2:15 PM - 2:30 PM
[PPS07-15] Depth variation in elemental and isotopic compositions of solar wind noble gases
Keywords:Solar wind, Noble gas, Lunar regolith, TOF-SNMS
Introduction
Since the 1970s, solar noble gases in extraterrestrial materials have been thought to have two distinct components: the solar wind (SW) and solar energetic particles (SEPs), detending on the depth from the material surface [1]. However, subsequent analysis of the target material of the Genesis mission disproved the existence of a distinct SEP component and instead attributed it to fractionated SW [2]. A challenge in previous noble gas measurements is the lack of an absolute depth scale and that might be one of the reasons that SEP component was believed long time.
In this study, noble gas depth profiling using time-of-flight secondary neutral mass spectrometry (TOF-SNMS) was applied to a lunar regolith grain. The 4He/20Ne and 20Ne/22Ne ratios were measured with nanometer-scale depth resolution to determine the depth at which the “SEP component” reside.
Materials and Methods
An ~80 µm sized ilmenite grain separated from the Apollo 17 soil 71501 (71501#6) was used. The grain was embedded in indium with its flat surface facing upward and coated with ~10 nm of gold. As a noble gas reference, a terrestrial ilmenite substrate implanted with noble gases was used (4He: 20 keV, 2 × 1015 cm−2; 20Ne: 60 keV, 1 × 1014 cm−2; 22Ne: 60 keV, 1 × 1013 cm−2).
A TOF-SNMS at Hokkaido University named LIMAS was used for depth profiling of noble gas isotopes (4He, 20, 22Ne, and 36, 40Ar) with major element of ilmenite (16O, 48Ti, and 56Fe) [3, 4]. Analyzed area was ~11 × 9 µm2 and the depth resolution relies on ion intensity which was 5–25 nm (Fig. 1).
An ion implantation simulation using SRIM [5] was conducted to simulate 4He, 20Ne, and 22Ne depth profiles of lunar ilmenite 71501#6. The SW speed distribution was assumed to be consistent with the present, based on the ACE SWICS 2.0 Level 2 Data from 1998:35 to 2011:233. The composition was based on the present-day SW [6]. To simulate SW irradiation at the Apollo 17 landing site, a geometric correction and passage through the geomagnetic tail were included.
Results
Fig. 1 shows the depth profiles of noble gas compositions in lunar samples obtained from both simulations and measurement. The measured 4He/20Ne ratio was ~250 at the surface, peaked at 650 at a depth of 30–40 nm, and then decreased to <450 at deeper sites. The 20Ne/22Ne ratio started at 15.5 at the surface and decreased to 11 at ~40 nm. The characteristics of these depth distributions are consistent with previous studies [1]. However, this is the first dataset to present the compositional depth profile based on an absolute depth scale.
Discussions
The 20Ne/22Ne measured profile agree with the simulation at the depth of 10 nm and decrease rapidly than the simulation. This discrepancy is likely be due to differences between the simulation and the actual implantation conditions such as the angle of incidence. Anyway, the existence of the “SEP-component” at depth of 40 nm indicates that the “SEP-component” is not the composition corresponding to SEPs, as previously thought, but rather a “fractionated solar wind”.
The measured 4He/20Ne profile shows lower values than the simulation, expect at a depth of 30–40 nm. The lower values beyond 40 nm are likely due to differences between the simulation and actual implantation, similar to the 20Ne/22Ne profile. In contrast, at depths shallower than 30 nm, the 4He concentration reaches ~1022 cm−3, which is close to the concentration of major element atoms in ilmenite (~2 × 1022 cm3). Therefore, the primary factor for the lower 4He/20Ne values in this region is likely the greater extent of He escape [7] compared to Ne.
References
[1] Benkert J. P. et al. (1993) JGR, 98, 13147. [2] Grimberg A. et al. (2006) Science, 314, 1133. [3] Bajo K. and Yurimoto H. (2024) J. Anal. Sci. Technol., 15, 19. [4] Otsuki Y. et al. under review. [5] Ziegler J. F. et al. (2010) Nucl. Instrum. Methods Phys. Res. B, 268, 1818. [6] Heber V. S. et al. (2009) GCA, 73, 7417. [7] Yurimoto H. et al. (2016) SIA, 48, 1181.
Since the 1970s, solar noble gases in extraterrestrial materials have been thought to have two distinct components: the solar wind (SW) and solar energetic particles (SEPs), detending on the depth from the material surface [1]. However, subsequent analysis of the target material of the Genesis mission disproved the existence of a distinct SEP component and instead attributed it to fractionated SW [2]. A challenge in previous noble gas measurements is the lack of an absolute depth scale and that might be one of the reasons that SEP component was believed long time.
In this study, noble gas depth profiling using time-of-flight secondary neutral mass spectrometry (TOF-SNMS) was applied to a lunar regolith grain. The 4He/20Ne and 20Ne/22Ne ratios were measured with nanometer-scale depth resolution to determine the depth at which the “SEP component” reside.
Materials and Methods
An ~80 µm sized ilmenite grain separated from the Apollo 17 soil 71501 (71501#6) was used. The grain was embedded in indium with its flat surface facing upward and coated with ~10 nm of gold. As a noble gas reference, a terrestrial ilmenite substrate implanted with noble gases was used (4He: 20 keV, 2 × 1015 cm−2; 20Ne: 60 keV, 1 × 1014 cm−2; 22Ne: 60 keV, 1 × 1013 cm−2).
A TOF-SNMS at Hokkaido University named LIMAS was used for depth profiling of noble gas isotopes (4He, 20, 22Ne, and 36, 40Ar) with major element of ilmenite (16O, 48Ti, and 56Fe) [3, 4]. Analyzed area was ~11 × 9 µm2 and the depth resolution relies on ion intensity which was 5–25 nm (Fig. 1).
An ion implantation simulation using SRIM [5] was conducted to simulate 4He, 20Ne, and 22Ne depth profiles of lunar ilmenite 71501#6. The SW speed distribution was assumed to be consistent with the present, based on the ACE SWICS 2.0 Level 2 Data from 1998:35 to 2011:233. The composition was based on the present-day SW [6]. To simulate SW irradiation at the Apollo 17 landing site, a geometric correction and passage through the geomagnetic tail were included.
Results
Fig. 1 shows the depth profiles of noble gas compositions in lunar samples obtained from both simulations and measurement. The measured 4He/20Ne ratio was ~250 at the surface, peaked at 650 at a depth of 30–40 nm, and then decreased to <450 at deeper sites. The 20Ne/22Ne ratio started at 15.5 at the surface and decreased to 11 at ~40 nm. The characteristics of these depth distributions are consistent with previous studies [1]. However, this is the first dataset to present the compositional depth profile based on an absolute depth scale.
Discussions
The 20Ne/22Ne measured profile agree with the simulation at the depth of 10 nm and decrease rapidly than the simulation. This discrepancy is likely be due to differences between the simulation and the actual implantation conditions such as the angle of incidence. Anyway, the existence of the “SEP-component” at depth of 40 nm indicates that the “SEP-component” is not the composition corresponding to SEPs, as previously thought, but rather a “fractionated solar wind”.
The measured 4He/20Ne profile shows lower values than the simulation, expect at a depth of 30–40 nm. The lower values beyond 40 nm are likely due to differences between the simulation and actual implantation, similar to the 20Ne/22Ne profile. In contrast, at depths shallower than 30 nm, the 4He concentration reaches ~1022 cm−3, which is close to the concentration of major element atoms in ilmenite (~2 × 1022 cm3). Therefore, the primary factor for the lower 4He/20Ne values in this region is likely the greater extent of He escape [7] compared to Ne.
References
[1] Benkert J. P. et al. (1993) JGR, 98, 13147. [2] Grimberg A. et al. (2006) Science, 314, 1133. [3] Bajo K. and Yurimoto H. (2024) J. Anal. Sci. Technol., 15, 19. [4] Otsuki Y. et al. under review. [5] Ziegler J. F. et al. (2010) Nucl. Instrum. Methods Phys. Res. B, 268, 1818. [6] Heber V. S. et al. (2009) GCA, 73, 7417. [7] Yurimoto H. et al. (2016) SIA, 48, 1181.