[P-PS06] Lunar Science and Exploration

[PPS06-P07] Water molecule creation by the solar wind on Mercury's surface modeled by the hydrogen irradiation to the anhydrous mineral

*Tomohiro Kitano¹, Tomoki Kimura¹, Misako Otsuki¹, Ryo Hoshino¹, Yusuke Nakauchi² (1.Tokyo University of Science, 2.Japan Aerospace Exploration Agency)

Keywords:water, Mercury, hydrogen irradiation

At Mercury and Moon, water molecules are created from surface minerals through the thermal and non-thermal chemical processes driven by the solar photon and solar wind irradiations. Recent studies suggested that there is the water ice in the polar permanent shadow regions at Mercury based on the neutron spectroscopies onboard spacecraft (Lawrence et al., 2013) and at Moon based on the infrared spectroscopies onboard spacecraft (Li et al., 2018). Although the source of polar ice is still unknown, the recent numerical simulations of water transport on Mercury (Jones et al., 2020) suggested the water creation process by the solar wind hydrogen irradiation to the surface material as a potential source of polar ice. However, the water creation on the Mercury's surface by the actual solar wind hydrogen irradiation has not yet been directly demonstrated by neither the observations nor experiment. This study demonstrates the water creation from Mercury's surface material by the solar wind based on the hydrogen ion and electron irradiation experiments to an anhydrous silicate mineral, Enstatite, which is a candidate for Mercury's surface material. The hydrogen ion and electron were irradiated with a flux of 1e+14-1e+15cm^-2s^-1 and fluence of 1e+18-1e+19cm^-2. Temporally stable water vapor release was confirmed for a long time (approximately 1e+4s) only during the hydrogen ion irradiation. The yield of water molecules by the hydrogen ion irradiation after removing the water originally adsorbed on the sample is estimated to be 0.37-0.38/incident ion. From the electron irradiation after the hydrogen irradiation, it was found that the electron irradiation enhance release of the water vapor in the mineral, which was originally created by the hydrogen irradiation. If we assume that the water molecules are uniformly released from Mercury's day-side hemisphere, the yield estimated from the hydrogen irradiation corresponds to a water creation rate of 1.37-1.41e+7kg/year. With the rate of the ice accumulation to the surface water creation from the surface estimated by the water transfer simulation in the Jones et al. (2020), the total amount of ice accumulated on Mercury surface through 3 billion years is estimated to be 4.1-4.2e+14kg from our experiment. This is a significant amount compared to the estimation of 1e+14-1e+15 kg by the radar observations in the previous studies (Eke et al., 2017; Deutsch et al., 2018; Susorney et al., 2019), which suggests that the water creation by the solar wind irradiation is an essential source process of the polar ice on Mercury.