14:00 〜 14:15
[PPS07-08] The Study of Mercury Sodium Exosphere: laboratory experiments of simulating thermal desorption(TD) process on Mercury surface
キーワード：水星のナトリウム外気圏、室内実験、熱脱着
Introduction
Based on the data from missions of Mariner 10 and Messenger, it is reported that the presence of Na, K, Mg, Ca, Al, Fe, and Mn in Mercury's exosphere were discovered and sodium is a very unique element in it. Mercury sodium exosphere is mainly supplied by particles released from surface through generation mechanisms of photon-stimulated desorption (PSD), thermal desorption (TD), ion sputtering, meteor impact vaporization (MIV).
The laboratory experimental study of generation progresses on Mercury surface is very important to understand Mercury exosphere. Yakshinskiy and Madey have studied photon-stimulated desorption and thermal desorption of Na from SiO2 films and a lunar sample (Yakshinskiy et al. 1999, 2000, 2004). Besides, Bennett et al. (2016) investigated Photon-stimulated desorption of Ca from the sample of calcium sulfide; Recently, Schaible et al. (2020) studied potential sources of Mercury’s exospheric sulfur, detected sulfur atoms desorpted from MgS.
Experiment process
From the results of Messenger Mission, Mercury’s major-element surface composition is markedly different from that of the Earth and Moon (Nittler et al. 2011, 2018). based on recent theoretical and experiment studies, it is showed that the formation of Mercury surface is mainly FeO-poor pyroxene (enstatite) and olivine (forsterite), Na-rich plagioclase, and Mg–Ca–Fe-sulfide assemblages (Nittler et al. 2019). Therefore in our experiments, we used Na-rich plagioclase, that is albite as our sample to simulate Mercury surface in a ultrahigh vacuum chamber (about 10-5Pa).
Our sample is Lithium pegmatite comes from Myoken Mountain, it is consisted of albite(Na-rich plagioclase), quartz, potash feldspar and lithium mica, lithium pyroxene, Lithium tourmaline, etc. The sample surface used in this experiment is mainly dominated by albite.
In the vacuum chamber, two Minco heaters are arranged to heat the sample surface efficiently. One heater is under a definite voltage and just change the other heater’s voltage to control the temperature. To measure sample surface temperature, thermistor is arranged near the surface of sample as temperature sensor.
Preliminary results
Residual gas analyzer (RGA) attached to vacuum chamber can monitor the pressure of specific particle. The temperature near the sample surface is heated up to about 250℃(523K) and at the temperature about 180℃(450K), the pressure of Na(23) starts to increase obviously, therefore it is verified that the initial threshold temperature of thermal desorption (TD) on assumed Mercury surface is about 450K. meanwhile this result is consistent with Yakshinskiy et al. (2000).
Future work
We have heated the temperature up to about 523K, next we consider to change the heating structure and heat the sample to higher temperature, in this way we can obtain a complete thermal desorption pattern for desorption of Na from albite. In addition, we also plan to simulate other generation processes, such as irradiation with UV light to simulate photon-stimulated desorption. Finally we plan to measure the velocity distribution and angular distribution of desorbed Na, combining Multiple Test Particle Monte Carlo Simulation to study Mercury sodium exosphere’s abundance, distribution, dynamics and variability.
Based on the data from missions of Mariner 10 and Messenger, it is reported that the presence of Na, K, Mg, Ca, Al, Fe, and Mn in Mercury's exosphere were discovered and sodium is a very unique element in it. Mercury sodium exosphere is mainly supplied by particles released from surface through generation mechanisms of photon-stimulated desorption (PSD), thermal desorption (TD), ion sputtering, meteor impact vaporization (MIV).
The laboratory experimental study of generation progresses on Mercury surface is very important to understand Mercury exosphere. Yakshinskiy and Madey have studied photon-stimulated desorption and thermal desorption of Na from SiO2 films and a lunar sample (Yakshinskiy et al. 1999, 2000, 2004). Besides, Bennett et al. (2016) investigated Photon-stimulated desorption of Ca from the sample of calcium sulfide; Recently, Schaible et al. (2020) studied potential sources of Mercury’s exospheric sulfur, detected sulfur atoms desorpted from MgS.
Experiment process
From the results of Messenger Mission, Mercury’s major-element surface composition is markedly different from that of the Earth and Moon (Nittler et al. 2011, 2018). based on recent theoretical and experiment studies, it is showed that the formation of Mercury surface is mainly FeO-poor pyroxene (enstatite) and olivine (forsterite), Na-rich plagioclase, and Mg–Ca–Fe-sulfide assemblages (Nittler et al. 2019). Therefore in our experiments, we used Na-rich plagioclase, that is albite as our sample to simulate Mercury surface in a ultrahigh vacuum chamber (about 10-5Pa).
Our sample is Lithium pegmatite comes from Myoken Mountain, it is consisted of albite(Na-rich plagioclase), quartz, potash feldspar and lithium mica, lithium pyroxene, Lithium tourmaline, etc. The sample surface used in this experiment is mainly dominated by albite.
In the vacuum chamber, two Minco heaters are arranged to heat the sample surface efficiently. One heater is under a definite voltage and just change the other heater’s voltage to control the temperature. To measure sample surface temperature, thermistor is arranged near the surface of sample as temperature sensor.
Preliminary results
Residual gas analyzer (RGA) attached to vacuum chamber can monitor the pressure of specific particle. The temperature near the sample surface is heated up to about 250℃(523K) and at the temperature about 180℃(450K), the pressure of Na(23) starts to increase obviously, therefore it is verified that the initial threshold temperature of thermal desorption (TD) on assumed Mercury surface is about 450K. meanwhile this result is consistent with Yakshinskiy et al. (2000).
Future work
We have heated the temperature up to about 523K, next we consider to change the heating structure and heat the sample to higher temperature, in this way we can obtain a complete thermal desorption pattern for desorption of Na from albite. In addition, we also plan to simulate other generation processes, such as irradiation with UV light to simulate photon-stimulated desorption. Finally we plan to measure the velocity distribution and angular distribution of desorbed Na, combining Multiple Test Particle Monte Carlo Simulation to study Mercury sodium exosphere’s abundance, distribution, dynamics and variability.