1000多年前,，一代文豪蘇軾寫出這樣的千古佳句：明月幾時(shí)有,，把酒問(wèn)青天,。天文學(xué)家對(duì)此孜孜以求,，毫不浪漫地要把地球這個(gè)伴侶的“芳齡”徹底弄清,。
關(guān)于月球的成因,，目前主要有4種流派——分裂說(shuō)、俘獲說(shuō),、同源說(shuō),、碰撞說(shuō)。不過(guò)，最主流的看法還是碰撞說(shuō),，這種觀點(diǎn)認(rèn)為月球形成于46億年前,。當(dāng)談起月球的年齡時(shí)，許多書本給出的都是這個(gè)答案,。不過(guò),，一個(gè)美國(guó)和丹麥科學(xué)家聯(lián)合研究小組的最新研究卻表明，月球可能要比此前認(rèn)為的“年輕”,。
根據(jù)碰撞說(shuō),，月球由大型天體與原始地球碰撞后拋射入太空的熔巖物質(zhì)形成。月球冷卻后,，熔巖形成了各種礦物質(zhì),。對(duì)由這些原始熔巖形成的物質(zhì)進(jìn)行分析，無(wú)疑能幫助科學(xué)家正確評(píng)估月球“芳齡”,。
1972年,，美國(guó)“阿波羅”16號(hào)飛船在月球笛卡爾環(huán)形山附近找到一種名為鐵方解鈣長(zhǎng)石的巖石，帶回地球后一直保存在美國(guó)航天局約翰遜航天中心,?？茖W(xué)家認(rèn)為，這種巖石是月球上“最老”的巖石種類之一,，應(yīng)該能幫助揭示月球年齡之謎,。
但直到最近，科學(xué)家才開(kāi)發(fā)出相關(guān)鑒定技術(shù),，他們運(yùn)用這種技術(shù)分析了鐵方解鈣長(zhǎng)石鉛和釹同位素，并由此斷定月球“芳齡”為43.6億歲,，比此前的答案年輕了2.4億歲,。
科學(xué)家說(shuō)，這一年齡與地球上發(fā)現(xiàn)的最古老礦石——在西澳大利亞發(fā)現(xiàn)的鋯石年齡相近,，表明地球和月球最古老的外殼幾乎同時(shí)形成,，都是在大碰撞后不久。
英國(guó)《自然》雜志網(wǎng)絡(luò)版17日刊登了這項(xiàng)研究成果,。參與研究的美國(guó)卡內(nèi)基科學(xué)研究所地球化學(xué)家理查德·卡爾森說(shuō)：“這塊樣品格外年輕,，要么意味著月球固化的時(shí)間比此前預(yù)計(jì)的晚很多，要么意味著我們需要徹底改變對(duì)月球化學(xué)史的認(rèn)識(shí),。”(生物谷 Bioon.com)
doi:10.1038/nature10328
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PMID:
Chronological evidence that the Moon is either young or did not have a global magma ocean
Lars E. Borg; James N. Connelly; Maud Boyet; Richard W. Carlson
Chemical evolution of planetary bodies, ranging from asteroids to the large rocky planets, is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth1, 2, 3. The Earth’s Moon is the archetypical example of this type of differentiation. Evidence for a lunar magma ocean is derived largely from the widespread distribution, compositional and mineralogical characteristics, and ancient ages inferred for the ferroan anorthosite (FAN) suite of lunar crustal rocks. The FANs are considered to be primary lunar flotation-cumulate crust that crystallized in the latter stages of magma ocean solidification. According to this theory, FANs represent the oldest lunar crustal rock type2, 3, 4. Attempts to date this rock suite have yielded ambiguous results, however, because individual isochron measurements are typically incompatible with the geochemical make-up of the samples, and have not been confirmed by additional isotopic systems5, 6, 7, 8, 9. By making improvements to the standard isotopic techniques, we report here the age of crystallization of FAN 60025 using the 207Pb–206Pb, 147Sm–143Nd and 146Sm–142Nd isotopic systems to be 4,360 ± 3 million years. This extraordinarily young age requires that either the Moon solidified significantly later than most previous estimates or the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect. If the latter is correct, then much of the lunar crust may have been produced by non-magma-ocean processes, such as serial magmatism10.
