11:45 AM - 12:00 PM
[MZZ45-04] Kinetics of evaporation of forsterite in H2-H2O gas mixture
Keywords:evaporation, kinetics, experiments, protoplanetary disk
Introduction:
Evaporation in protoplanetary disks plays a critical role in volatile loss from planetary materials. To understand the kinetics of this process, forsterite (Mg2SiO4) has been actively studied as a model substance of silicate materials. Previous experiments showed that evaporation of forsterite in low-pressure H2 follows half order kinetics with respect to PH2 [1,2], suggesting that gas in protoplanetary disks composed mainly of H2 affects the evaporation kinetics. H2O is another major volatile molecule in disks, whose canonical abundance relative to H2 is 10−4-10−3 inside the snowline and can be enhanced by factors of a few or more due to radial drift of icy pebbles [3]. Since the relative abundance of H2O/H2 changes the thermodynamic stability of forsterite, it is a potential controlling factor of its evaporation kinetics. Tsuchiyama et al. [4] theoretically investigated this effect by applying the Hertz-Knudsen equation, which equates the evaporation rate to the condensation rate in equilibrium based on the detailed balance, and predicted that the evaporation rate is proportional PH2/PH2O under certain pressures of H2 and H2O. However, it is uncertain whether this equilibrium approach is sufficient to describe evaporation in non-equilibrium conditions. Furthermore, for understanding the mechanism of this process, experimental kinetic data are essential. In this study, we experimentally investigate the evaporation rates of forsterite in H2-H2O gas mixture and evaluate the dependence on temperatures and ambient gas compositions to discuss the reaction mechanism.
Methods:
Forsterite single crystal samples were evaporated with the furnace at 1350-1600 K for 3-48 hours. To evaluate the amounts of evaporation, the weights of the samples were measured before and after each experiment. During the experiments, H2 gas was introduced at a constant flow rate into the reaction chamber, which was simultaneously evacuated and thereby kept at 1 Pa. The H2 gas supply line passed over liquid water to convey H2O gas into the chamber. PH2/PH2O of the flowing gas was measured with the differentially pumped quadrupole mass spectrometer, ranging from ~150 to 200.
Results and Discussion:
The weights of the samples decreased constantly with time at each condition, and thus the evaporation rates were determined with the weight losses per unit time and surface area. The evaporation rates at PH2/PH2O ~150-200 were smaller than those in the absence of H2O by 1 order of magnitude at 1350 K and by factors of ~2 at 1450 and 1600 K. At 1350 K, the evaporation rates positively correlated with PH2/PH2O, indicating the decrease of the evaporation rates with increasing PH2O. These results clearly showed that evaporation of forsterite is suppressed under H2O-enriched disk conditions (×10 of the solar abundance).
The evaporation rates in pure H2 gas followed a linear Arrhenius relation with the activation energy of 345 ± 34 kJ mol−1, which is consistent with the previous experiments [1]. On the other hand, the Arrhenius plots of the evaporation in H2-H2O mixed gas did not yield a single line with the decrease of the gradient at higher temperatures (Fig. 1). We calculated the activation energy with the data below 1450 K and at PH2/PH2O ~190 and obtained 497 ± 51 kJ mol−1, which is ~150 kJ mol−1 larger than that in the H2O-free condition. To discuss the mechanism, we interpreted the results with the statistical evaporation rate theory that we previously derived based on transition state theory [5]. It was suggested from the obtained activation energies that there is a potential energy barrier for this reaction which is largely attributed to the formation of H2O from forsterite rather than the decomposition of other metal-bearing products.
[1] Tsuchiyama et al. (1998) Mineral. J. 20, 113. [2] Takigawa et al. (2009) ApJ 97, L97. [3] Booth et al. (2017) MNRAS 469, 3994. [4] Tsuchiyama et al. (1999) GCA 63, 2451. [5] Inada et al. JCP, under review.
Evaporation in protoplanetary disks plays a critical role in volatile loss from planetary materials. To understand the kinetics of this process, forsterite (Mg2SiO4) has been actively studied as a model substance of silicate materials. Previous experiments showed that evaporation of forsterite in low-pressure H2 follows half order kinetics with respect to PH2 [1,2], suggesting that gas in protoplanetary disks composed mainly of H2 affects the evaporation kinetics. H2O is another major volatile molecule in disks, whose canonical abundance relative to H2 is 10−4-10−3 inside the snowline and can be enhanced by factors of a few or more due to radial drift of icy pebbles [3]. Since the relative abundance of H2O/H2 changes the thermodynamic stability of forsterite, it is a potential controlling factor of its evaporation kinetics. Tsuchiyama et al. [4] theoretically investigated this effect by applying the Hertz-Knudsen equation, which equates the evaporation rate to the condensation rate in equilibrium based on the detailed balance, and predicted that the evaporation rate is proportional PH2/PH2O under certain pressures of H2 and H2O. However, it is uncertain whether this equilibrium approach is sufficient to describe evaporation in non-equilibrium conditions. Furthermore, for understanding the mechanism of this process, experimental kinetic data are essential. In this study, we experimentally investigate the evaporation rates of forsterite in H2-H2O gas mixture and evaluate the dependence on temperatures and ambient gas compositions to discuss the reaction mechanism.
Methods:
Forsterite single crystal samples were evaporated with the furnace at 1350-1600 K for 3-48 hours. To evaluate the amounts of evaporation, the weights of the samples were measured before and after each experiment. During the experiments, H2 gas was introduced at a constant flow rate into the reaction chamber, which was simultaneously evacuated and thereby kept at 1 Pa. The H2 gas supply line passed over liquid water to convey H2O gas into the chamber. PH2/PH2O of the flowing gas was measured with the differentially pumped quadrupole mass spectrometer, ranging from ~150 to 200.
Results and Discussion:
The weights of the samples decreased constantly with time at each condition, and thus the evaporation rates were determined with the weight losses per unit time and surface area. The evaporation rates at PH2/PH2O ~150-200 were smaller than those in the absence of H2O by 1 order of magnitude at 1350 K and by factors of ~2 at 1450 and 1600 K. At 1350 K, the evaporation rates positively correlated with PH2/PH2O, indicating the decrease of the evaporation rates with increasing PH2O. These results clearly showed that evaporation of forsterite is suppressed under H2O-enriched disk conditions (×10 of the solar abundance).
The evaporation rates in pure H2 gas followed a linear Arrhenius relation with the activation energy of 345 ± 34 kJ mol−1, which is consistent with the previous experiments [1]. On the other hand, the Arrhenius plots of the evaporation in H2-H2O mixed gas did not yield a single line with the decrease of the gradient at higher temperatures (Fig. 1). We calculated the activation energy with the data below 1450 K and at PH2/PH2O ~190 and obtained 497 ± 51 kJ mol−1, which is ~150 kJ mol−1 larger than that in the H2O-free condition. To discuss the mechanism, we interpreted the results with the statistical evaporation rate theory that we previously derived based on transition state theory [5]. It was suggested from the obtained activation energies that there is a potential energy barrier for this reaction which is largely attributed to the formation of H2O from forsterite rather than the decomposition of other metal-bearing products.
[1] Tsuchiyama et al. (1998) Mineral. J. 20, 113. [2] Takigawa et al. (2009) ApJ 97, L97. [3] Booth et al. (2017) MNRAS 469, 3994. [4] Tsuchiyama et al. (1999) GCA 63, 2451. [5] Inada et al. JCP, under review.