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ItemCaCO3 Polymorphs as Mineral Catalysts for Prebiotic Phosphorylation of Uridine(Georgia Institute of Technology, 2022-12) Schaible, Micah J. ; Castañeda, Alma D. ; Menor-Salvan, Cesar ; Pasek, Matthew A. ; Burcar, Bradley T. ; Orlando, Thomas M.Establishing plausible routes for the abiotic formation of nucleotides is a challenging problem because the phosphorylation of organic molecules is thermodynamically unfavorable in water, and because common phosphorous-containing minerals such as apatite are highly insoluble. Reactions of reduced phases such as the meteoritic mineral schreibersite with ammonia containing solutions can form stable amino-derivatives of phosphates/phosphite, and carbonate-rich lakes have been suggested as environments where phosphate species and organic molecules could accumulate in significant abundances, thus promoting an ideal environment for abiotic phosphorylation. This work reports the catalytic properties of three CaCO3 polymorphs – calcite, aragonite, and vaterite – on diamidophosphate (DAP)-induced phosphorylation of the uridine nucleoside during a 24-hour dry-down reaction. It is shown that the phosphorylation reaction is accelerated in solutions containing CaCO3 compared to those with no mineral present. For un-buffered solutions with no mineral present, the primary products formed are uridine monophosphates (UMP), with yields making up 22.3 ± 3.9% of the total detected species, while solutions containing calcite and aragonite formed primarily UMP dimers (yields of 15.3 ± 1.1% and 14.8 ± 1.3%, respectively). Vaterite showed a strong preference for forming cyclic UMP (cUMP) (26.3 ± 0.3% yield), and no higher order polymers were observed using any carbonate mineral. Reactions containing CaSO4∙2H2O (gypsum) showed a preference for forming cUMP, though not as strong as vaterite, while those containing CaCl2 (calcium chloride) and CaWO4 (scheelite) did not yield any phosphorylated products other than UMPs. These results suggest that CaCO3 minerals could have played an important role in facilitating prebiotic phosphorylation in aqueous environments that undergo drying cycles.
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ItemPhoton Stimulated Desorption of MgS as a Potential Source of Sulfur in Mercury’s Exosphere - Data Files(Georgia Institute of Technology, 2020-06-14) Schaible, Micah J. ; Sarantos, Menelaos ; Anzures, Brendan A. ; Parman, Stephen W. ; Orlando, Thomas M.Mercury has a relatively high sulfur content on its surface, and a signal consistent with S+ was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon stimulated desorption (PSD) of neutral sulfur atoms (S0) from MgS substrates was studied using resonance enhanced multiphoton ionization (REMPI) and time-of-flight (TOF) mass spectrometry. Though the PSD process is inherently non-thermal, the measured velocity distributions were fit using flux weighted Maxwellian distributions with translation energies ˂E> expressed as translational “temperatures” Tt = ˂E>/µkB. A bi-modal distribution consisting of both thermal (Tt = 300 K) and supra-thermal (Tt >1000 K) components in roughly a 2:1 ratio was found to best fit the data. The experimental PSD cross-section, ~4×10-22 cm2, and integrated velocity distributions were used to calculate the PSD source rate of S0 into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely a primary source to S0 in Mercury’s exosphere at low (<1000 km) altitudes.