Asymmetric impacts of near-Earth asteroids on the Moon
Asymmetric impacts of near-Earth asteroids on the Moon
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DOI：
10.1051/0004-6361/200912901
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发表时间：
2009-07
影响因子：
6.5
通讯作者：
Takashi Ito;R. Malhotra
中科院分区：
文献类型：
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作者：
Takashi Ito;R. Malhotra
上下文最近对月球陨石坑的研究揭示了月球表面放射状陨石坑的不对称分布。这种不对称性与月球的同步旋转有关：与月球的后半球相比，前半球上的环形山密度更高。放射状陨石坑通常代表最年轻的撞击。目标。本文的目的是检验以下假设：（一）近地小行星群是造成射线状陨石坑的撞击物的来源;（二）这一抛射体群的撞击在数量上解释了观测到的不对称性。方法.我们对代表近地小行星的大量试验粒子的轨道演变进行了数值模拟，以便直接确定它们对月球的撞击通量。模拟分两个阶段进行。在第一阶段中，我们获得了近地小行星在地球活动范围内的遭遇统计数据。在第二阶段，我们计算了月球表面相遇粒子的直接撞击通量;后一阶段的计算被限制在地球活动范围内。根据已知近地小行星的去偏轨道分布产生了稳态合成近地小行星群。结果我们发现近地小行星撞击月球的流量确实存在不对称性：顶点与顶点之比为1.32 ± 0.01。然而，观察到的射线状陨石坑分布的不对称性明显更明显：顶点与前顶点的比率为1.65 ± 0.16。我们的研究结果表明，存在一个未被发现的人口较慢（低冲击速度）的弹丸，如人口的对象几乎与地球共轨;年轻的月球陨石坑需要更多的观测研究，以确保这一结论。
Context. Recent lunar crater studies have revealed an asymmetric distribution of rayed craters on the lunar surface. The asymmetry is related to the synchronous rotation of the Moon: there is a higher density of rayed craters on the leading hemisphere compared with the trailing hemisphere. Rayed craters represent generally the youngest impacts. Aims. The purpose of this paper is to test the hypotheses that (i) the population of Near-Earth asteroids (NEAs) is the source of the impactors that have made the rayed craters; and (ii) that impacts by this projectile population account quantitatively for the observed asymmetry. Methods. We carried out numerical simulations of the orbital evolution of a large number of test particles representing NEAs in order to determine directly their impact flux on the Moon. The simulations were done in two stages. In the first stage we obtained encounter statistics of NEAs on the Earth’s activity sphere. In the second stage we calculated the direct impact flux of the encountering particles on the surface of the Moon; the latter calculations were confined within the activity sphere of the Earth. A steady-state synthetic population of NEAs was generated from a debiased orbital distribution of the known NEAs. Results. We find that the near-Earth asteroids do have an asymmetry in their impact flux on the Moon: apex-to-antapex ratio of 1.32 ± 0.01. However, the observed rayed crater distribution’s asymmetry is significantly more pronounced: apex-to-antapex ratio of 1.65 ± 0.16. Our results suggest the existence of an undetected population of slower (low impact velocity) projectiles, such as a population of objects nearly coorbiting with Earth; more observational studies of young lunar craters is needed to secure this conclusion.