Presentation #212.04 in the session “Laboratory Astrophysics Division (LAD): Planetary Bodies and Exotic Molecules”.
Observations of Mercury’s exosphere have found that Na and He are the two dominant confirmed neutral species. The Na exosphere provides clues to the surface mineralogy and is an important constraint for formation models of Mercury. A number of models have been developed to explain the Na exosphere. However, the models make questionable assumptions about the sputter yield from ion irradiation, due to the lack of experimental data for regolith-like loose powders. Spectrophotometry is a combined analysis of the spectral and photometric properties of a surface, enabling understanding composition and regolith structure. MESSENGER’s MACSC and MDIS instruments provided spectral and photometric observations. However, to better interpret these observations, we need to understand how solar wind ions affect the spectrophotometry of Na-bearing minerals such as plagioclase feldspars, expected to be common on Mercury. We have developed a novel apparatus to perform solar wind-like ion irradiation of loose powders and measure sputter yields and spectral changes. We will first study loose powders of plagioclase feldspar. Sputtered particles will be collected by catcher foils surrounding the samples. The foils will be analyzed ex-situ using a high precision, automated quartz crystal microbalance system to derive the sputter yield and its angular distribution. We will also analyze the foils ex-situ using synchrotron based X-ray photoelectron spectroscopy to determine relative elemental abundances for the sputtered particles and the energy distribution of the sputtered particles. Spectra spanning 350-2500 nm will be collected in-vacuo and in-situ as a function of ion fluence. The results will provide needed inputs for models of Mercury’s surface spectrophotometry and models of the production of the Hermean exosphere. This project is supported, in part, by the NASA Solar System Workings program grant 80NSSC18K0521.