch band at 9.2 μm causing the peak height to decrease and the FWHM to increase, and both yield and Si-O peak height decreased exponentially with increasing fluence. These measurements allow constraints to be placed on solar wind contribution to observational and theoretical models of water on the lunar surface.