11:00 AM - 11:15 AM
[PPS03-07] Collisional evolution of asteroid Ryugu in the main-belt
Keywords:Ryugu, Asteroid family, Collisional evolution, Crater chronology
The near-Earth asteroid Ryugu, which was explored by Hayabusa2, is thought to have originated from the fragments formed by the destruction of its parent body in the inner main belt and thrown into the near-Earth orbit after its orbit was disturbed by the ν6 resonance. In this study, considering the orbital evolution from the orbit of the parent body to the near-Earth orbit of Ryugu we calculated the collision frequency of Ryugu in each orbit. Based on the results, we examined how the collision frequency changed with the orbital change and where the impact bodies came from.
Ryugu is thought to be originated from New Polana or Eulalia family (Bottke et al., 2015, Sugita et al., 2019). Because the orbital element of Ryugu was close to these families when Ryugu was in the main-belt, it is expected that Ryugu would collide with them more frequently than other main-belt asteroids. Then, the collisions with these families may supply Ryugu with rocks that formed at different depths in the parent body of Ryugu or rocks from the other parent body. Rocks such as Bright boulders (Tatsumi et al., 2021), which spectrums are different from their surroundings in Ryugu, may be derived from them. However, no studies have quantitatively estimated the collision frequency with New Polana or Eulalia family. In this study, we evaluate the possibility that Ryugu contains the rocks with different formation processes by calculating the frequency of collisions with the families and the resulting destruction of Ryugu.
Additionally, a previous studies have estimated the surface age of Ryugu using the crater chronology model constructed based on the average collision frequency in the main belt (Sugita et al., 2019, Morota et al., Cho et al., 2021). However, as mentioned above, the collision frequency of Ryugu may increase due to collisions with the asteroid families, and in the “Transition orbit”, in which eccentricity was high after disturbed by the ν6 resonance, the collision frequency also increases. Therefore, we attempted to estimate the more accurate surface age by calculating the collision and crater frequency in each orbit.
We numerically calculated the “intrinsic collision probabilities” (Wetherill, 1967) in the main belt, transition orbit, and near-Earth orbit, and estimated the collision frequency with bodies larger than a certain diameter by using the size-frequency distribution of asteroids (Bottke et al., 2005). As a result, it was found that the crater frequency is about twice as high in the transition orbit as in the main belt. Furthermore, about 40% of the bodies impacting Ryugu in the main belt came from New Polana or Eulalia family. Based on the calculated cratering frequency the surface age of Ryugu is estimated to be 4.2 to 6.6 million years, slightly younger than previous estimates (8.9 ± 2.5 million years) (Sugita et al., 2019).
The destruction of Ryugu by collisions with the asteroid families was calculated to be occurred about once every 4 billion years with New Polana family and about once every 1 billion years with Eulalia family. Based on these calculations, the probability that the Ryugu experienced the collisional destruction with the asteroid families is calculated to be more than 20% with New Polana family and more than 60% with Eulalia family. This probability could be larger when assuming that Ryugu is a rubble pile body and its intensity is small, so it is quite possible that Ryugu has experienced the collisional destruction with the asteroid families. This also suggests that Ryugu contains rocks that formed at different depths in the parent body of Ryugu or rocks from the other parent body.
Ryugu is thought to be originated from New Polana or Eulalia family (Bottke et al., 2015, Sugita et al., 2019). Because the orbital element of Ryugu was close to these families when Ryugu was in the main-belt, it is expected that Ryugu would collide with them more frequently than other main-belt asteroids. Then, the collisions with these families may supply Ryugu with rocks that formed at different depths in the parent body of Ryugu or rocks from the other parent body. Rocks such as Bright boulders (Tatsumi et al., 2021), which spectrums are different from their surroundings in Ryugu, may be derived from them. However, no studies have quantitatively estimated the collision frequency with New Polana or Eulalia family. In this study, we evaluate the possibility that Ryugu contains the rocks with different formation processes by calculating the frequency of collisions with the families and the resulting destruction of Ryugu.
Additionally, a previous studies have estimated the surface age of Ryugu using the crater chronology model constructed based on the average collision frequency in the main belt (Sugita et al., 2019, Morota et al., Cho et al., 2021). However, as mentioned above, the collision frequency of Ryugu may increase due to collisions with the asteroid families, and in the “Transition orbit”, in which eccentricity was high after disturbed by the ν6 resonance, the collision frequency also increases. Therefore, we attempted to estimate the more accurate surface age by calculating the collision and crater frequency in each orbit.
We numerically calculated the “intrinsic collision probabilities” (Wetherill, 1967) in the main belt, transition orbit, and near-Earth orbit, and estimated the collision frequency with bodies larger than a certain diameter by using the size-frequency distribution of asteroids (Bottke et al., 2005). As a result, it was found that the crater frequency is about twice as high in the transition orbit as in the main belt. Furthermore, about 40% of the bodies impacting Ryugu in the main belt came from New Polana or Eulalia family. Based on the calculated cratering frequency the surface age of Ryugu is estimated to be 4.2 to 6.6 million years, slightly younger than previous estimates (8.9 ± 2.5 million years) (Sugita et al., 2019).
The destruction of Ryugu by collisions with the asteroid families was calculated to be occurred about once every 4 billion years with New Polana family and about once every 1 billion years with Eulalia family. Based on these calculations, the probability that the Ryugu experienced the collisional destruction with the asteroid families is calculated to be more than 20% with New Polana family and more than 60% with Eulalia family. This probability could be larger when assuming that Ryugu is a rubble pile body and its intensity is small, so it is quite possible that Ryugu has experienced the collisional destruction with the asteroid families. This also suggests that Ryugu contains rocks that formed at different depths in the parent body of Ryugu or rocks from the other parent body.