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[PPS03-P03] Icy Pebble Accretion and the Origin of Volatile-Rich Asteroids in the Main Belt
Keywords:Asteroids, Protoplanetary disk, Planet formation, Early Solar System
Among the known asteroids, the main belt asteroids exhibit a radial distribution of surface composition1, ranging from anhydrous bodies to those rich in hydrated minerals, ice, and even ammonia2. This compositional trend is most pronounced in the largest asteroids (D > 120 km)3, which are believed to be primordial with their properties likely determined during the accretion epoch4. Their ancient origins raise fundamental questions about early Solar System evolution and planet formation. A widely accepted hypothesis suggests that they may have originated in the outer Solar System, accreted in a cold, distant reservoir, and were later implanted into the main belt during giant planet migration after the protosolar disk dissipation5,6. Alternatively, volatiles may have migrated inward in the disk through pebble drift, a process where small, icy pebbles drift inward from the outer Solar System and accrete onto forming planetesimals in the main belt7,8. The latter scenario has not been quantitatively tested in detail. However, recent advances in asteroid observations enable us to verify theoretical predictions2,9. In this study, we test this scenario in which rocky asteroids formed in situ and accumulated volatiles via icy pebble accretion as H2O and NH3 snowlines migrated inward.
Using a simplified protoplanetary-disk model to describe radially-inward drift with an assumed snowline migration scenario10, we treat the turbulence strength of the disk, radial pebble flux, and pebble size (characterized by the dimensionless stopping time: Stokes number, St) as free parameters. We employ analytical expressions to calculate pebble accretion on rocky planetesimals11,12. The model results are compared to topographic (the minimum thickness of the volatile-containing later) and mass constraints for bodies in the main belt derived from observations2.
Results indicate that volatile delivery to asteroids via icy pebble accretion requires a moderate pebble flux (<20 MEarth /Myr). Water accretion requires small pebbles with St<10-3 (<1 mm), and ammonia accretion necessitates solids with St~10-5 (~10 μm) . However, such small particle sizes may be challenging, as the slow radial drift with small St in the model requires a high dust-to-gas surface density ratio (~1), and is inconsistent with observations of protoplanetary disks, which reveal that the disks contain larger dust grains ( >10 μm)13. If we adopt St>=10-2 which is commonly used in pebble accretion models for forming other planetary bodies12, then most asteroids with diameters of 100–200 km can only accumulate a thin veneer of icy pebbles (less than 1 km thick), which is insufficient to cover its surface.
References:
1. Gradie, J. & Tedesco, E. Compositional Structure of the Asteroid Belt. Science 216, 1405–1407 (1982).
2. Rivkin, A. S. et al. The Nature of Low-albedo Small Bodies from 3 μm Spectroscopy: One Group that Formed within the Ammonia Snow Line and One that Formed beyond It. Planet. Sci. J. 3, 153 (2022).
3. DeMeo, F. E. & Carry, B. Solar System evolution from compositional mapping of the asteroid belt. Nature 505, 629–634 (2014).
4. Bottke, W. F. et al. The fossilized size distribution of the main asteroid belt. Icarus 175, 111–140 (2005).
5. Hopp, T. et al. Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Sci. Adv. 8, eadd8141 (2022).
6. Takir, D., Neumann, W., Raymond, S. N., Emery, J. P. & Trieloff, M. Late accretion of Ceres-like asteroids and their implantation into the outer main belt. Nat. Astron. 7, 524–533 (2023).
7. Nara, Y., Okuzumi, S. & Kurokawa, H. Delivery of ammonia ice to Ceres by pebble accretion. AASDivision Extreme Sol. Syst. Abstr. 51, 317.19 (2019).
8. De Sanctis, M. C. et al. Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres. Nature 528, 241–244 (2015).
9. Usui, F., Hasegawa, S., Ootsubo, T. & Onaka, T. AKARI/IRC near-infrared asteroid spectroscopic survey: AcuA-spec. Publ. Astron. Soc. Jpn. 71, 1 (2019).
10. Oka, A., Nakamoto, T. & Ida, S. EVOLUTION OF SNOW LINE IN OPTICALLY THICK PROTOPLANETARY DISKS: EFFECTS OF WATER ICE OPACITY AND DUST GRAIN SIZE. Astrophys. J. 738, 141 (2011).
11. Visser, R. G. & Ormel, C. W. On the growth of pebble-accreting planetesimals. Astron. Astrophys. 586, A66 (2016).
12. Ormel, C. W. The Emerging Paradigm of Pebble Accretion. in Formation, Evolution, and Dynamics of Young Solar Systems 197–228 (Springer International Publishing, Cham, 2017). doi:10.1007/978-3-319-60609-5.
13. Andrews, S. M. Observations of Protoplanetary Disk Structures. Annu. Rev. Astron. Astrophys. 58, 483–528 (2020).
Using a simplified protoplanetary-disk model to describe radially-inward drift with an assumed snowline migration scenario10, we treat the turbulence strength of the disk, radial pebble flux, and pebble size (characterized by the dimensionless stopping time: Stokes number, St) as free parameters. We employ analytical expressions to calculate pebble accretion on rocky planetesimals11,12. The model results are compared to topographic (the minimum thickness of the volatile-containing later) and mass constraints for bodies in the main belt derived from observations2.
Results indicate that volatile delivery to asteroids via icy pebble accretion requires a moderate pebble flux (<20 MEarth /Myr). Water accretion requires small pebbles with St<10-3 (<1 mm), and ammonia accretion necessitates solids with St~10-5 (~10 μm) . However, such small particle sizes may be challenging, as the slow radial drift with small St in the model requires a high dust-to-gas surface density ratio (~1), and is inconsistent with observations of protoplanetary disks, which reveal that the disks contain larger dust grains ( >10 μm)13. If we adopt St>=10-2 which is commonly used in pebble accretion models for forming other planetary bodies12, then most asteroids with diameters of 100–200 km can only accumulate a thin veneer of icy pebbles (less than 1 km thick), which is insufficient to cover its surface.
References:
1. Gradie, J. & Tedesco, E. Compositional Structure of the Asteroid Belt. Science 216, 1405–1407 (1982).
2. Rivkin, A. S. et al. The Nature of Low-albedo Small Bodies from 3 μm Spectroscopy: One Group that Formed within the Ammonia Snow Line and One that Formed beyond It. Planet. Sci. J. 3, 153 (2022).
3. DeMeo, F. E. & Carry, B. Solar System evolution from compositional mapping of the asteroid belt. Nature 505, 629–634 (2014).
4. Bottke, W. F. et al. The fossilized size distribution of the main asteroid belt. Icarus 175, 111–140 (2005).
5. Hopp, T. et al. Ryugu’s nucleosynthetic heritage from the outskirts of the Solar System. Sci. Adv. 8, eadd8141 (2022).
6. Takir, D., Neumann, W., Raymond, S. N., Emery, J. P. & Trieloff, M. Late accretion of Ceres-like asteroids and their implantation into the outer main belt. Nat. Astron. 7, 524–533 (2023).
7. Nara, Y., Okuzumi, S. & Kurokawa, H. Delivery of ammonia ice to Ceres by pebble accretion. AASDivision Extreme Sol. Syst. Abstr. 51, 317.19 (2019).
8. De Sanctis, M. C. et al. Ammoniated phyllosilicates with a likely outer Solar System origin on (1) Ceres. Nature 528, 241–244 (2015).
9. Usui, F., Hasegawa, S., Ootsubo, T. & Onaka, T. AKARI/IRC near-infrared asteroid spectroscopic survey: AcuA-spec. Publ. Astron. Soc. Jpn. 71, 1 (2019).
10. Oka, A., Nakamoto, T. & Ida, S. EVOLUTION OF SNOW LINE IN OPTICALLY THICK PROTOPLANETARY DISKS: EFFECTS OF WATER ICE OPACITY AND DUST GRAIN SIZE. Astrophys. J. 738, 141 (2011).
11. Visser, R. G. & Ormel, C. W. On the growth of pebble-accreting planetesimals. Astron. Astrophys. 586, A66 (2016).
12. Ormel, C. W. The Emerging Paradigm of Pebble Accretion. in Formation, Evolution, and Dynamics of Young Solar Systems 197–228 (Springer International Publishing, Cham, 2017). doi:10.1007/978-3-319-60609-5.
13. Andrews, S. M. Observations of Protoplanetary Disk Structures. Annu. Rev. Astron. Astrophys. 58, 483–528 (2020).