Presentation Information
[PPS03-P03]Investigating boulder properties on asteroid Ryugu through an integrated analysis of Hayabusa2 multi-instrument data
*Seima Ishida1, Koki Tsutsui1, Eri Tatsumi2, Mayumi Ichikawa3, Koki Yumoto3, Antonin Wargnier3, Tomoki Nakamura1 (1.Tohoku University, 2.Instituto de Astrofisica de Canarias, 3.JAXA)
Keywords:
Asteroid Ryugu,Hayabusa2,Remote sensing,Boulder
After arriving at the near-Earth asteroid Ryugu, Hayabusa2 conducted comprehensive remote-sensing observations, including visible to near-infrared reflectance spectroscopy and thermal-infrared imaging, and successfully collected regolith samples from two sites and returned them to Earth [1]. Mineralogical and chemical analyses of the returned samples have substantially updated and refined our understanding of the formation process and evolutionary history of Ryugu regolith [2][3]. In contrast, the returned samples are limited to particles up to ~10 mm in size [4], and meter-scale boulders—which dominate much of Ryugu’s surface [5]—could not be collected, leaving their material properties poorly constrained. Moreover, the thermal inertia inferred for the returned particles is higher than the thermal inertia inferred for Ryugu boulders from remote sensing [6][7], suggesting that microscopic physical properties obtained in the laboratory cannot be straightforwardly extrapolated to the boulder scale [8]. Accordingly, regolith-based insights alone are insufficient to characterize boulders, and it is crucial to constrain boulder properties directly from remote-sensing observations. Several peculiarities have been reported for Ryugu boulders—such as those with markedly higher reflectance than the surrounding regolith and candidates that may have been delivered from other bodies—and understanding these characteristics in detail provides an essential basis for discussing Ryugu’s surface formation and evolution history [5][9].
The Hayabusa2 instruments ONC-T, NIRS3, and TIR acquired quantitative datasets including multi-band visible reflectance, near-infrared spectra, and thermal emission on Ryugu [5][10][11]. While many studies have focused on analyses specialized to individual instruments, integrated investigations that combine multi-instrument feature sets for the same targets—and simultaneously evaluate the correspondence between reflectance-spectral properties and thermophysical properties—remain limited. Such cross-instrument, integrated interpretation is particularly important for boulders, for which laboratory analyses of returned samples are not possible.
In this study, we extract features that may reflect boulder material properties by consistently co-registering the observational footprints of ONC-T/NIRS3/TIR using GIS products. We analyze 6,241 boulders with ONC-T, while the TIR and NIRS3 analyses are restricted to 1,621 boulders due to their larger observational footprints. Because spatial resolution and observing conditions differ among these instruments, representative values were estimated using the overlap between boulder polygons and each instrument’s footprints. The extracted features used for study include, among others, (i) visible reflectance and spectral slope from ONC-T, (ii) absorption band depths and band centers from NIRS3 near-infrared reflectance spectra, and (iii) thermal inertia derived from TIR [11]. We statistically analyze the spatial distributions (latitude/longitude dependence) and correlations among these features, and evaluate boulder characteristics by comparing them with the global-scale spatial trends of each feature. In this presentation, we demonstrate the multifaceted constraints on boulder properties enabled by the three-instrument integration and clarify the relationships between reflectance-spectral properties and thermophysical properties that could be missed in single-instrument analyses. This work provides a basis for discussing the formation and evolution of Ryugu’s surface—including boulders—in a manner consistent with the material picture established from the returned regolith samples.
Acknowledgments
This work was supported by the Hayabusa2# International Visibility Enhancement Project. We used analysis code provided by Senior Associate Professor Naru Hirata, and we gratefully acknowledge his contribution.
References: [1] Tsuda et al. (2020) Acta Astronautica, 171, 42–54. [2] Nakamura et al. (2022) Science, 379, eabn8671. [3] Yokoyama et al. (2023) Science, 379, eabn7850. [4] Yada et al. (2022) Nature Astronomy, 6, 214–220. [5] Sugita et al. (2019) Science, 364, eaaw0422. [6] Ishizaki et al. (2023) International Journal of Thermophysics, 44, 51. [7] Grott et al. (2019) Nature Astronomy, 3, 971–976. [8] Hamm et al. (2023) Geophysical Research Letters, 50, e2023GL104795. [9] Tatsumi et al. (2021) Nature Astronomy, 5, 39–45. [10] Kitazato et al. (2019) Science, 364, 272–275. [11] Shimaki et al. (2020) Icarus, 348, 113835.
The Hayabusa2 instruments ONC-T, NIRS3, and TIR acquired quantitative datasets including multi-band visible reflectance, near-infrared spectra, and thermal emission on Ryugu [5][10][11]. While many studies have focused on analyses specialized to individual instruments, integrated investigations that combine multi-instrument feature sets for the same targets—and simultaneously evaluate the correspondence between reflectance-spectral properties and thermophysical properties—remain limited. Such cross-instrument, integrated interpretation is particularly important for boulders, for which laboratory analyses of returned samples are not possible.
In this study, we extract features that may reflect boulder material properties by consistently co-registering the observational footprints of ONC-T/NIRS3/TIR using GIS products. We analyze 6,241 boulders with ONC-T, while the TIR and NIRS3 analyses are restricted to 1,621 boulders due to their larger observational footprints. Because spatial resolution and observing conditions differ among these instruments, representative values were estimated using the overlap between boulder polygons and each instrument’s footprints. The extracted features used for study include, among others, (i) visible reflectance and spectral slope from ONC-T, (ii) absorption band depths and band centers from NIRS3 near-infrared reflectance spectra, and (iii) thermal inertia derived from TIR [11]. We statistically analyze the spatial distributions (latitude/longitude dependence) and correlations among these features, and evaluate boulder characteristics by comparing them with the global-scale spatial trends of each feature. In this presentation, we demonstrate the multifaceted constraints on boulder properties enabled by the three-instrument integration and clarify the relationships between reflectance-spectral properties and thermophysical properties that could be missed in single-instrument analyses. This work provides a basis for discussing the formation and evolution of Ryugu’s surface—including boulders—in a manner consistent with the material picture established from the returned regolith samples.
Acknowledgments
This work was supported by the Hayabusa2# International Visibility Enhancement Project. We used analysis code provided by Senior Associate Professor Naru Hirata, and we gratefully acknowledge his contribution.
References: [1] Tsuda et al. (2020) Acta Astronautica, 171, 42–54. [2] Nakamura et al. (2022) Science, 379, eabn8671. [3] Yokoyama et al. (2023) Science, 379, eabn7850. [4] Yada et al. (2022) Nature Astronomy, 6, 214–220. [5] Sugita et al. (2019) Science, 364, eaaw0422. [6] Ishizaki et al. (2023) International Journal of Thermophysics, 44, 51. [7] Grott et al. (2019) Nature Astronomy, 3, 971–976. [8] Hamm et al. (2023) Geophysical Research Letters, 50, e2023GL104795. [9] Tatsumi et al. (2021) Nature Astronomy, 5, 39–45. [10] Kitazato et al. (2019) Science, 364, 272–275. [11] Shimaki et al. (2020) Icarus, 348, 113835.