Presentation Information
[PPS12-12]Constraints on the internal structure and evolutionary timescale of the parent body based on the lithological diversity of Ryugu and CI chondrites and thermal evolution modeling
*Sojiro Yamazaki1, Takashi Mikouchi1, Michael Zolensky2 (1.The University of Tokyo, 2.NASA Johnson Space Center)
Keywords:
Ryugu,CI chondrite,Thermal evolution model
Introduction: This study investigates the thermal evolution of the parent bodies of Ryugu samples and CI chondrites, which record the thermal and chemical evolution of primitive asteroids in the early Solar System [1–3]. Based on mineralogy and degree of aqueous alteration, lithologies are classified into six types (I–VI). Type I is olivine-rich and least altered, Type II shows weak alteration, Type III contains dolomite and represents moderate alteration, Type IV includes dolomite and magnesite, Type V has large carbonate aggregates, and Type VI contains little carbonate [4]. The close match in lithology types and their proportions between Ryugu and CI chondrites suggests formation in similar parent bodies followed by disruption and re-accumulation into rubble-pile asteroids [1].
Methods: We developed a thermal evolution model incorporating accretion and internal heating. Mass growth was calculated from planetesimal surface density and collisional cross section [5], and temperature evolution was solved using a 1-D spherical heat conduction equation. The model includes radiogenic heating by 26Al, latent heat of ice melting, and serpentinization reaction heat, with impact heating and radiative cooling as surface boundary conditions. Lithologies were assigned from the peak temperature at each location (Table 1).
Results and Discussion: We varied bulk water/rock ratio (W/R), accretion onset time, and body size (Fig. 1). Small bodies (~10 km) experience insufficient 26Al heating and are dominated by low-temperature lithologies (I–II). Larger bodies (50–100 km) develop temperature gradients because early-accreted material is strongly heated while late-accreted material remains cooler, producing diverse lithologies (Fig. 2). Accretion timing is critical. Early accretion (~1 Myr after CAI formation) causes excessive heating and suppresses carbonates, producing Type VI. Late accretion (~3 Myr) yields insufficient heating and mostly anhydrous lithologies. Moderate onset (~1.5–2 Myr) allows diverse lithologies. Lower W/R enhances heating due to higher rock fraction, whereas higher W/R suppresses heating, and latent heat of ice melting further limits temperature rise.
Conclusions: A parent body with radius ~50–100 km, accretion onset ~1.5–2 Myr after CAI formation, and W/R ~ 0.6–0.9 well reproduces the lithology distribution of Ryugu and CI chondrites. Progressive accretion generates temperature differences between early and late materials, naturally producing internal heterogeneity. Volatile-rich planetesimals of this scale were likely common and later disrupted and re-accumulated to form rubble-pile asteroids (Fig. 3). Thus, this model provides a unified scenario linking accretion, heating, and collisional evolution for Ryugu- and CI-like bodies, and potentially Bennu.
[1] Nakamura T. et al. (2023), Science 379, 787.
[2] Iizuka, T. et al. (2025) Nature, 450, 525–528.
[3] Yokoyama T. et al., 2022, Science 379, 6634.
[4] Mikouchi, T. et al. (2025) 87th Meteoritical Society Meeting (MetSoc), #5211.
[5] Thommes, E. et al. (2001) Astrophysical Ages and Time Scales, ASP Conference Series, Vol. 245.
Methods: We developed a thermal evolution model incorporating accretion and internal heating. Mass growth was calculated from planetesimal surface density and collisional cross section [5], and temperature evolution was solved using a 1-D spherical heat conduction equation. The model includes radiogenic heating by 26Al, latent heat of ice melting, and serpentinization reaction heat, with impact heating and radiative cooling as surface boundary conditions. Lithologies were assigned from the peak temperature at each location (Table 1).
Results and Discussion: We varied bulk water/rock ratio (W/R), accretion onset time, and body size (Fig. 1). Small bodies (~10 km) experience insufficient 26Al heating and are dominated by low-temperature lithologies (I–II). Larger bodies (50–100 km) develop temperature gradients because early-accreted material is strongly heated while late-accreted material remains cooler, producing diverse lithologies (Fig. 2). Accretion timing is critical. Early accretion (~1 Myr after CAI formation) causes excessive heating and suppresses carbonates, producing Type VI. Late accretion (~3 Myr) yields insufficient heating and mostly anhydrous lithologies. Moderate onset (~1.5–2 Myr) allows diverse lithologies. Lower W/R enhances heating due to higher rock fraction, whereas higher W/R suppresses heating, and latent heat of ice melting further limits temperature rise.
Conclusions: A parent body with radius ~50–100 km, accretion onset ~1.5–2 Myr after CAI formation, and W/R ~ 0.6–0.9 well reproduces the lithology distribution of Ryugu and CI chondrites. Progressive accretion generates temperature differences between early and late materials, naturally producing internal heterogeneity. Volatile-rich planetesimals of this scale were likely common and later disrupted and re-accumulated to form rubble-pile asteroids (Fig. 3). Thus, this model provides a unified scenario linking accretion, heating, and collisional evolution for Ryugu- and CI-like bodies, and potentially Bennu.
[1] Nakamura T. et al. (2023), Science 379, 787.
[2] Iizuka, T. et al. (2025) Nature, 450, 525–528.
[3] Yokoyama T. et al., 2022, Science 379, 6634.
[4] Mikouchi, T. et al. (2025) 87th Meteoritical Society Meeting (MetSoc), #5211.
[5] Thommes, E. et al. (2001) Astrophysical Ages and Time Scales, ASP Conference Series, Vol. 245.