Determination of sputtering and desorption cross sections for metal species relevant to the exosphere of Mercury & other airless bodies: Laboratory measurements

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The surface of Mercury is covered by a rocky crust made up mostly of silicate minerals, and daytime temperatures are significantly warmer than those found on Earth. Its very thin atmosphere is generated mostly by the interaction of the solar wind with the planetary surface, as well as bombardment by meteors and the radioactive decay of elements within the planet’s crust. This soil-derived “exosphere” contains metal-volatile species of Na, K, Ca, Al, Mg, Fe, and Mn, which have been observed by telescopes on Earth and by orbiting spacecraft. There are only a few experimental measurements for the release of sodium and potassium from the surface that are available, with no systematic data for all metal-volatile species. This team will provide critical laboratory measurements needed to interpret observations of Mercury’s exosphere, (2) improve theoretical models of the exosphere, and (3) provide predictions for as yet unobserved airless bodies, such as asteroids or comets. During each year of this grant, the PI will sponsor two Community College transfer students enrolled as STEM majors to spend the summer performing research on Planetary Science. The team will carry out laboratory measurements of thermal desorption, UV photo-desorption, and ion-sputtering cross sections for metal-volatile species ejected from a simulated Mercurian surface. Cross sections for important volatile species will be measured in the laboratory for both adsorbed and intrinsic (incorporated into the lattice) metal volatiles [Na, K, Ca, Al, Mg, Fe, and Mn] from mineral surfaces and carbonaceous meteorites under conditions relevant to Mercury. Measurements will be performed as a function of temperature, substrate material, substrate roughness, and ion irradiation history. 1 keV/am H+ and He+, the primary components of the solar wind, will be used as solar-wind analogs, allowing systematic investigation of chemical sputtering; 2 keV H2+ will also be used to examine any molecular sputtering effects. Lyman-alpha or other UV photons will be used for photon-stimulated desorption measurements. X-ray photoelectron spectroscopy will be used to quantify the surface abundance of metal volatiles for desorption and sputtering measurements, and the ejected neutral species and/or ions will be measured by a mass spectrometer. This combination of techniques will provide complementary quantitative information on whether surface depletions are due exclusively to loss into vacuum, or if diffusion into the bulk plays a role. The goal is to determine the surface binding energies for the metal volatiles to the planetary regolith, and quantitatively evaluate the relative weight for each of the physical mechanisms contributing to the exospheres of Mercury and other airless bodies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.