10:45 AM - 12:15 PM
[PPS08-P07] Three-dimensional distributions of solar wind helium on the surfaces of Itokawa regolith particles
Keywords:Itokawa, solar wind, helium, space weathering, LIMAS
The Hayabusa spacecraft brought back regolith particles from the asteroid Itokawa. Previous studies revealed that the majority of Itokawa particles had been exposed to solar wind [1, 2, 3]. The solar wind is a stream of plasma that mainly consists of hydrogen and helium (He) with an average speed of ~400 km/s, penetrating <1 μm from the surface of irradiated materials. It has been suggested that solar wind implantation plays an important role in the space weathering of materials on the surface of airless bodies [4]. Some Itokawa particles have unique structures produced by space weathering such as blisters, melt splashes, and vapor redeposition layers [2, 5]. To better understand the relationship between space weathering and solar wind implantation, we studied three-dimensional distributions of solar wind 4He on Itokawa particles using the secondary neutral mass spectrometry instrument called LIMAS (Hokkaido University) [6, 7].
Five Itokawa particles, previously reported to have typical structures due to space weathering [5, 8], were mounted on Indium. The sample surfaces were observed by a field-emission scanning electron microscope (FE-SEM; JEOL JSM-7000F, Hokkaido University). After Au-coating of 10 nm in thickness, we performed the depth profiling analysis using LIMAS. The pulsed primary beam (Ga+) scanned selected areas (~15 × 25 μm2) on the sample surfaces. The sputtering rates and the relative sensitivity factors of 4He+ normalized to 16O+ were determined using an artificially 4He-implanted olivine standard before each sample analysis.
The FE-SEM observation revealed that the almost entire surface of the RA-QD02-0307 particle shows space-weathering structures such as blisters and melt splashes. The LIMAS analysis showed that the sample surface is enriched in He, indicating that the particle experienced solar wind exposure. The 4He peak depth was ~30 nm from the surface. This is ~10 nm deeper than the peak depth of the solar wind 4He+ implantation simulation using the TRIM program [9]. The deeper peak depth may be explained by (1) the formation of a vapor redeposition layer (~10 nm in thickness [2]) during solar wind exposure, or (2) decrease in He concentration at depths of less than ~30 nm related to blister formation. In either case, the result shows that the distribution of implanted solar wind He on the Itokawa regolith particle changed as space weathering progressed.
In other samples, He was detected in a limited part of the scan area of the LIMAS analysis, indicating that the solar wind He distribution in a single regolith particle is heterogeneous. This is consistent with the previous study reporting heterogeneous distributions of blisters on Itokawa regolith particles [5].
References: [1] Nagao K. et al. (2011) Science 333, 1128–1131. [2] Noguchi T. et al. (2014) Meteoritics & Planet. Sci. 49, 188–214. [3] Daly L. et al. (2021) Nat. Astron. 5, 1275–1285. [4] Pieters C. M. and Noble S. K. (2016) J. Geophys. Res. Planets 121, 1865–1884. [5] Matsumoto T. et al. (2016) Geochim. Cosmochim. Acta 187, 195–217. [6] Ebata S. et al. (2012) Surf. Interface Anal. 44, 635–640. [7] Bajo K. et al. (2015) Geochem. J. 49, 559–566. [8] Matsumoto T. et al. (2018) Icarus 303, 22–33. [9] Ziegler J. F. http://www.srim.org.
Five Itokawa particles, previously reported to have typical structures due to space weathering [5, 8], were mounted on Indium. The sample surfaces were observed by a field-emission scanning electron microscope (FE-SEM; JEOL JSM-7000F, Hokkaido University). After Au-coating of 10 nm in thickness, we performed the depth profiling analysis using LIMAS. The pulsed primary beam (Ga+) scanned selected areas (~15 × 25 μm2) on the sample surfaces. The sputtering rates and the relative sensitivity factors of 4He+ normalized to 16O+ were determined using an artificially 4He-implanted olivine standard before each sample analysis.
The FE-SEM observation revealed that the almost entire surface of the RA-QD02-0307 particle shows space-weathering structures such as blisters and melt splashes. The LIMAS analysis showed that the sample surface is enriched in He, indicating that the particle experienced solar wind exposure. The 4He peak depth was ~30 nm from the surface. This is ~10 nm deeper than the peak depth of the solar wind 4He+ implantation simulation using the TRIM program [9]. The deeper peak depth may be explained by (1) the formation of a vapor redeposition layer (~10 nm in thickness [2]) during solar wind exposure, or (2) decrease in He concentration at depths of less than ~30 nm related to blister formation. In either case, the result shows that the distribution of implanted solar wind He on the Itokawa regolith particle changed as space weathering progressed.
In other samples, He was detected in a limited part of the scan area of the LIMAS analysis, indicating that the solar wind He distribution in a single regolith particle is heterogeneous. This is consistent with the previous study reporting heterogeneous distributions of blisters on Itokawa regolith particles [5].
References: [1] Nagao K. et al. (2011) Science 333, 1128–1131. [2] Noguchi T. et al. (2014) Meteoritics & Planet. Sci. 49, 188–214. [3] Daly L. et al. (2021) Nat. Astron. 5, 1275–1285. [4] Pieters C. M. and Noble S. K. (2016) J. Geophys. Res. Planets 121, 1865–1884. [5] Matsumoto T. et al. (2016) Geochim. Cosmochim. Acta 187, 195–217. [6] Ebata S. et al. (2012) Surf. Interface Anal. 44, 635–640. [7] Bajo K. et al. (2015) Geochem. J. 49, 559–566. [8] Matsumoto T. et al. (2018) Icarus 303, 22–33. [9] Ziegler J. F. http://www.srim.org.