3:30 PM - 4:00 PM
[PPS03-01] Hayabusa2 mission summary: Proximity observations and returned samples tell us about the history of Ryugu
★Invited Papers
Keywords:asteroids, planetary exploration, crater, carbonaceous chondrites, formation of the Solar System
The Hayabusa2 nominal mission will end in March 2022. The formation of the Solar System and the subsequent history of asteroid Ryugu will be discussed based on the proximity observations of Ryugu and analysis of the return sample.
From the initial analysis, it was confirmed that the Rhygu sample is similar to CI chondrites in terms of mineral assemblages and composition, and isotopic ratios [1,2,3]. The presence of grains with large anomalies in the stable isotope ratios of hydrogen, carbon, and nitrogen was confirmed [4], suggesting that some of the particles produced in the low-temperature environment of a molecular cloud are retained without undergoing isotopic equilibrium on the body. The total carbon is comparable to that of CI chondrites, whereas the carbonate content is considerably higher, and therefore the organic carbon content is somewhat lower. The presence of CO2 in fluid inclusions in a large phyrrhotite crystal [3], the IR absorption of NH in ammonium salts or organic nitrogen compounds [5], and the very low content of chondrules and CAI [3] suggest that the parent body of Ryugu may have originated outside the snow lines of CO2 and NH3 (outside Jupiter's orbit). Aqueous alteration in the parent body is estimated to have proceeded in a fluid with a temperature of ~310 K generated by the decay heat of 26Al in the interior of an ~100-km sized icy body at 5-6 million years after the formation of CAI in the protosolar disk [6]. The presence of less altered grains in Ryugu particles suggests the difference in temperature between the near-surface and interior of the icy parent body at the stage of aqueous alteration [3]. Some of the less altered grains have particularly high porosity [3], suggesting the formation of planetesimals from fluffy ice and stone particles. It is noteworthy that franboidal magnetite grains with a remanent magnetization of ~100 μT were found [7], suggesting the influence of the dynamo of the parent body or the magnetic field of a giant planet such as proto-Jupiter.
These results are consistent with the scenario that carbonaceous chondrite parent bodies are icy planetesimals with diameters of ~100 km that formed in the outer Solar System and were brought to the inner Solar System through scattering by giant planets.
However, based on the reflection spectra and orbital analysis of Ryugu, its parent body is considered to be one of the collisional families in the inner asteroid belt [8]. Therefore, it is necessary to verify whether the scattering of giant planets could bring planetesimals to such an inner region of the asteroid belt. In any case, it is a great discovery that the parent bodies (at least one of them) of CI chondrites are collisional families in the inner asteroid belt.
Much has also been learned about the post-formation history of Ryugu. From the analysis of the YORP effect using the shape models, Ryugu is now slowly spinning down and had a fast rotation in the past (~10 Ma)[9]. The analysis of the tilt angle distribution on the surface of Ryugu suggests that the top shape was formed by surface landslides when the rotation period was 3.5 to 3.75 hours [10]. Following a previous study [11], which suggested that the east-west asymmetry of the crater rims of Ryugu was caused by the Coriolis force acting on crater ejecta during a high-speed rotation era, the analysis with precise topographic correction revealed that the three large craters of Ryugu have significant higher rims both on the west and equatorial sides [12]. These craters are more susceptible to the Coriolis force during high-speed rotation than other craters, and the average tilt angle of the each surrounding terrain is larger than that of other craters. These results suggest that the top shape of Ryugu was formed during high-speed rotation about 10 million years ago, and that the crater formation proceeded during the spin-down phase. In the future, it is necessary to decipher the history of Ryugu from the returned sample analysis.
[1] Yurimoto et al. 2022, LPSC abstract #1377; [2] Yokoyama et al. 2022, LPAC abstract #1273; [3] Nakamura et al. 2022, LPSC abstract #1753; [4] Yabuta et al. 2022, LPSC abstract #2241; [5] Pilorget et al. 2021, Nat. Astron. doi:10.1038/s41550-021-01549-z; [6] Nagashima et al. 2022, LPSC abstract #1689;
[7] Kimura 2022, LPSC abstract #1101; [8] Sugita et al. 2019, Science 364, aaw0422; [9] Kanamaru et al. 2021, JGRE 126, 12; [10] Watanabe et al. 2019, Science 364, 268; [11] Hirata et al. 2021, Icarus 354, 114073; [12] Yamada 2022 Master Thesis, Nagoya Univ.
From the initial analysis, it was confirmed that the Rhygu sample is similar to CI chondrites in terms of mineral assemblages and composition, and isotopic ratios [1,2,3]. The presence of grains with large anomalies in the stable isotope ratios of hydrogen, carbon, and nitrogen was confirmed [4], suggesting that some of the particles produced in the low-temperature environment of a molecular cloud are retained without undergoing isotopic equilibrium on the body. The total carbon is comparable to that of CI chondrites, whereas the carbonate content is considerably higher, and therefore the organic carbon content is somewhat lower. The presence of CO2 in fluid inclusions in a large phyrrhotite crystal [3], the IR absorption of NH in ammonium salts or organic nitrogen compounds [5], and the very low content of chondrules and CAI [3] suggest that the parent body of Ryugu may have originated outside the snow lines of CO2 and NH3 (outside Jupiter's orbit). Aqueous alteration in the parent body is estimated to have proceeded in a fluid with a temperature of ~310 K generated by the decay heat of 26Al in the interior of an ~100-km sized icy body at 5-6 million years after the formation of CAI in the protosolar disk [6]. The presence of less altered grains in Ryugu particles suggests the difference in temperature between the near-surface and interior of the icy parent body at the stage of aqueous alteration [3]. Some of the less altered grains have particularly high porosity [3], suggesting the formation of planetesimals from fluffy ice and stone particles. It is noteworthy that franboidal magnetite grains with a remanent magnetization of ~100 μT were found [7], suggesting the influence of the dynamo of the parent body or the magnetic field of a giant planet such as proto-Jupiter.
These results are consistent with the scenario that carbonaceous chondrite parent bodies are icy planetesimals with diameters of ~100 km that formed in the outer Solar System and were brought to the inner Solar System through scattering by giant planets.
However, based on the reflection spectra and orbital analysis of Ryugu, its parent body is considered to be one of the collisional families in the inner asteroid belt [8]. Therefore, it is necessary to verify whether the scattering of giant planets could bring planetesimals to such an inner region of the asteroid belt. In any case, it is a great discovery that the parent bodies (at least one of them) of CI chondrites are collisional families in the inner asteroid belt.
Much has also been learned about the post-formation history of Ryugu. From the analysis of the YORP effect using the shape models, Ryugu is now slowly spinning down and had a fast rotation in the past (~10 Ma)[9]. The analysis of the tilt angle distribution on the surface of Ryugu suggests that the top shape was formed by surface landslides when the rotation period was 3.5 to 3.75 hours [10]. Following a previous study [11], which suggested that the east-west asymmetry of the crater rims of Ryugu was caused by the Coriolis force acting on crater ejecta during a high-speed rotation era, the analysis with precise topographic correction revealed that the three large craters of Ryugu have significant higher rims both on the west and equatorial sides [12]. These craters are more susceptible to the Coriolis force during high-speed rotation than other craters, and the average tilt angle of the each surrounding terrain is larger than that of other craters. These results suggest that the top shape of Ryugu was formed during high-speed rotation about 10 million years ago, and that the crater formation proceeded during the spin-down phase. In the future, it is necessary to decipher the history of Ryugu from the returned sample analysis.
[1] Yurimoto et al. 2022, LPSC abstract #1377; [2] Yokoyama et al. 2022, LPAC abstract #1273; [3] Nakamura et al. 2022, LPSC abstract #1753; [4] Yabuta et al. 2022, LPSC abstract #2241; [5] Pilorget et al. 2021, Nat. Astron. doi:10.1038/s41550-021-01549-z; [6] Nagashima et al. 2022, LPSC abstract #1689;
[7] Kimura 2022, LPSC abstract #1101; [8] Sugita et al. 2019, Science 364, aaw0422; [9] Kanamaru et al. 2021, JGRE 126, 12; [10] Watanabe et al. 2019, Science 364, 268; [11] Hirata et al. 2021, Icarus 354, 114073; [12] Yamada 2022 Master Thesis, Nagoya Univ.