12:00 PM - 12:15 PM
[PPS03-12] Investigation of planetesimal formation and migration explaining the isotopic distribution in the Solar System
Keywords:Asteroid Ryugu, Isotopic trichotomy of Solar System materials, Planetary growth, Gravitational scattering
Dust particles and chronological information preserved in meteorites and Ryugu and Bennu samples from the early Solar System are not consistent with current planetary formation theories.
First, there is the CAI storage problem. There is an approximately 2 Myr age difference between the formations of CAIs and chondrules, whereas these components are mechanically well-mixed down to scales of a few centimeters. Theoretically, dust aggregates of ~1 mm diameter in the protoplanetary disk gas are expected to fall toward the Sun within a few thousand years. Therefore, without mechanisms such as pressure bumps to retain dust in specific regions of the disk, it would be difficult to keep CAIs in particle form for >1 Myr. Scenarios where dust forms planetesimals before falling, however, raise another question: if chondrules accumulate later on planetesimals containing CAIs, can the two components mix homogeneously?
Another issue is the isotopic trichotomy observed in Solar System materials. Specifically, the parent bodies of meteorites, Ryugu, and Bennu are systematically divided into three distinct isotopic sources: NC, CC, and CI. Explaining this phenomenon poses a significant challenge. Theoretically, during the growth phase, dust particles migrate radially across large distances within the protoplanetary disk. Even if there were spatial isotopic gradients in the early disk, mechanical mixing would likely progress significantly before these particles accumulated into parent planetesimals.
One hypothesis suggests that multiple pressure bumps, created by the formation of giant gas planets, acted as reservoirs for NC, CC, and CI materials, respectively. However, this raises the issue of timing in gas planet formation. Theoretically, once planetesimals form, planetary growth progresses relatively quickly, with Jupiter expected to form within 1 Myr (Kobayashi & Tanaka, 2023). If this is the case, the resultant early dissipation of the disk gas would conflict with evidence that magnetite grains, which precipitated during aqueous alteration in planetesimals ~4 Myr after CAI formation, record strong magnetic fields thought to originate from the gas disk (Nakamura et al., 2021). If Jupiter's formation was delayed, it must be evaluated whether the NC-CC isotopic distinction found in iron meteorites formed within 1 Myr of Solar System formation can still be explained.
Understanding planetesimal formation is key to addressing these issues. The growth from dust to planetesimals is a bottleneck in planetary formation theory, with no current models accounting for droplet particles like CAIs and chondrules. It is necessary to explore accretion processes involving both small matrix grains and larger droplet particles. While droplet particles hinder planetesimal formation due to their inability to grow via simple accretion, they aid in long-term retention and can contribute to growth after planetesimals form. Comparing these results with meteorite parent bodies may help resolve the CAI storage problem and the origins of isotopic trichotomy.
Numerical simulations track dust growth and infall to evaluate the homogenization of initial isotopic distributions. By assigning isotopic ratio indices to dust based on their radial positions and calculating their growth and migration, the average indices of planetesimals in each region are determined. Simulations are compared for disks with and without pressure bumps, and the isotopic ratios are analyzed based on body mass.
Furthermore, the mass distribution of dust accumulated by planetesimals and the gravitational scattering processes of planetesimals caused by the outward migration of giant planets analyzed through orbital calculations, allowing an evaluation of the conditions that eventually brought planetesimals to the asteroid belt. The development of accretion growth simulation methods capable of handling not only aggregates of submicron grains but also mm-sized droplet particles is also discussed.
First, there is the CAI storage problem. There is an approximately 2 Myr age difference between the formations of CAIs and chondrules, whereas these components are mechanically well-mixed down to scales of a few centimeters. Theoretically, dust aggregates of ~1 mm diameter in the protoplanetary disk gas are expected to fall toward the Sun within a few thousand years. Therefore, without mechanisms such as pressure bumps to retain dust in specific regions of the disk, it would be difficult to keep CAIs in particle form for >1 Myr. Scenarios where dust forms planetesimals before falling, however, raise another question: if chondrules accumulate later on planetesimals containing CAIs, can the two components mix homogeneously?
Another issue is the isotopic trichotomy observed in Solar System materials. Specifically, the parent bodies of meteorites, Ryugu, and Bennu are systematically divided into three distinct isotopic sources: NC, CC, and CI. Explaining this phenomenon poses a significant challenge. Theoretically, during the growth phase, dust particles migrate radially across large distances within the protoplanetary disk. Even if there were spatial isotopic gradients in the early disk, mechanical mixing would likely progress significantly before these particles accumulated into parent planetesimals.
One hypothesis suggests that multiple pressure bumps, created by the formation of giant gas planets, acted as reservoirs for NC, CC, and CI materials, respectively. However, this raises the issue of timing in gas planet formation. Theoretically, once planetesimals form, planetary growth progresses relatively quickly, with Jupiter expected to form within 1 Myr (Kobayashi & Tanaka, 2023). If this is the case, the resultant early dissipation of the disk gas would conflict with evidence that magnetite grains, which precipitated during aqueous alteration in planetesimals ~4 Myr after CAI formation, record strong magnetic fields thought to originate from the gas disk (Nakamura et al., 2021). If Jupiter's formation was delayed, it must be evaluated whether the NC-CC isotopic distinction found in iron meteorites formed within 1 Myr of Solar System formation can still be explained.
Understanding planetesimal formation is key to addressing these issues. The growth from dust to planetesimals is a bottleneck in planetary formation theory, with no current models accounting for droplet particles like CAIs and chondrules. It is necessary to explore accretion processes involving both small matrix grains and larger droplet particles. While droplet particles hinder planetesimal formation due to their inability to grow via simple accretion, they aid in long-term retention and can contribute to growth after planetesimals form. Comparing these results with meteorite parent bodies may help resolve the CAI storage problem and the origins of isotopic trichotomy.
Numerical simulations track dust growth and infall to evaluate the homogenization of initial isotopic distributions. By assigning isotopic ratio indices to dust based on their radial positions and calculating their growth and migration, the average indices of planetesimals in each region are determined. Simulations are compared for disks with and without pressure bumps, and the isotopic ratios are analyzed based on body mass.
Furthermore, the mass distribution of dust accumulated by planetesimals and the gravitational scattering processes of planetesimals caused by the outward migration of giant planets analyzed through orbital calculations, allowing an evaluation of the conditions that eventually brought planetesimals to the asteroid belt. The development of accretion growth simulation methods capable of handling not only aggregates of submicron grains but also mm-sized droplet particles is also discussed.