11:00 AM - 11:15 AM
[PPS06-12] Growth and Thermal Evolution of Icy Planetesimals
Keywords:planetesimal, icy body, evolution
Recent studies suggest that the thermal evolution of icy planetesimals was highly diverse. For instance, samples obtained by the asteroid explorer Hayabusa2 indicate that aqueous alteration occurred at a low temperature of approximately 300 K and that Ryugu's material is compositionally homogeneous. This suggests that Ryugu's parent body was a low-temperature, undifferentiated icy planetesimal. In contrast, iron meteorites with isotopic compositions similar to those of carbonaceous chondrites have also been identified. Since the parent bodies of carbonaceous chondrites are thought to be icy planetesimals, the existence of such iron meteorites suggests that some icy planetesimals experienced internal heating beyond the melting point of metal. These findings indicate that icy planetesimals followed thermally diverse evolutionary pathways; however, a comprehensive theoretical explanation remains lacking. The evolution of icy planetesimals involves a complex interplay of various physical and chemical processes. These include the coexistence of rock and water, leading to variations in material properties; the high mobility of hydrous minerals, which may facilitate convective heat transport; phase transitions such as ice melting, aqueous alteration, and dehydration reactions; and the formation of internal structures driven by accretional growth and impact heating. Despite this complexity, previous numerical studies on the thermal evolution of icy planetesimals have only partially incorporated these fundamental processes.
In this study, we develop a new numerical model that accounts for all these processes and simulate the internal thermal evolution of an icy planetesimal that accretes from an initial state of 70 K and a radius of 1 km. The simulations track the evolution over 100 million years following the formation of calcium-aluminum-rich inclusions (CAIs). We systematically vary parameters such as final radius (10–1000 km), timing of accretion onset (1.0 or 2.0 Myr after CAI formation), accretion duration (0.4 or 4.0 Myr), and accretion mode (β = 0 or 2) to examine differences in internal temperature distribution and peak temperature. The results reveal that while larger final radii generally lead to higher internal temperatures, the heating history strongly depends on accretion characteristics. For example, rapid early accretion results in significant heating and diverse internal structural transformations. Specifically, a planetesimal that begins accretion 1.0 Myr after CAI formation, undergoes linear growth (β = 0) over 0.4 Myr, and reaches a final radius of 100 km can attain a core temperature exceeding the Fe-FeS eutectic melting point (~1,250 K). In contrast, a planetesimal with a final radius of only 10 km remains below 400 K. If the same accretion parameters are applied to a runaway growth mode (β = 2), the maximum temperature for a 100 km body is limited to 800 K. Delaying the accretion onset to 2.0 Myr after CAI formation results in a lower peak temperature of 600 K for a 100 km planetesimal. When the accretion duration is extended to 4.0 Myr, only planetesimals exceeding 500 km in radius reach core temperatures above 1,000 K, while impact heating effects become apparent near the surface. These results suggest that if the timescale for substantial growth is too long, the accumulation of isotopically depleted materials leads to insufficient internal heating.
Overall, our findings demonstrate that the thermal evolution of icy planetesimals is highly sensitive to variations in final radius, accretion timing, duration, and mode. By comparing our results with observational data from sample analyses and asteroid missions, we infer that a planetesimal that began accretion 1.0 Myr after CAI formation, underwent linear growth over 4.0 Myr, and reached a radius of 50 km likely evolved into the parent body of Ryugu. In contrast, a planetesimal that accreted over 0.4 Myr and grew beyond 200 km may have become the parent body of iron meteorites. Furthermore, a body that accreted 1.0 Myr after CAI formation and grew to a present-day radius of 250 km via either linear or runaway growth over 0.4 Myr, or a body that accreted 2.0 Myr after CAI formation and reached the same size through runaway growth, exhibits an internal structure similar to that of Saturn's moon Enceladus. This study provides a unified explanation for the thermal evolution of icy planetesimals and offers predictions that can be tested through direct exploration. Future comparisons with observational data from NASA's Lucy mission, which is set to explore Jupiter Trojans, will further refine our understanding of the thermal history of icy planetesimals.
In this study, we develop a new numerical model that accounts for all these processes and simulate the internal thermal evolution of an icy planetesimal that accretes from an initial state of 70 K and a radius of 1 km. The simulations track the evolution over 100 million years following the formation of calcium-aluminum-rich inclusions (CAIs). We systematically vary parameters such as final radius (10–1000 km), timing of accretion onset (1.0 or 2.0 Myr after CAI formation), accretion duration (0.4 or 4.0 Myr), and accretion mode (β = 0 or 2) to examine differences in internal temperature distribution and peak temperature. The results reveal that while larger final radii generally lead to higher internal temperatures, the heating history strongly depends on accretion characteristics. For example, rapid early accretion results in significant heating and diverse internal structural transformations. Specifically, a planetesimal that begins accretion 1.0 Myr after CAI formation, undergoes linear growth (β = 0) over 0.4 Myr, and reaches a final radius of 100 km can attain a core temperature exceeding the Fe-FeS eutectic melting point (~1,250 K). In contrast, a planetesimal with a final radius of only 10 km remains below 400 K. If the same accretion parameters are applied to a runaway growth mode (β = 2), the maximum temperature for a 100 km body is limited to 800 K. Delaying the accretion onset to 2.0 Myr after CAI formation results in a lower peak temperature of 600 K for a 100 km planetesimal. When the accretion duration is extended to 4.0 Myr, only planetesimals exceeding 500 km in radius reach core temperatures above 1,000 K, while impact heating effects become apparent near the surface. These results suggest that if the timescale for substantial growth is too long, the accumulation of isotopically depleted materials leads to insufficient internal heating.
Overall, our findings demonstrate that the thermal evolution of icy planetesimals is highly sensitive to variations in final radius, accretion timing, duration, and mode. By comparing our results with observational data from sample analyses and asteroid missions, we infer that a planetesimal that began accretion 1.0 Myr after CAI formation, underwent linear growth over 4.0 Myr, and reached a radius of 50 km likely evolved into the parent body of Ryugu. In contrast, a planetesimal that accreted over 0.4 Myr and grew beyond 200 km may have become the parent body of iron meteorites. Furthermore, a body that accreted 1.0 Myr after CAI formation and grew to a present-day radius of 250 km via either linear or runaway growth over 0.4 Myr, or a body that accreted 2.0 Myr after CAI formation and reached the same size through runaway growth, exhibits an internal structure similar to that of Saturn's moon Enceladus. This study provides a unified explanation for the thermal evolution of icy planetesimals and offers predictions that can be tested through direct exploration. Future comparisons with observational data from NASA's Lucy mission, which is set to explore Jupiter Trojans, will further refine our understanding of the thermal history of icy planetesimals.