The dynamical evolution of lunar impact ejecta
The dynamical evolution of lunar impact ejecta
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DOI：
10.1006/icar.1995.1193
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发表时间：
1995-12-01
期刊：
影响因子：
3.2
通讯作者：
Levison, HF
中科院分区：
文献类型：
--
作者：
Gladman, BJ;Burns, JA;Levison, HF
已知月球陨石数量的增加清楚地表明，月球撞击抛射物向地球的传递比以前认为的要常见得多。为了更好地了解传递机制的时间尺度，更好地约束发射环境，我们对撞击事件期间从月球表面发射的物质的动态演化进行了一系列数值模拟。发射速度选择在2.3到3.5公里/秒之间，因为2.38公里/秒是从月球逃逸的正式速度。在我们研究的第一阶段，我们将物理学建模为一个由太阳、地球、月球和撞击碎片组成的四体问题，跟踪粒子直到它撞击地球或月球，或者它逃离地月系统进入日心轨道。在这个阶段，撞击地球或月球的物质的比例是初始喷射速度的一个强函数。我们模拟的第二阶段跟随逃逸粒子群在类地行星区域的后续演化。在这个阶段，我们包括了所有行星到土星的引力效应；这些粒子虽然彼此之间没有相互作用，但在长达1000万年的时间里，它们是用全n体处理进化而来的。尽管在一些细节上有所不同，但我们的结果证实了以前使用蒙特卡罗方法的几个计算结果，这些计算结果显示了地球喷射出的许多粒子的快速（< 1 Myr）吸积。我们计算出，大约三分之一的喷射物质会迅速到达地球；事实上，很大一部分最缓慢喷射的物质在不到10kyr的时间内返回。当这些粒子继续从地球的引力场中散射出去时，它们的偏心率和倾斜度会上升，而它们的半长轴会扩散，直到它们开始穿过其他类地行星的轨道，与金星的碰撞变得很常见。在大约1myr之后，粒子进入平衡状态，大致均匀地分布在整个太阳系内部，由于行星的持续吸积和被驱使到太阳掠食状态，它们的数量缓慢下降。我们将返回地球的模拟粒子的年龄谱与来自月球陨石的可用数据进行比较。我们得出结论，逃逸的月球陨石坑喷出物的速度分布（数量与发射速度）必须足够陡峭，以至于很少有粒子以超过3.0 km/s的速度发射。我们还表明，返回的物体是均匀地在地球表面传递的。(C) 1995学术出版社，Inc。
The increasing numbers of known lunar meteorites make it clear that the delivery of lunar impact ejecta to the Earth is an occurrence much more common than previously thought, To better understand the time scales of the delivery mechanism and to better constrain the launch circumstances, we have conducted a series of numerical simulations of the dynamical evolution of material that is launched off the lunar surface during impact events. Launch velocities were chosen between 2.3 and 3.5 km/sec, since 2.38 km/sec is the formal escape speed from the Moon. During the first stage of our study we model the physics as a four-body problem consisting of the Sun, Earth, Moon, and impact fragment, The particle is followed until it impacts the Earth or Moon, or it escapes the Earth-Moon system into heliocentric orbit. The fraction of material that strikes the Earth or Moon in this stage is a strong function of the initial ejection velocity, The second stage of our simulation follows the swarm of escaping particles during their subsequent evolution in the terrestrial planet region. During this stage we include the gravitational effects of all the planets out to Saturn; the particles, although not interacting with each other, are evolved using a full N-body treatment for up to 10 million years, Although differing in some details, our results confirm several previous calculations, which used Monte Carlo methods and showed a rapid ( < 1 Myr) accretion of many of the ejected particles by the Earth. We calculate that about one-third of the ejected material reaches the Earth rapidly; in fact, a very large fraction of the most slowly ejected material returns in less than 10 kyr. As the particles continue to scatter off the gravitational field of the Earth, their eccentricities and inclina-tions rise while their semimajor axes spread until they begin to cross the orbits of other terrestrial planets, Collisions with Venus become common. After about 1 Myr, the particles then settle into an equilibrium state, distributed roughly uniformly throughout the inner solar system, with their numbers slowly declining due to continued accretion by the planets and by being driven to a Sun-grazing state. We compare the age spectrum of the simulated particles that return to reimpact the Earth with the available data from the lunar meteorites. We conclude that the velocity distribution (in number versus launch speed) of the escaping lunar crater ejecta must be steep enough that few particles are launched with speeds greater than 3.0 km/sec. We also show that the returning objects are delivered uniformly over the surface of the Earth. (C) 1995 Academic Press, Inc.