09:00 〜 09:15
[PPS03-01] Spectral evolutions of primitive asteroids
inferred from remote sensing and sample analyses of Ryugu and Bennu
★Invited Papers
キーワード：リュウグウ、べヌー、小惑星、宇宙風化、分光スペクトル
Introduction: Our understanding of the diversity among primitive asteroids heavily relies on ground-based telescope observations of their reflectance spectra. However, the interpretation of these spectra is often challenging because primitive asteroids typically exhibit featureless spectra. In addition, it has been long known that processes such as solar wind irradiations and micrometeorite bombardments (i.e., space weathering) can modify the spectra of surface materials, masking the signature of the bulk asteroid.
Now that we have samples returned from the two near-Earth primitive asteroids, Ryugu and Bennu, we may be able to establish the fundamental linkage between material properties and spectral information. Comparing the two asteroids is crucial in this regard because they not only differ in global spectra, but high-resolution observations by spacecraft have revealed that their spectral evolution occurred in opposite directions; Ryugu reddened [1] while Bennu blued [2]. However, the essential difference between Ryugu and Bennu that led to such spectral difference remains unidentified. This is because these asteroids share many physical properties as a top-shaped rubble pile [3, 4], and their compositions are both consistent with low-petrologic-type carbonaceous chondrites [5, 6]. To address this issue, we aimed to obtain further constraints on the spectral relation between Ryugu and Bennu using cross-calibrated remote-sensing data and spectral images of returned samples.
Method: We performed spectral and photometric analyses of 123 craters on Ryugu and 565 craters on Bennu, with diameters ranging from 4–300 m, using multi-band images (0.48–0.85 µm) obtained by the Optical Navigation Camera (ONC) onboard Hayabusa2 and MapCam onboard OSIRIS-REx. Systematic errors between the radiometric calibrations of the two imagers were corrected through cross calibration using the Moon [7]. Moon. We investigated the correlation of crater spectra with their size to estimate spectral evolution trends because smaller craters are likely fresher due to their shorter lifetimes. We conducted multi-band spectroscopy of 252 Ryugu samples at the JAXA curation facility using spectral filters compatible with ONC.
Results: Our analyses show that the spectral distributions of craters on Ryugu and Bennu follow a common trend line in the reflectance–spectral slope diagram. In addition, the spectra of fresh craters on both asteroids are indistinguishable within the cross-calibration accuracy. The findings suggest that Ryugu and Bennu initially had similar spectra, but they evolved in opposite directions along a common trend line.
The spectral evolution processes include (1) space weathering, (2) solar heating, and (3) grain size/porosity evolution. The following multiple lines of evidence may be more consistent with (3).
・The spectral evolution trends on Ryugu and Bennu align along a common trend line.
・Despite Ryugu samples retaining a higher abundance of water compared to the asteroid surface [8] and likely being fresher in terms of (1) and (2), we observed that their spectral slope is more consistent with the older end of the observed spectral evolution trend.
・The spectral transition from coarse (~1 mm) to fine (<300 µm) grained aggregates of Ryugu samples qualitatively aligns with the observed spectral evolution trend.
・Analyses of crater photometry showed that older craters have steeper phase curves than fresher ones on Ryugu, while older craters have shallower phase curves on Bennu.
Discussion: Our results suggest that the spectral difference between Ryugu and Bennu resulted from their opposite grain size/porosity evolution; surface materials on Ryugu became fine-grained or porous, while those on Bennu became coarser (i.e., fines were more efficiently lost) or compact. Our model calculation, based on [9], shows that such opposite evolution may be simply explained by their difference in asteroid size. This hypothesis implies that asteroids with different spectral types can have similar compositions, and thus Ryugu/Bennu-like materials may be widespread in the solar system.
References: [1] Sugita et al. (2019). Science, 364(6437). [2] DellaGiustina et al. (2020). Science, 370(6517). [3] Watanabe et al. (2019). Science, 364(6437). [4] Lauretta et al. (2019). Nature, 568(7750). [5] Yokoyama et al. (2022) Science, eabn7850. [6] Zega et al. (2024). 55th LPSC. [7] Yumoto et al. Icarus. in rev. [8] Pilorget et al. (2022). Nat. Astron., 6(2). [9] Hsu et al. (2022). Nat. Astron., 6(9).
Now that we have samples returned from the two near-Earth primitive asteroids, Ryugu and Bennu, we may be able to establish the fundamental linkage between material properties and spectral information. Comparing the two asteroids is crucial in this regard because they not only differ in global spectra, but high-resolution observations by spacecraft have revealed that their spectral evolution occurred in opposite directions; Ryugu reddened [1] while Bennu blued [2]. However, the essential difference between Ryugu and Bennu that led to such spectral difference remains unidentified. This is because these asteroids share many physical properties as a top-shaped rubble pile [3, 4], and their compositions are both consistent with low-petrologic-type carbonaceous chondrites [5, 6]. To address this issue, we aimed to obtain further constraints on the spectral relation between Ryugu and Bennu using cross-calibrated remote-sensing data and spectral images of returned samples.
Method: We performed spectral and photometric analyses of 123 craters on Ryugu and 565 craters on Bennu, with diameters ranging from 4–300 m, using multi-band images (0.48–0.85 µm) obtained by the Optical Navigation Camera (ONC) onboard Hayabusa2 and MapCam onboard OSIRIS-REx. Systematic errors between the radiometric calibrations of the two imagers were corrected through cross calibration using the Moon [7]. Moon. We investigated the correlation of crater spectra with their size to estimate spectral evolution trends because smaller craters are likely fresher due to their shorter lifetimes. We conducted multi-band spectroscopy of 252 Ryugu samples at the JAXA curation facility using spectral filters compatible with ONC.
Results: Our analyses show that the spectral distributions of craters on Ryugu and Bennu follow a common trend line in the reflectance–spectral slope diagram. In addition, the spectra of fresh craters on both asteroids are indistinguishable within the cross-calibration accuracy. The findings suggest that Ryugu and Bennu initially had similar spectra, but they evolved in opposite directions along a common trend line.
The spectral evolution processes include (1) space weathering, (2) solar heating, and (3) grain size/porosity evolution. The following multiple lines of evidence may be more consistent with (3).
・The spectral evolution trends on Ryugu and Bennu align along a common trend line.
・Despite Ryugu samples retaining a higher abundance of water compared to the asteroid surface [8] and likely being fresher in terms of (1) and (2), we observed that their spectral slope is more consistent with the older end of the observed spectral evolution trend.
・The spectral transition from coarse (~1 mm) to fine (<300 µm) grained aggregates of Ryugu samples qualitatively aligns with the observed spectral evolution trend.
・Analyses of crater photometry showed that older craters have steeper phase curves than fresher ones on Ryugu, while older craters have shallower phase curves on Bennu.
Discussion: Our results suggest that the spectral difference between Ryugu and Bennu resulted from their opposite grain size/porosity evolution; surface materials on Ryugu became fine-grained or porous, while those on Bennu became coarser (i.e., fines were more efficiently lost) or compact. Our model calculation, based on [9], shows that such opposite evolution may be simply explained by their difference in asteroid size. This hypothesis implies that asteroids with different spectral types can have similar compositions, and thus Ryugu/Bennu-like materials may be widespread in the solar system.
References: [1] Sugita et al. (2019). Science, 364(6437). [2] DellaGiustina et al. (2020). Science, 370(6517). [3] Watanabe et al. (2019). Science, 364(6437). [4] Lauretta et al. (2019). Nature, 568(7750). [5] Yokoyama et al. (2022) Science, eabn7850. [6] Zega et al. (2024). 55th LPSC. [7] Yumoto et al. Icarus. in rev. [8] Pilorget et al. (2022). Nat. Astron., 6(2). [9] Hsu et al. (2022). Nat. Astron., 6(9).