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arXiv 1804.04772astro-ph.EP
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再撞击碎片对月球岩浆海凝固的影响

Effect of Re-impacting Debris on the Solidification of the Lunar Magma Ocean

Viranga Perera, Alan P. Jackson, Linda T. Elkins-Tanton, Erik Asphaug

摘要:

斜长岩构成了月球地壳的大部分,被认为是在月球岩浆海(LMO)凝固过程中形成的,这些岩石可能会漂浮到月球表面。这些早期的漂浮外壳会在剩余的LMO上形成一个热毯,延长凝固时间。月球斜长岩的地质年代学表明,LMO冷却,或在一个或多个晚期事件的重熔和再结晶的时间尺度很长。为了更好地解释这一地质年代学,我们模拟了LMO固化的情况下,月球正在不断轰击返回的弹丸释放出的月球形成的巨大的影响。在巨大的撞击之后,超过一个月球质量的物质逃离了地月系统进入日心轨道,其中大部分在返回轨道上返回了1亿年。如果足够大的话,这些射弹会在月球新生的漂浮地壳上刺穿一些洞,使LMO暴露在太空中,并导致更快的冷却。我们使用月球的热演化模型来模拟这些场景,该模型允许在隔离LMO的漂浮地壳中产生(通过撞击坑)和演化(固化和填充)孔。为了有效地产生孔,可以显著加速岩浆海洋的凝固,从而将冷却时间减少5倍以上。如果孔的产生效率低,但抛射体动能向热能的冲击转换有效,则LMO凝固可以在一定程度上延长,将冷却时间延长50%或更多。

英文摘要:

Anorthosites that comprise the bulk of the lunar crust are believed to have formed during solidification of a Lunar Magma Ocean (LMO) in which these rocks would have floated to the surface. This early flotation crust would have formed a thermal blanket over the remaining LMO, prolonging solidification. Geochronology of lunar anorthosites indicates a long timescale of LMO cooling, or re-melting and re-crystallization in one or more late events. To better interpret this geochronology, we model LMO solidification in a scenario where the Moon is being continuously bombarded by returning projectiles released from the Moon-forming giant impact. More than one lunar mass of material escaped the Earth-Moon system onto heliocentric orbits following the giant impact, much of it to come back on returning orbits for a period of 100 Myr. If large enough, these projectiles would have punctured holes in the nascent floatation crust of the Moon, exposing the LMO to space and causing more rapid cooling. We model these scenarios using a thermal evolution model of the Moon that allows for production (by cratering) and evolution (solidification and infill) of holes in the flotation crust that insulates the LMO. For effective hole production, solidification of the magma ocean can be significantly expedited, decreasing the cooling time by more than a factor of 5. If hole production is inefficient, but shock conversion of projectile kinetic energy to thermal energy is efficient, then LMO solidification can be somewhat prolonged, lengthening the cooling time by 50% or more.

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