新一期英国《自然》杂志刊登研究报告说，地球可能曾经拥有两个月亮，只是后来这两颗星球发生碰撞合二为一，成为今天我们所看见的月球。
美国加利福尼亚大学等机构研究人员报告说，他们提出的这个新假说，可以较好地解释月球上的一些奇特现象。月球由于自转和公转上的特点，始终以同一面对着地球，而其近地一侧和远地一侧有完全不同的地貌，如近地一侧的地貌更为平坦，而远地一侧则有许多凹坑和高山；在地质成分上，近地一侧含有比远地一侧丰富得多的钾、磷和稀土等。
研究人员认为，月球起源于40多亿年前一颗星球与地球的相撞，飘散在太空中的物质逐渐形成了今天月球的主体，但与过去认为的只形成了一个月球不同，当时还形成了另一个较小的星球。这个较小的月亮直径只有约1000公里，是今天月球体积的约三十分之一。它存在了数千万年，这段时间里能在地球上看见两个月亮。
两个月亮最终相撞，由于相撞时两者速度相对较低，结果合二为一。计算机模型显示，这次撞击应该发生在今天月球的远地一侧，并因此造成了月球远地一侧更加起伏不平的地貌，这次撞击还将大量的钾、磷和稀土等元素推向了月球的近地一侧。
关于月球近地一侧和远地一侧差异巨大的问题，科研人员曾提出许多理论解释，如认为其原因在于地球引力或月球内部岩浆的运动特点等，本次研究又为此增添了一种新理论。不过，这些理论都还需要进一步的证据支持，因此科研人员也期盼着各国的探月计划能够提供更多线索。(生物谷 Bioon.com)
doi:10.1038/nature10289
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Forming the lunar farside highlands by accretion of a companion moon
M. Jutzi; E. Asphaug
The most striking geological feature of the Moon is the terrain and elevation dichotomy1 between the hemispheres: the nearside is low and flat, dominated by volcanic maria, whereas the farside is mountainous and deeply cratered. Associated with this geological dichotomy is a compositional and thermal variation2, 3, with the nearside Procellarum KREEP (potassium/rare-earth element/phosphorus) Terrane and environs interpreted as having thin, compositionally evolved crust in comparison with the massive feldspathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating4, asymmetric convective processes5 or asymmetric crystallization of the magma ocean6) or external effects (such as the event that formed the South Pole/Aitken basin1 or asymmetric cratering7). Here we consider its origin as a late carapace added by the accretion of a companion moon. Companion moons are a common outcome of simulations8 of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable9, a ~1,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact10. Most of the Moon’s magma ocean would solidify on this timescale11, 12, whereas the companion moon would evolve more quickly into a crust and a solid mantle derived from similar disk material, and would presumably have little or no core. Its likely fate would be to collide with the Moon at ~2–3 km s−1, well below the speed of sound in silicates. According to our simulations, a large moon/Moon size ratio (~0.3) and a subsonic impact velocity lead to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness consistent with the dimensions of the farside highlands1, 13 and in agreement with the degree-two crustal thickness profile4. The collision furthermore displaces the KREEP-rich layer to the opposite hemisphere, explaining the observed concentration2, 3.
