講演情報
[PPS04-P26]Dynamical Evolution and Accretion of Ejecta by the Moon-forming Impact
*松本 侑士1、黒澤 耕介2,3、伊藤 孝士4、小久保 英一郎4 (1.神戸大学大学院理学研究科、2.神戸大学大学院人間発達環境学研究科、3.千葉工業大学惑星探査研究センター、4.国立天文台天文シミュレーションプロジェクト)
キーワード：
天体力学、惑星表面、クレータ
The Moon-forming impact is the most recent known giant impact in the terrestrial planet region and is expected to have generated ejecta with a total mass comparable to that of the Moon. This ejecta is subsequently accreted by the planets and the Moon, playing an important role in shaping planetary surfaces through impact cratering.
The accretion rates of the ejecta are expected to depend on their initial velocities, because debris with higher relative velocities with respect to Earth has a lower probability of being accreted by Earth and can acquire high eccentricities, allowing it to become orbit-crossing with other planets. However, the velocity distribution of the ejecta depends on the Moon-forming impact model, which remains poorly constrained. In addition, although previous studies have investigated ejecta accretion, Mercury and the Moon—both of which have short orbital periods—have not been explicitly considered.
We investigate the dynamical evolution and accretion of ejecta produced by the Moon-forming impact using N-body simulations over 100 Myr that include the planets of the solar system and the Moon. We find that low–initial-velocity ejecta is preferentially accreted by Earth. The accretion fractions onto Mercury and the Moon are typically 2.4% and 0.87%, respectively, and these fractions tend to increase with increasing initial ejecta velocity. For Mercury, this trend arises because high–initial-velocity ejecta is more strongly scattered and can reach Mercury’s orbit. For the Moon, higher relative velocities of the ejecta with respect to the Earth–Moon system increase the Moon-to-Earth impact ratio owing to weaker gravitational focusing by Earth.
The accretion rates of the ejecta are expected to depend on their initial velocities, because debris with higher relative velocities with respect to Earth has a lower probability of being accreted by Earth and can acquire high eccentricities, allowing it to become orbit-crossing with other planets. However, the velocity distribution of the ejecta depends on the Moon-forming impact model, which remains poorly constrained. In addition, although previous studies have investigated ejecta accretion, Mercury and the Moon—both of which have short orbital periods—have not been explicitly considered.
We investigate the dynamical evolution and accretion of ejecta produced by the Moon-forming impact using N-body simulations over 100 Myr that include the planets of the solar system and the Moon. We find that low–initial-velocity ejecta is preferentially accreted by Earth. The accretion fractions onto Mercury and the Moon are typically 2.4% and 0.87%, respectively, and these fractions tend to increase with increasing initial ejecta velocity. For Mercury, this trend arises because high–initial-velocity ejecta is more strongly scattered and can reach Mercury’s orbit. For the Moon, higher relative velocities of the ejecta with respect to the Earth–Moon system increase the Moon-to-Earth impact ratio owing to weaker gravitational focusing by Earth.