新一期英國(guó)《自然》雜志刊登研究報(bào)告說(shuō),，地球可能曾經(jīng)擁有兩個(gè)月亮,，只是后來(lái)這兩顆星球發(fā)生碰撞合二為一,，成為今天我們所看見(jiàn)的月球。
美國(guó)加利福尼亞大學(xué)等機(jī)構(gòu)研究人員報(bào)告說(shuō),，他們提出的這個(gè)新假說(shuō)，可以較好地解釋月球上的一些奇特現(xiàn)象,。月球由于自轉(zhuǎn)和公轉(zhuǎn)上的特點(diǎn),，始終以同一面對(duì)著地球，而其近地一側(cè)和遠(yuǎn)地一側(cè)有完全不同的地貌,，如近地一側(cè)的地貌更為平坦,，而遠(yuǎn)地一側(cè)則有許多凹坑和高山；在地質(zhì)成分上,，近地一側(cè)含有比遠(yuǎn)地一側(cè)豐富得多的鉀,、磷和稀土等。
研究人員認(rèn)為,，月球起源于40多億年前一顆星球與地球的相撞,，飄散在太空中的物質(zhì)逐漸形成了今天月球的主體，但與過(guò)去認(rèn)為的只形成了一個(gè)月球不同,，當(dāng)時(shí)還形成了另一個(gè)較小的星球,。這個(gè)較小的月亮直徑只有約1000公里，是今天月球體積的約三十分之一,。它存在了數(shù)千萬(wàn)年,，這段時(shí)間里能在地球上看見(jiàn)兩個(gè)月亮。
兩個(gè)月亮最終相撞,，由于相撞時(shí)兩者速度相對(duì)較低,，結(jié)果合二為一。計(jì)算機(jī)模型顯示,，這次撞擊應(yīng)該發(fā)生在今天月球的遠(yuǎn)地一側(cè),，并因此造成了月球遠(yuǎn)地一側(cè)更加起伏不平的地貌，這次撞擊還將大量的鉀,、磷和稀土等元素推向了月球的近地一側(cè),。
關(guān)于月球近地一側(cè)和遠(yuǎn)地一側(cè)差異巨大的問(wèn)題，科研人員曾提出許多理論解釋?zhuān)缯J(rèn)為其原因在于地球引力或月球內(nèi)部巖漿的運(yùn)動(dòng)特點(diǎn)等,，本次研究又為此增添了一種新理論,。不過(guò),，這些理論都還需要進(jìn)一步的證據(jù)支持，因此科研人員也期盼著各國(guó)的探月計(jì)劃能夠提供更多線索,。(生物谷 Bioon.com)
doi:10.1038/nature10289
PMC:
PMID:
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.
