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Orlando,
Thomas M.
Orlando,
Thomas M.
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ItemTemperature Programmed Desorption Comparison of Lunar Regolith to Lunar Regolith Simulants LMS-1 and LHS-1 - Data Files(Georgia Institute of Technology, 2022) Clendenen, Ashley Rebekah ; Aleksandrov, Aleksandr ; Jones, Brant M. ; Loutzenhiser, Peter G. ; Britt, Daniel T. ; Orlando, Thomas M.Water and molecular hydrogen evolution from Apollo sample 14163 and lunar regolith simulants LMS-1 and LHS-1 were examined using Temperature Programmed Desorption (TPD) in ultra-high vacuum. LMS-1, LHS-1, and Apollo 14163 released water upon heating, whereas only the Apollo sample directly released measurable quantities of molecular hydrogen. The resulting H2O and H2 TPD curves were fit using a model which considers desorption at the vacuum grain interface, transport in the void space between grain-grain boundaries, molecule formation via recombination reactions and sub-surface diffusion. The model yielded a most probable H2O formation and desorption effective activation energy of ~150 kJ mol-1 for all samples. The probability distribution widths were ~100 - 400, ~100 - 350, and ~100 - 300 kJ mol-1 for LMS-1, LHS-1, and Apollo 14163, respectively. In addition to having the narrowest energy distribution width, the Apollo sample released the least amount to water (103 ppm) relative to LMS-1 (176 ppm) and LHS-1 (195 ppm). Since essentially no molecular hydrogen was observed from the simulants, the results indicate that LMS-1 and LHS-1 display water surface binding and transport interactions similar to actual regolith but not the desorption chemistry associated with the implanted hydrogen from the solar wind. Overall, these terrestrial surrogates are useful for understanding the surface and interface interactions of lunar regolith grains, which are largely dominated by the terminal hydroxyl sites under both solar wind bombardment and terrestrial preparation conditions.
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ItemThermal Evolution of Water and Hydrogen from Apollo Lunar Regolith - Data Files(Georgia Institute of Technology, 2021) Jones, Brant M. ; Aleksandrov, Aleksandr ; Hibbitts, Charles ; Orlando, Thomas M.The evolution of water and molecular hydrogen from Apollo lunar sample 15221, a mature mare soil, was examined by temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions. Desorption at the grain/vacuum interface with re-adsorption as water transports though the void space of the grains and activated sub-surface diffusion were found to reproduce the experimental TPD signal. Signal from the grain/vacuum interface yielded the second order desorption activation energies and site probability distributions. Water from sample 15221 exhibited a broad distribution of activation energies peaking at 130 kJ mol-1 extending up to 350 kJ mol-1 at zero coverage limit with an onset of 110 kJ mol-1 at full coverage. Our results suggest that water and hydrogen originating from lunar regolith contributes a minor amount to the observed mass in the LCROSS impact event. The abnormal amount of molecular hydrogen observed in the ejecta plume of the LCROSS impact may indicate that the radiolytic production of H2 from electron and galatic cosmic rays of physisorbed water is a contributor to the vast quantity of molecular hydrogen detected.
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ItemA New In Situ Quasi-continuous Solar-wind Source of Molecular Water on Mercury - Data Files(Georgia Institute of Technology, 2020) Jones, Brant M. ; Sarantos, Menelaos ; Orlando, Thomas M.Radar observations of Mercury and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft data indicate the probable existence of water ice in the permanently shadowed polar regions. Generally, water is accepted to be of exogenous origin through delivery via comets and meteoritic impact. However, a continuous water formation process that involves thermal transformation of chemically stable mineral-bound hydroxyl groups produced by implanted solar wind protons is readily available on the surface of Mercury. At typical temperatures prevailing on Mercury’s dayside surface, H2O can be produced from reactions involving OH groups on or within the H-saturated regolith grain interfaces. Similar reactions will also occur due to micrometeorite impact events on both the day and nightside. Once produced, H2O is released into the exosphere and then transported and processed via Jeans escape, photodissociation, dissociative adsorption, or condensation. Water reaching cold traps will be bound over geological periods. This simple water cycle will produce a highly chemically reduced surface and can contribute significant amounts of H2O over geological time periods. The overall process is an important but hitherto unnoticed source term that will contribute to the accumulation of water in the permanently shadowed regions of Mercury.
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ItemEfficient Intermolecular Energy Exchange and Soft Ionization of Water at Nanoplatelet Interfaces - Data Files(Georgia Institute of Technology, 2020) Jones, Brant M. ; Hu, Hang ; Alexsandrov, Alexandr ; Smith, William ; Clark, Aurora E. ; Li, Xiaosong ; Orlando, Thomas M.X-ray, energetic photon and electron irradiation can ionize and electronically excite target atoms and molecules. These excitations undergo complicated relaxation and energy transfer processes that ultimately determine the manifold of system responses to the deposited excess energy. In weakly bound gas- and solution-phase samples, intermolecular Coulomb decay (ICD) and electron-transfer mediated decay (ETMD) can occur with neighboring atoms or molecules leading to efficient transfer of the excess energy to the surroundings. In ionic solids such as metal-oxides, intra- and inter-atomic Auger decay produces localized final states that lead to lattice damage and typically removal of cations from the substrate. The relative importance of Auger stimulated damage (ASD) versus ICD and ETMD in micro-solvated nanoparticle interfaces is not known. Though ASD is generally expected, essentially no lattice damage resulting from ionization and electronic excitation of micro-solvated boehmite (AlOOH) nanoplatelets has been observed. Efficient energy transfer and soft ionization of interfacial molecules is likely a general phenomenon at gas-oxyhydroxide nanoparticle interfaces where the density of states of the ionized chemisorbed species significantly overlap the core hole states of the solid.
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ItemInvestigation of Water Interactions With Apollo Lunar Regolith Grains - Data Files(Georgia Institute of Technology, 2019) Jones, Brant M. ; Aleksandrov, Aleksandr ; Dyar, M. Darby ; Hibbitts, Charles ; Orlando, Thomas M.Desorption activation energies of chemisorbed water on Apollo lunar samples 14163 and 10084 were determined by temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions. Desorption at the grain/vacuum interface and desorption/transport of water though the porous medium with re-adsorption were found to reproduce the experimental TPD signal. Signal from the grain/vacuum interface yielded desorption activation energies and site probability distributions. Highland sample 14163 exhibited a broad distribution of binding site energies peaking at 60 kJ mol-1 while mare sample 10084 exhibited a narrower distribution of binding site energies peaking at 65 kJ mol-1. Water desorption from the lunar surface over a typical lunar day was simulated with the measured coverage dependent activation energies of the mare and highland samples. The resulting desorption profile of water through a lunar temperature cycle is in general agreement with Lunar Reconnaissance Orbiter (LRO) Lyman-α Mapping Project (LAMP) spacecraft-based observations of trends for both highland and mare assuming ~1% submonolayer coverage and that photostimulated desorption is neglected.