09:15 〜 09:30
[PPS03-02] 宇宙風化によるリュウグウ帰還試料の近赤外域分光学的不均一性
キーワード：小惑星、リュウグウ、宇宙風化
Introduction: Asteroid taxonomy is classified based on reflectance spectral profiles in Vis to NIR wavelength. On the other hand, surface materials of an asteroid have been altered due to space weathering and the reflectance spectral profiles have also been changed [1]. Therefore, it is necessary to understand the mechanism of spectral changes due to space weathering.
C-type asteroid 162173 Ryugu is the target body of the JAXA Hayabusa2 project, and after remote sensing of the asteroid and two samplings there, two kinds of Ryugu samples were brought back to Earth in 2020. During the first sampling, “Surface” materials were collected and stored into Chamber A. During the second sampling, after artificial crater formation by the SCI [2], “Subsurface” materials were collected and stored into Chamber C. Initial description of these Ryugu samples have been carried out at the JAXA Curation Center [3].
MicrOmega, an near infrared hyperspectral microscope, is one of the instruments for initial description. MicrOmega can acquire NIR spectra (0.99-3.65 µm) [4]. Common profiles of most Ryugu samples are a continuum around 2.0 µm and three absorption bands (at 2.7, 3.1, and 3.4 µm) [5][6][7].
Space weathering simulations on carbonaceous meteorites have reported a gentler slope in NIR after laser irradiation [8]. Remote sensing using Vis light on the asteroid Ryugu has reported a redder slope due to space weathering [9]. It remains poorly understood how the NIR slope changes by space weathering on the asteroid Ryugu.
The 2.7 µm absorption band corresponds to OH groups [5]. Space weathering reduces its depth [10]. Peak positions and depths were estimated by fitting analysis of the short wavelength side [6].
In this study, the 2.0 µm slope and the 2.7 µm absorption band are quantitatively analyzed. Then, by comparing surface and subsurface samples,spectral changes due to space weathering on the asteroid Ryugu were moved on to understanding.
Methods: Spectral data of 163 (99 in Chamber A, 64 in Chamber C) in the MicrOmega-Curation DARTS Server were analyzed.
The 2.0 µm slope was quantitatively calculated by fitting a linear function. For the analysis of the 2.7 µm absorption band, the peak position and depth were calculated by fitting multiple Gaussian functions.
Results: The 2.7 µm absorption band is best fitted with six absorption bands in all samples. In a relationship between the peak position and depth of the composite waveform, the Chamber A samples are distributed in two regions: α, where the peak position is shorter and the depth is greater, and β, where the peak position is longer and the depth is smaller. No similar trend was observed in Chamber C samples: C in Figure 1. The 2.0 µm slope tends to be gentler in the order α > C > β (α: (6.05 ± 1.31) × 10-5 [%/cm-1], β: (5.83 ± 1.42) × 10-5 [%/cm-1], and C: (6.01 ± 1.51) × 10-5 [%/cm-1]). Thus, the peak position and depth of the 2.7 µm absorption band becomes longer and decreases as the 2.0 µm slope becomes gentler.
Discussion: NIR spectrometry by NIRS3 reported that the ratio of the 2.4 µm to the 2.2 µm reflectance is reduced due to space weathering [11]. Space weathering has been reported to reduce the depth of the 2.7 µm absorption band [10]. Therefore, for the Ryugu samples, space weathering seems to affect more in the order α < C < β.
References: [1] Hasegawa, S. et al. (2022) ApJL, 939, L9. [2] Arakawa, M. et al. (2020) Science 368, 67–71. [3] Yada, T. et al. (2021) Nat Astron, 6,214–220. [4] Bibring, J.-P. et al. (2017) Space Sci Rev, 208, 401–412. [5] Pilorget, C. et al. (2021) Nat Astron, 6, 221–225. [6] Le Pivert-Jolivet, T. et al. (2023) Nat Astron, doi:10.1038/s41550-023-02092-9. [7] Loizeau, D. et al. (2023) Nat Astron, 7, 391–397. [8] Hiroi, T., Sugita, S. (2010) Planetary People, 19, 1, 36-47. [9] Sugita, S. et al. (2019) Science, 364, 252. [10] Noguchi, T. et al. (2022) Nat Astron, doi:10.1038/s41550-022-01841-6. [11] Matsuoka, M. et al. (2023) Commun Earth Environ, 4, 335.
C-type asteroid 162173 Ryugu is the target body of the JAXA Hayabusa2 project, and after remote sensing of the asteroid and two samplings there, two kinds of Ryugu samples were brought back to Earth in 2020. During the first sampling, “Surface” materials were collected and stored into Chamber A. During the second sampling, after artificial crater formation by the SCI [2], “Subsurface” materials were collected and stored into Chamber C. Initial description of these Ryugu samples have been carried out at the JAXA Curation Center [3].
MicrOmega, an near infrared hyperspectral microscope, is one of the instruments for initial description. MicrOmega can acquire NIR spectra (0.99-3.65 µm) [4]. Common profiles of most Ryugu samples are a continuum around 2.0 µm and three absorption bands (at 2.7, 3.1, and 3.4 µm) [5][6][7].
Space weathering simulations on carbonaceous meteorites have reported a gentler slope in NIR after laser irradiation [8]. Remote sensing using Vis light on the asteroid Ryugu has reported a redder slope due to space weathering [9]. It remains poorly understood how the NIR slope changes by space weathering on the asteroid Ryugu.
The 2.7 µm absorption band corresponds to OH groups [5]. Space weathering reduces its depth [10]. Peak positions and depths were estimated by fitting analysis of the short wavelength side [6].
In this study, the 2.0 µm slope and the 2.7 µm absorption band are quantitatively analyzed. Then, by comparing surface and subsurface samples,spectral changes due to space weathering on the asteroid Ryugu were moved on to understanding.
Methods: Spectral data of 163 (99 in Chamber A, 64 in Chamber C) in the MicrOmega-Curation DARTS Server were analyzed.
The 2.0 µm slope was quantitatively calculated by fitting a linear function. For the analysis of the 2.7 µm absorption band, the peak position and depth were calculated by fitting multiple Gaussian functions.
Results: The 2.7 µm absorption band is best fitted with six absorption bands in all samples. In a relationship between the peak position and depth of the composite waveform, the Chamber A samples are distributed in two regions: α, where the peak position is shorter and the depth is greater, and β, where the peak position is longer and the depth is smaller. No similar trend was observed in Chamber C samples: C in Figure 1. The 2.0 µm slope tends to be gentler in the order α > C > β (α: (6.05 ± 1.31) × 10-5 [%/cm-1], β: (5.83 ± 1.42) × 10-5 [%/cm-1], and C: (6.01 ± 1.51) × 10-5 [%/cm-1]). Thus, the peak position and depth of the 2.7 µm absorption band becomes longer and decreases as the 2.0 µm slope becomes gentler.
Discussion: NIR spectrometry by NIRS3 reported that the ratio of the 2.4 µm to the 2.2 µm reflectance is reduced due to space weathering [11]. Space weathering has been reported to reduce the depth of the 2.7 µm absorption band [10]. Therefore, for the Ryugu samples, space weathering seems to affect more in the order α < C < β.
References: [1] Hasegawa, S. et al. (2022) ApJL, 939, L9. [2] Arakawa, M. et al. (2020) Science 368, 67–71. [3] Yada, T. et al. (2021) Nat Astron, 6,214–220. [4] Bibring, J.-P. et al. (2017) Space Sci Rev, 208, 401–412. [5] Pilorget, C. et al. (2021) Nat Astron, 6, 221–225. [6] Le Pivert-Jolivet, T. et al. (2023) Nat Astron, doi:10.1038/s41550-023-02092-9. [7] Loizeau, D. et al. (2023) Nat Astron, 7, 391–397. [8] Hiroi, T., Sugita, S. (2010) Planetary People, 19, 1, 36-47. [9] Sugita, S. et al. (2019) Science, 364, 252. [10] Noguchi, T. et al. (2022) Nat Astron, doi:10.1038/s41550-022-01841-6. [11] Matsuoka, M. et al. (2023) Commun Earth Environ, 4, 335.