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[PPS08-P10] Depth profiling of solar wind noble gases in lunar soils by isotope nanoscope
Keywords:Lunar regolith, Solar wind, Noble gas
Lunar and asteroidal surfaces are irradiated by solar wind which is a plasma flow of hydrogen (95%) and helium (5%). Solar wind is implanted into the rock surface of the regolith, of which project ranges correspond to their kinetic energy. Therefore, we can estimate the energy distribution of the solar wind by analyzing the helium depth profile of extraterrestrial materials [1]. Solar wind noble gases are useful to investigate properties of the solar wind because of the extremely low concentrations in the rock. Studies of solar wind noble gases have been conducted on extraterrestrial samples such as lunar and asteroid samples from Itokawa (e.g., [2], [3]). However, in-situ depth profiling of the noble gases for single grain has not been performed. In this study, we report the in-situ depth profiling of a lunar regolith sample using the isotope nanoscope LIMAS.
A femto-second laser of LIMAS for post-ionization ionizes all the elements, and 70% of helium can be ionized in a laser focused region [4]. Gallium liquid metal ion beam focuses and rasters on a sample surface, of which beam diameter was ~1 µm and scanning area was ~10 × 16 µm2. These features allows in-situ analysis of the particle surface [5]. Ilmenite grain from Apollo 17 sample 71501 were used. The size of the grain was about 100 µm and embedded it into indium. Scanning electron microscope (SEM; JEOL, JSM-7000F) was used to observe the microstructure on the sample surface. Helium depth profiling was performed by LIMAS. The measured ions were 4He+, and the major elements of ilmenite, 56Fe2+, 48Ti2+, and 16O+. The depth profile of 4He has a link to the distribution of 4He implantation energy. We estimated the energy distribution of 4He implantation by using TRIM code (Transport of Ions in Matter; [6]).
The SEM observation for sample surface showed crater structures and blisters. This indicates that the sample surface was exposed to the outermost layer of the lunar surface, and the surface of the grain would be irradiated by solar wind for a long time. The helium depth profile (Fig. 1) obtained by LIMAS shows a peak at about 30 nm from the sample surface and decreased gradually to 300 nm depth. The peak at 30 nm corresponds to the project range of 4He to ilmenite with an energy of ~4 keV (corresponding to ~450 km/s), which is consistent with the present solar wind observed by Advanced Composition Explorer (ACE) [7]. The profile demonstrates excessive components larger than 5 keV relative to the present solar wind, which is remarkable above 15 keV. These high energy component could be explained by one of the following processes. (1) The ancient sun was emitting more energetic components than that of the present Sun. (2) Helium near the surface degassed due to exceeding the retention limit of He, and as a result, the fast component looks relatively large.
[1] Bajo et al. (2015) Geochemical Journal. 49. 559. [2] Nagao et al. (2011) Science. 333. 1128. [3] Wieler (2016) Chemie der Erde. 76. 463. [4] Yurimoto et al. (2016) Surface and Interface Analysis. 48. 1181. [5] Otsuki et al. (2021) Journal of the Mass Spectrometry Society of Japan. 69. 197. [6] Ziegler et al. SRIM - The Stopping and Range of Ions in Matter. (http://www.srim.org/). [7] Reisenfeld et al. (2013) Space Science Reviews. 175. 125.
A femto-second laser of LIMAS for post-ionization ionizes all the elements, and 70% of helium can be ionized in a laser focused region [4]. Gallium liquid metal ion beam focuses and rasters on a sample surface, of which beam diameter was ~1 µm and scanning area was ~10 × 16 µm2. These features allows in-situ analysis of the particle surface [5]. Ilmenite grain from Apollo 17 sample 71501 were used. The size of the grain was about 100 µm and embedded it into indium. Scanning electron microscope (SEM; JEOL, JSM-7000F) was used to observe the microstructure on the sample surface. Helium depth profiling was performed by LIMAS. The measured ions were 4He+, and the major elements of ilmenite, 56Fe2+, 48Ti2+, and 16O+. The depth profile of 4He has a link to the distribution of 4He implantation energy. We estimated the energy distribution of 4He implantation by using TRIM code (Transport of Ions in Matter; [6]).
The SEM observation for sample surface showed crater structures and blisters. This indicates that the sample surface was exposed to the outermost layer of the lunar surface, and the surface of the grain would be irradiated by solar wind for a long time. The helium depth profile (Fig. 1) obtained by LIMAS shows a peak at about 30 nm from the sample surface and decreased gradually to 300 nm depth. The peak at 30 nm corresponds to the project range of 4He to ilmenite with an energy of ~4 keV (corresponding to ~450 km/s), which is consistent with the present solar wind observed by Advanced Composition Explorer (ACE) [7]. The profile demonstrates excessive components larger than 5 keV relative to the present solar wind, which is remarkable above 15 keV. These high energy component could be explained by one of the following processes. (1) The ancient sun was emitting more energetic components than that of the present Sun. (2) Helium near the surface degassed due to exceeding the retention limit of He, and as a result, the fast component looks relatively large.
[1] Bajo et al. (2015) Geochemical Journal. 49. 559. [2] Nagao et al. (2011) Science. 333. 1128. [3] Wieler (2016) Chemie der Erde. 76. 463. [4] Yurimoto et al. (2016) Surface and Interface Analysis. 48. 1181. [5] Otsuki et al. (2021) Journal of the Mass Spectrometry Society of Japan. 69. 197. [6] Ziegler et al. SRIM - The Stopping and Range of Ions in Matter. (http://www.srim.org/). [7] Reisenfeld et al. (2013) Space Science Reviews. 175. 125.