9:30 AM - 9:45 AM
[PPS03-03] Collisional probability between Bennu and the Vesta family in the inner main belt: Implication for the supply process of V-type boulders in the asteroid Bennu.
Keywords:Bennu, Collisional evolution, Vesta family, Exogenic boulders
Near-Earth asteroid Bennu contains bright boulders and some of them are spectrally like the main-belt asteroid Vesta (DellaGiustina+ 2021, Tatsumi+ 2021). On the other hand, no such boulders have been identified in Ryugu (Tatsumi+ 2021). They are thought to have originated from the collisions with asteroids from the Vesta family, but it is controversial when they were supplied. Investigating this process is crucial for understanding material mixing in the main belt. In this study, we aim to clarify the timing and mechanisms of Vesta-derived material delivery to Bennu by estimating the collision probability with the Vesta family.
There are two major hypotheses for the delivery of V-type boulders to Bennu: (1) catastrophic collision of Bennu's predecessor with a Vesta-family asteroid, or (2) cratering impact of Bennu's parent body with Vesta-family asteroids (DellaGiustina+ 2021). Given the relatively small number of Vesta-family asteroids (~1% in the main belt) and the low probability of the catastrophic collision, DellaGiustina+ 2021 suggested that scenario (2) is more likely than scenario (1). However, immediately following the formation of asteroid families, a large number of small asteroids are typically produced. If the Vesta family temporarily dominated the main belt in small diameters, the collisional frequency with Vesta-family could have been significantly higher, making scenario (1) more plausible than previously thought. To evaluate the feasibility of this scenario, we calculated the probability that Bennu's predecessors experienced catastrophic collisions with Vesta-family.
The probability was assessed using the following steps:
(1) Bennu is assumed to have originated from New Polana family. Its semi-major axis is shifted from the family center to ν6 resonance.
(2) The intrinsic collisional probabilities with Vesta family and other main belt asteroids are calculated at each orbit using the method of Wetherill (1967).
(3) The size frequency distribution (SFD) of Vesta family is assumed, and the collisional frequency with the asteroids capable of catastrophically disrupting Bennu's predecessor is estimated.
(4) A stochastic approach is applied, using a random number to determine whether catastrophic disruption occurs at each timestep based on the calculated frequency. If the disruption occurs, the fragments are generated.
(5) This processes is repeated, and only asteroids whose final diameter is approximately equal to that of Bennu are extracted.
(6) For each extracted asteroid, the occurrence of a catastrophic collision with a Vesta-family asteroid is examined, and the probability of Bennu's predecessor experiencing such an event is derived.
In step (3), we assumed the cumulative SFD of the fragments produced by the formation of Vesta family is consistent with the observation for > 1 km (Nesvorny+ 2008), and extrapolated with a slope of -3 for < 1 km. The SFD was also modeled to decrease over time, starting with the smaller-diameter asteroids, following the equilibrium SFD of the main belt (Bottke+ 2005).
Using the method described above, the result shows that the probability for Bennu's predecessors to have experienced catastrophic collisions with Vesta family asteroids is about 30%. This probability was initially expected to be low because the number of currently observed Vesta family accounts for only about 1% of the total number of main-belt asteroids (JPL Small-Body Database), but our result indicates that such collisions were still feasible. This is because the assumption that the slope of Vesta family's SFD was larger than that of main belt immediately after the formation of Vesta family, results in that the number of Vesta family asteroids in 10~100 m diameter which would destroy Bennu's predecessor is larger than that of the present main belt.
These results support the hypothesis that the V-type boulders were supplied to Bennu by the catastrophic collisions. Therefore, this possibility should be considered when discussing the supply process of V-type boulders. Also, comparing the collisional evolution of Ryugu and Bennu, the difference between them (V-type boulders have been found on Bennu but not on Ryugu) may reflect whether they have experienced the catastrophic collision with Vesta family.
There are two major hypotheses for the delivery of V-type boulders to Bennu: (1) catastrophic collision of Bennu's predecessor with a Vesta-family asteroid, or (2) cratering impact of Bennu's parent body with Vesta-family asteroids (DellaGiustina+ 2021). Given the relatively small number of Vesta-family asteroids (~1% in the main belt) and the low probability of the catastrophic collision, DellaGiustina+ 2021 suggested that scenario (2) is more likely than scenario (1). However, immediately following the formation of asteroid families, a large number of small asteroids are typically produced. If the Vesta family temporarily dominated the main belt in small diameters, the collisional frequency with Vesta-family could have been significantly higher, making scenario (1) more plausible than previously thought. To evaluate the feasibility of this scenario, we calculated the probability that Bennu's predecessors experienced catastrophic collisions with Vesta-family.
The probability was assessed using the following steps:
(1) Bennu is assumed to have originated from New Polana family. Its semi-major axis is shifted from the family center to ν6 resonance.
(2) The intrinsic collisional probabilities with Vesta family and other main belt asteroids are calculated at each orbit using the method of Wetherill (1967).
(3) The size frequency distribution (SFD) of Vesta family is assumed, and the collisional frequency with the asteroids capable of catastrophically disrupting Bennu's predecessor is estimated.
(4) A stochastic approach is applied, using a random number to determine whether catastrophic disruption occurs at each timestep based on the calculated frequency. If the disruption occurs, the fragments are generated.
(5) This processes is repeated, and only asteroids whose final diameter is approximately equal to that of Bennu are extracted.
(6) For each extracted asteroid, the occurrence of a catastrophic collision with a Vesta-family asteroid is examined, and the probability of Bennu's predecessor experiencing such an event is derived.
In step (3), we assumed the cumulative SFD of the fragments produced by the formation of Vesta family is consistent with the observation for > 1 km (Nesvorny+ 2008), and extrapolated with a slope of -3 for < 1 km. The SFD was also modeled to decrease over time, starting with the smaller-diameter asteroids, following the equilibrium SFD of the main belt (Bottke+ 2005).
Using the method described above, the result shows that the probability for Bennu's predecessors to have experienced catastrophic collisions with Vesta family asteroids is about 30%. This probability was initially expected to be low because the number of currently observed Vesta family accounts for only about 1% of the total number of main-belt asteroids (JPL Small-Body Database), but our result indicates that such collisions were still feasible. This is because the assumption that the slope of Vesta family's SFD was larger than that of main belt immediately after the formation of Vesta family, results in that the number of Vesta family asteroids in 10~100 m diameter which would destroy Bennu's predecessor is larger than that of the present main belt.
These results support the hypothesis that the V-type boulders were supplied to Bennu by the catastrophic collisions. Therefore, this possibility should be considered when discussing the supply process of V-type boulders. Also, comparing the collisional evolution of Ryugu and Bennu, the difference between them (V-type boulders have been found on Bennu but not on Ryugu) may reflect whether they have experienced the catastrophic collision with Vesta family.