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Forming the lunar farside highlands by accretion of a companion moon

机译：通过伴生月亮的形成形成月球远端高原

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摘要

月球是有两个截然不同半球的卫星。始终朝向rn我们的“近地面”高度低、平坦、且为暗色，而rn“远地面”则多山并有很深的陨石坑。Marlin rnJutzi和Erik Asphaug提出，月球的这种“二分rn状况”也许是晚期吸积另一相伴卫星所造成rn的。相伴卫星是巨大撞击的一个常见结果，而rn原始“月盘”模拟及新的计算表明，以亚音速rn撞击速度与一个相伴卫星相撞会导致一个“吸rn积堆”而不是一个陨石坑，这样产生的一个半rn球层是与“远地面”高原地形的尺寸和壳层结rn构一致的。%The most striking geological feature of the Moon is the terrain and elevation dichotomy~1 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 variation~(2-3), 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 feld-spathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating~4, asymmetric convective processes~5 or asymmetric crystallization of the magma ocean~6) or external effects (such as the event that formed the South Pole/Aitken basin1 or asymmetric cratering~7). Here we consider its origin as a late carapace added by the accretion of a companion moon. Companion moons are a common outcome of simulations~8 of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable~9, a ~l,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact~(10). Most of the Moon's magma ocean would solidify on this timescale~(11-12), 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 highlands~(1,13) 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 concentration~(2,3).
机译：月球是有两个截然不同半球的卫星。始终朝向rn我们的“近地面”高度低、平坦、且为暗色，而rn“远地面”则多山并有很深的陨石坑。Marlin rnJutzi和Erik Asphaug提出，月球的这种“二分rn状况”也许是晚期吸积另一相伴卫星所造成rn的。相伴卫星是巨大撞击的一个常见结果，而rn原始“月盘”模拟及新的计算表明，以亚音速rn撞击速度与一个相伴卫星相撞会导致一个“吸rn积堆”而不是一个陨石坑，这样产生的一个半rn球层是与“远地面”高原地形的尺寸和壳层结rn构一致的。%The most striking geological feature of the Moon is the terrain and elevation dichotomy~1 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 variation~(2-3), 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 feld-spathic highlands. The lunar dichotomy may have been caused by internal effects (for example spatial variations in tidal heating~4, asymmetric convective processes~5 or asymmetric crystallization of the magma ocean~6) or external effects (such as the event that formed the South Pole/Aitken basin1 or asymmetric cratering~7). Here we consider its origin as a late carapace added by the accretion of a companion moon. Companion moons are a common outcome of simulations~8 of Moon formation from a protolunar disk resulting from a giant impact, and although most coplanar configurations are unstable~9, a ~l,200-km-diameter moon located at one of the Trojan points could be dynamically stable for tens of millions of years after the giant impact~(10). Most of the Moon's magma ocean would solidify on this timescale~(11-12), 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 highlands~(1,13) 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 concentration~(2,3).

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  • 来源
    《Nature》 |2011年第7358期|p.69-72|共4页
  • 作者

    M. Jutzi; E. Asphaug;

  • 作者单位

    Earth and Planetary Sciences Department, University of California, Santa Cruz, 1156 Highstreet, Santa Cruz, California 95060, USA,Physics Institute, University of Bern, Sidlerstrasse 5,3012 Bern,Switzerland;

    Earth and Planetary Sciences Department, University of California, Santa Cruz, 1156 Highstreet, Santa Cruz, California 95060, USA;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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