Inada and Tachibana, Evaporation kinetics of forsterite in H2-H2O gas mixtures, submitted to GCA

Effects of ambient gas are important for understanding evaporation of planetary materials in the protosolar disk. In this study, we experimentally investigated the evaporation kinetics of forsterite in H2-H2O gas mixtures to elucidate the dependences on H2O abundance at various temperature and total pressure conditions (1350-1600 K, 0.1-1 Pa). We found that the evaporation rate decreases with the increase ofwith the reaction order −0.88 ± 0.25 under high conditions (10−3at 1 Pa and 1400 K). The activation energy increased when the evaporation is suppressed by H2O (510 ± 41 kJ mol−1, while 358 ± 11 kJ mol−1 without the effect of H2O). These results are mostly consistent with the previous theoretical prediction based on the Hertz-Knudsen equation. We also interpreted the present and previous experimental data, especially the reaction orders, by the mechanism via surface oxygen vacancies analogous to the Mars and van Krevelen (MvK) mechanism of catalytic redox reactions. This provided the molecular understanding of the consistency between the experiments and the Hertz-Knudsen equation. Based on these findings, we evaluated conditions of the protosolar disk where H2O affects evaporation by assuming the kinetics consistent with the Hertz-Knudsen and using the parameters determined from the present experiments. This showed that H2O may have suppressed evaporation of silicate materials at 1200 K and/or H2O-enriched disk conditions.