09:00 〜 09:15
[PPS03-06] An overview of preliminary analyses on bulk and individual Ryugu Samples Returned by Hayabusa2
キーワード：リュウグウ、C型小惑星、はやぶさ2、キュレーション、CIコンドライト
Hayabusa2 spacecraft, which accomplished successful touchdowns onto the near-Earth C-type asteroid Ryugu surface in Feb. and Jul. of 2019 [1, 2], returned its re-entry capsule encapsulating Ryugu’s surface samples back to the Earth on Dec. 6, 2020.
The recovered sample container was immediately transferred to the Extraterrestrial Sample Curation Center of JAXA after its landing to the Earth and introduced into the clean chamber (CC) of vacuum or purified nitrogen conditions without exposing to air. The samples were recovered from the catcher into the sapphire dishes and analyzed for primary descriptions. Firstly, bulk samples removed from the chamber A and C of the sample catcher, which corresponds to the first and second touchdown samples respectively, were analyzed for their weights by a balance, optical micrographs by an optical microscope, bulk infrared spectra by an FT-IR, infrared spectral images by an infrared imager MicrOmega, and visible reflectance spectra by a six-bands filtered microscope. Then individual particles have been removed from the bulk samples with a vacuum tweezer and placed onto sapphire dishes one by one and analyzed in the same manner with the bulk samples. So far, 404 of individual Ryugu particles of > 1mm in size have been handpicked from bulk samples from chambers A and C, weighed and observed with the optical microscope.
As reported in [3], the average of their bulk densities calculated based on estimated volumes and measured weights is smaller than any other meteorites. Their Infrared and visible spectra of bulk samples from chambers A and C are comparable to those of global Ryugu obtained by the onboard instruments of Hayabusa2, NIRS3 and ONC-T [4, 5], respectively, indicating their representativeness of surface regolith of the asteroid Ryugu. Besides, their visible spectra are darker than any known meteorites and their infrared spectra show adsorption features in 2.72 µm and 3.4 µm, corresponding to presences of O-H like hydrous minerals, C-H like organic materials and/or carbonates, respectively [3, 6]. No high temperature component like chondrule or Ca-, Al-rich inclusion (CAI) is observed from the surfaces of bulk and individual Ryugu samples by the optical microscope, thus they are most similar to CI chondrites among all known meteorites, although their bulk densities are smaller and their reflectance spectra are darker than those of CI chondrites. Individual Ryugu particles show angular and rounded shape features, and smooth and rough features in their surface morphologies [7]. The ratio of the former is 4:6, and that of the latter is 2:8. The angular shapes of the particles might have result from their recent destruction on the asteroid’s surface or during and/or after the sampling, whereas the rounded ones might not have experienced such destruction recently and preserved surfaces of regolith gardening on the asteroid’s surface. Further investigation is needed for confirmation of the relationship between their shapes or morphologies and duration of surface exposure on the asteroid.
Hayabusa2 brought back one of the most primitive planetary samples from Ryugu, implying the relationship between CI chondrites found on the Earth and Cb-type asteroids in spectral type. Results of initial analyses and analyses by phase 2 curation teams will bring us important implications about the early solar nebular and bodies in there and nature of water and organics in outer solar system to be transported to the inner region where the proto-Earth should have existed.
References: [1] Watanabe S. et al. (2019) Science 364, 268. [2] Tsuda Y. et al. (2020) Acta Astron. 171, 42. [3] Yada T. et al. (2021) Nat. Astron. doi: 10.1038/s41550-021-01550-6. [4] Kitazato K. et al. (2019) Science 364, 272. [5] Sugita S. et al. (2019) Science 364, eaaw0422. [6] Pilorget C. et al. (2021) Nat. Astron. doi: 10.1038/s41550-021-01549-z. [7] Miyazaki A. et al. (2022) Lunar Planet. Sci. LIII, #1816.
The recovered sample container was immediately transferred to the Extraterrestrial Sample Curation Center of JAXA after its landing to the Earth and introduced into the clean chamber (CC) of vacuum or purified nitrogen conditions without exposing to air. The samples were recovered from the catcher into the sapphire dishes and analyzed for primary descriptions. Firstly, bulk samples removed from the chamber A and C of the sample catcher, which corresponds to the first and second touchdown samples respectively, were analyzed for their weights by a balance, optical micrographs by an optical microscope, bulk infrared spectra by an FT-IR, infrared spectral images by an infrared imager MicrOmega, and visible reflectance spectra by a six-bands filtered microscope. Then individual particles have been removed from the bulk samples with a vacuum tweezer and placed onto sapphire dishes one by one and analyzed in the same manner with the bulk samples. So far, 404 of individual Ryugu particles of > 1mm in size have been handpicked from bulk samples from chambers A and C, weighed and observed with the optical microscope.
As reported in [3], the average of their bulk densities calculated based on estimated volumes and measured weights is smaller than any other meteorites. Their Infrared and visible spectra of bulk samples from chambers A and C are comparable to those of global Ryugu obtained by the onboard instruments of Hayabusa2, NIRS3 and ONC-T [4, 5], respectively, indicating their representativeness of surface regolith of the asteroid Ryugu. Besides, their visible spectra are darker than any known meteorites and their infrared spectra show adsorption features in 2.72 µm and 3.4 µm, corresponding to presences of O-H like hydrous minerals, C-H like organic materials and/or carbonates, respectively [3, 6]. No high temperature component like chondrule or Ca-, Al-rich inclusion (CAI) is observed from the surfaces of bulk and individual Ryugu samples by the optical microscope, thus they are most similar to CI chondrites among all known meteorites, although their bulk densities are smaller and their reflectance spectra are darker than those of CI chondrites. Individual Ryugu particles show angular and rounded shape features, and smooth and rough features in their surface morphologies [7]. The ratio of the former is 4:6, and that of the latter is 2:8. The angular shapes of the particles might have result from their recent destruction on the asteroid’s surface or during and/or after the sampling, whereas the rounded ones might not have experienced such destruction recently and preserved surfaces of regolith gardening on the asteroid’s surface. Further investigation is needed for confirmation of the relationship between their shapes or morphologies and duration of surface exposure on the asteroid.
Hayabusa2 brought back one of the most primitive planetary samples from Ryugu, implying the relationship between CI chondrites found on the Earth and Cb-type asteroids in spectral type. Results of initial analyses and analyses by phase 2 curation teams will bring us important implications about the early solar nebular and bodies in there and nature of water and organics in outer solar system to be transported to the inner region where the proto-Earth should have existed.
References: [1] Watanabe S. et al. (2019) Science 364, 268. [2] Tsuda Y. et al. (2020) Acta Astron. 171, 42. [3] Yada T. et al. (2021) Nat. Astron. doi: 10.1038/s41550-021-01550-6. [4] Kitazato K. et al. (2019) Science 364, 272. [5] Sugita S. et al. (2019) Science 364, eaaw0422. [6] Pilorget C. et al. (2021) Nat. Astron. doi: 10.1038/s41550-021-01549-z. [7] Miyazaki A. et al. (2022) Lunar Planet. Sci. LIII, #1816.