09:15 〜 09:30
[PPS03-02] Ryugu帰還試料のMicrOmega測定と炭素質隕石のUV照射実験から明らかになった短時間太陽UV暴露の影響
キーワード：近赤外分光、リュウグウ、宇宙風化、紫外線
Introduction:
The C-type asteroid 162173 Ryugu is the target body of the JAXA’s Hayabusa2 project, and two types of samples, expected to exhibit different degrees of space weathering, were returned to Earth in 2020. The 1st touchdown (TD1) sampled surface materials into chamber A, and the 2ndtouchdown (TD2) sampled subsurface materials into chamber C after the Small Carry-on Impactor (SCI) impact. After return, initial descriptions of these Ryugu samples had been carried out at the JAXA Curation Center without exposure to the atmosphere [1]. Initial description by the NIR hyperspectral microscope MicrOmega, developed by IAS, revealed that the 2.7 µm band originating from OH groups shifts and weakens with space weathering [2], [3].
Space weathering changes the optical properties of airless bodies like asteroids, the Moon, and Mercury, causing reddening, darkening, and attenuation of absorption bands. While laser and ion irradiation corresponding to micrometeorite impacts and solar wind have been studied, Solar UV radiation effects are less explored. In experiments with olivine and enstatite pellets, UV irradiation reduced visible reflectance by 40% and 20%, respectively. Solar UV is increasingly considered a dominant factor in space weathering during the first 1–10 years, before solar wind and micrometeorite impacts become more significant [4].
This study investigates the cause of space weathering on Ryugu samples through UV irradiation experiments on carbonaceous chondrites.
Methods:
This study analyzed MicrOmega spectral data from 207 millimeter-sized particles (119 from chamber A, 88 from chamber C) archived in DARTS. We analyzed the 2.7 µm band using multiple Gaussian functions fitting.
This study used an ivuna particle (~2.5 mg, ~1.5 mm diameter), compositionally similar to Ryugu, for UV irradiation. UV light is irradiated in a nitrogen-filled small chamber using a 300W Xenon lamp (Asahi MAX-303) (250–385 nm). Spectra of ivuna before and after UV irradiation were measured using a JASCO FT/IR-6100 under nitrogen condition.
Results:
Six Gaussian components were used for best fitting to the 2.7 µm band. The composite waveform shows a decrease in band depth as the peak position shifts towards longer wavelengths. The chamber C particles tend to have features that overlap both the shorter and longer band center groups in chamber A, but are around the middle in Figure 1a.
We examined the effect of solar UV radiation on the 2.7 µm band of chamber C which were exposure by Solar UV ray after the SCI impact for ~3 months. UV irradiation experiment on the ivuna for about a month at 1AU shows the 2.7 µm band shifts by ~1 nm and its depth decreases by ~3% in Figure 1b.
Discussion:
The 2.7 µm band shift and depth reduction in Ryugu particles are expected to be attributed to dehydroxylation caused by space weathering, with a trend progressing from the shorter band center group in chamber A to chamber C, and further to the longer band center group in chamber A. The chamber C particles, having been exposed to solar UV radiation for ~3 months, are likely more dehydroxylated than the shorter band center group in chamber A. The UV irradiation experiment on ivuna suggests that chamber C contains particles affected by solar UV exposure during the SCI impact to TD2.
References:
[1] T. Yada et al., Nat Astron, vol. 6, no. 2, pp. 214–220, Dec. 2021, doi: 10.1038/s41550-021-01550-6.
[2] C. Pilorget et al., Nat Astron, vol. 6, no. 2, pp. 221–225, Dec. 2021, doi: 10.1038/s41550-021-01549-z.
[3] T. Le Pivert-Jolivet et al., Nat Astron, Oct. 2023, doi: 10.1038/s41550-023-02092-9.
[4] S. Sho et al., Journal of Evolving Space Activities, vol. 2, no. 145, doi: 10.57350/jesa.145.
The C-type asteroid 162173 Ryugu is the target body of the JAXA’s Hayabusa2 project, and two types of samples, expected to exhibit different degrees of space weathering, were returned to Earth in 2020. The 1st touchdown (TD1) sampled surface materials into chamber A, and the 2ndtouchdown (TD2) sampled subsurface materials into chamber C after the Small Carry-on Impactor (SCI) impact. After return, initial descriptions of these Ryugu samples had been carried out at the JAXA Curation Center without exposure to the atmosphere [1]. Initial description by the NIR hyperspectral microscope MicrOmega, developed by IAS, revealed that the 2.7 µm band originating from OH groups shifts and weakens with space weathering [2], [3].
Space weathering changes the optical properties of airless bodies like asteroids, the Moon, and Mercury, causing reddening, darkening, and attenuation of absorption bands. While laser and ion irradiation corresponding to micrometeorite impacts and solar wind have been studied, Solar UV radiation effects are less explored. In experiments with olivine and enstatite pellets, UV irradiation reduced visible reflectance by 40% and 20%, respectively. Solar UV is increasingly considered a dominant factor in space weathering during the first 1–10 years, before solar wind and micrometeorite impacts become more significant [4].
This study investigates the cause of space weathering on Ryugu samples through UV irradiation experiments on carbonaceous chondrites.
Methods:
This study analyzed MicrOmega spectral data from 207 millimeter-sized particles (119 from chamber A, 88 from chamber C) archived in DARTS. We analyzed the 2.7 µm band using multiple Gaussian functions fitting.
This study used an ivuna particle (~2.5 mg, ~1.5 mm diameter), compositionally similar to Ryugu, for UV irradiation. UV light is irradiated in a nitrogen-filled small chamber using a 300W Xenon lamp (Asahi MAX-303) (250–385 nm). Spectra of ivuna before and after UV irradiation were measured using a JASCO FT/IR-6100 under nitrogen condition.
Results:
Six Gaussian components were used for best fitting to the 2.7 µm band. The composite waveform shows a decrease in band depth as the peak position shifts towards longer wavelengths. The chamber C particles tend to have features that overlap both the shorter and longer band center groups in chamber A, but are around the middle in Figure 1a.
We examined the effect of solar UV radiation on the 2.7 µm band of chamber C which were exposure by Solar UV ray after the SCI impact for ~3 months. UV irradiation experiment on the ivuna for about a month at 1AU shows the 2.7 µm band shifts by ~1 nm and its depth decreases by ~3% in Figure 1b.
Discussion:
The 2.7 µm band shift and depth reduction in Ryugu particles are expected to be attributed to dehydroxylation caused by space weathering, with a trend progressing from the shorter band center group in chamber A to chamber C, and further to the longer band center group in chamber A. The chamber C particles, having been exposed to solar UV radiation for ~3 months, are likely more dehydroxylated than the shorter band center group in chamber A. The UV irradiation experiment on ivuna suggests that chamber C contains particles affected by solar UV exposure during the SCI impact to TD2.
References:
[1] T. Yada et al., Nat Astron, vol. 6, no. 2, pp. 214–220, Dec. 2021, doi: 10.1038/s41550-021-01550-6.
[2] C. Pilorget et al., Nat Astron, vol. 6, no. 2, pp. 221–225, Dec. 2021, doi: 10.1038/s41550-021-01549-z.
[3] T. Le Pivert-Jolivet et al., Nat Astron, Oct. 2023, doi: 10.1038/s41550-023-02092-9.
[4] S. Sho et al., Journal of Evolving Space Activities, vol. 2, no. 145, doi: 10.57350/jesa.145.