Lunar history

　　月球的歷史

　　EARTHS moon has a split personality. One half of its surfacethe half which, thanks to thevagaries of orbital mechanics, always faces Earthis dominated by dark, smooth expanses ofancient, frozen lava known as maria . The contrast between the darkness of themaria and the brightness of the surrounding highlands forms a pattern popularly known,depending on the culture of the observer, as the man in the moon, the rabbit on the moon orone of many other optical illusions.

　　地球的衛(wèi)星---月球有著分裂的人格。其表面的一半 ---由于軌道力學的變幻莫測，這一半總是面對地球---一直處于黑暗之中。這片黑暗的地方被稱為月海，不僅平坦廣闊，而且由古老而凍結的火山巖形成。根據(jù)觀察者不同的文化，月海的黑暗和周圍群山的明亮對比形成了一種眾所周知的模式，比如嫦娥，月兔或許多其它視錯覺之一。

　　When astronomers got their first glimpses of the moons far side, however, they saw astrikingly different landscape. Early lunar probes revealed a surface that was mountainous,rugged, heavily cratered and virtually devoid of maria. To quote Bill Anders, one of theastronauts on Apollo 8 and thus one of the first three people to see the far side of the moondirectly, it looks like a sand pile my kids have been playing in...all beat up, no definition, just alot of bumps and holes.

　　然而，當天文學家首次瞥見月球較遠的那側時，他們看到了一處截然不同的景觀。早期月球探測器顯示了這塊多山，崎嶇不平，擁有大量隕石坑以及幾乎毫無大海的地域的表面。引用阿波羅8號宇航員之一，也是首次直接看到月球較遠那側的三個人之一的比爾?安德斯的話說，那塊地方就像孩子們玩的沙堆一樣到處都是破破爛爛的，無法解釋，就是很多坑坑洼洼。

　　Selenologists, as those who study the moon callthemselves, have kicked around many theories toexplain the differences between the moons twofaces: Earth partly shielding the moon frommeteoritic impacts; uneven heating from beneath;the collision that excavated a 2,500km-wide craterat the lunar south pole, one of the biggest in thesolar system. None seems satisfactory. But in apaper just published in Nature Martin Jutzi and ErikAsphaug of the University of California, Santa Cruz,examine yet another idea. This is that the mountainsof the lunar far side may be debris left over from an even bigger collision than the one at thesouth polea collision that left lumps on the lunar surface rather than digging holes in it.

　　作為研究月球而稱他們自己為月球學家的那些人已經非正式地討論了許多理論來解釋月球的兩種截然不同表面的區(qū)別：地球部分地保護了月球免受隕石的碰撞;月球內部不均勻的加熱;在月球南極空砸出一個2500公里寬的隕石坑---太陽系最大的隕石坑之一---的那次碰撞。好像沒人對些理論滿意。但是，加州大學圣克魯斯分校的馬丁?徹奇希和埃里克?阿薩福格剛剛在《自然雜志》發(fā)表的一篇論文檢驗了迄今為止的另一個猜想。這個猜想就是月球較遠那側的群山甚至可能是比月球南極那次碰撞更大的碰撞所留下的巖屑---這次更大碰撞在月球表面留下了隆起，而非砸坑。

　　Moonstruck

　　沉迷于幻想

　　The most widely accepted theory of how the moon formed suggests it is the result of a cosmicsmash-up between Earth and an object roughly the size of Mars that happened about 4.5 billionyears ago during the solar systems turbulent youth. This impact would have sprayed hugequantities of material into space, forming a ring around Earth similar to, but much thicker than,those that now adorn Saturn.

　　月球如何形成的最廣泛接受的理論表明，大約發(fā)生在45億年前的年輕的太陽系動蕩期間，地球和一個大約火星大小的物體間的宇宙大碰撞的結果而產生了月球。這次碰撞已將大量的物質散發(fā)到了太空，從而形成了環(huán)繞地球的光環(huán)，類似于那些現(xiàn)在裝飾土星的光環(huán)，但是要厚得多。

　　The moon is thought to have condensed rapidly from this ring. Computer simulation of theprocess suggests, however, that several smaller moons may have formed at the same time.Some would then have collided with Earth or the moon, while others would have been ejectedfrom the system altogether. But one or more of them often end up, according to thesimulations, in what are known as Trojan orbits, where they followed the proto-moon endlesslyaround Earth.

　　月球被認為是從這個光環(huán)的氣體凝聚而成。然而，計算機模擬的過程表明，這次大碰撞可能在同一時間形成了幾個更小的衛(wèi)星。然后，有些衛(wèi)星就與地球或月球相撞，而其它衛(wèi)星則完全被太陽系噴射出來。但，根據(jù)計算機多次模擬，其中的一個或多個衛(wèi)星往往停留在被稱為特洛伊星座的軌道上而終結。這些衛(wèi)星在那兒伴隨著永無休止地圍繞地球的原月。

　　Or, rather, almost endlessly. For the hypothesis of Dr Jutzi and Dr Asphaug is that a largemoonlet did eventually catch up and collide with the proto-moonat a relatively low speedgiven that they shared an orbitand produced the highlands on its far side as a result.

　　或者，更確切些，幾乎是沒有休止的。因為徹奇希博士和阿薩福格博士的假說就是一個大的小衛(wèi)星最終趕了上來并與這個原月發(fā)生了碰撞---考慮到它們共享一條軌道，是以相對低的速度碰撞---結果，就在月球較遠的那側產生了那些群山。

　　To test their theory, the two researchers simulated such a collision on a computer. They foundthat, assuming the second moon was indeed smaller than the main one, and was made ofsimilar stuff , then a comparatively low-speed collision at around 2.4 kilometres asecond would mean that much more material from the moonlet was deposited on the lunarsurface than was blasted off by the impact. Most of the excess rock, moreover, would havestayed near the point of impact, covering about half of the moon with a thicker,mountainous layer of crustjust what is seen on the real moon.

　　為了測試他們的理論，這兩位研究者在計算機上模擬了這樣一次大碰撞。他們發(fā)現(xiàn)，假設第二個月球確實比最大的月球要小，并且由相似的物質組成。然后，以大約每秒2.4公里的相對低速碰撞意味著，小衛(wèi)星上更多的物質被沉積在月球表面而不是因碰撞而噴射出來。此外，過多的巖石將留落在碰撞點附近，且以更厚、多山的外殼層占了月球約為一半的面積 ---正如真正月球上所見到的一樣。

　　Of course, computer simulations can say only whatcould have happened, not what did. For that,physical evidence is required. Some suggestiveevidence does exist, though. For a start, the cruston the far side of the moon is indeed much thickerthan that on the nearside. Also, because the secondmoon would have been smaller than the main one, itwould have cooled and solidified more quickly,meaning that its rocks would be older. Sure enough,different parts of the present moons crust,recovered by various missions to the lunar surface,differ in age by up to 200m years, a fact that has long puzzled selenologists but which theimpact theory neatly explains. Moreover, such a huge collision on the far side would haveshifted much of the magma ocean that then underlay the moons surface to the near side,which led to the formation of the maria.

　　當然，計算機模擬只能估計已經發(fā)生的情形，而不是確實發(fā)生的情況。為此，實物證據(jù)是需要的。雖然，某些提示性的證據(jù)確實存在。因為從一開始，較遠那側的月球外殼確實比較近側的外殼要厚實的多。此外，因為這第二個月球應該比最大的月球要小的多，所以這個月球應該冷卻、凝固要快得多。這也意味著這個月球的巖石要年久的多。果真如此，現(xiàn)在月球外殼的不同部分，各次去月球表面的探測任務發(fā)現(xiàn)，這些巖石年代的不同達到 2億年之久。一個長期困擾月球學家的客觀事實被碰撞理論巧妙地解釋了。此外，在月球較遠那側如此巨大的一次碰撞應該迅速移動很多巖漿洋，然后在月球表面一直鋪到近地側，這就導致了月海的形成。

　　More evidence may surface when the Gravity Recovery and Interior Laboratory mission, anAmerican spacecraft designed to map the moons gravitational field, arrives in lunar orbit inaround six months time. That should yield a detailed map of the lunar crust, and how itsthickness varies from place to place. But the best way to check the new hypothesis would befor selenologists to get their hands on rocks from the far side.

　　當引力恢復和室內實驗任務時，更多的證據(jù)可能會出現(xiàn)。一架設計用來勘測月球引力場的美國航天器，大約會在六個月內到達月球軌道。這次探測任務應該會產生出一份詳細的月球外殼圖，以及各個地方的厚度如何變化。但是，對月球學家來說，檢查這個新假說的最好方式就是找到月球較遠那側的巖石。

　　Sadly, that is unlikely to happen soon. Neither America nor Russia has any sample-recoverymissions planned for the moon. China does . But, like the American andSoviet missions of the 1960s and 1970s, the Chinese are expected to opt for the safety of alanding on the near side. Selenologists may therefore have to wait a long time before they cantest the Jutzi-Asphaug model properly.

　　可嘆的是，這種情況不可能很快發(fā)生。無論是美國還是俄羅斯都沒有任何計劃去月球恢復取樣的任務。中國倒有一個。但是，像20世紀60年代和70年代的美國和蘇聯(lián)的任務一樣，中國人選擇在近地側安全著陸。因此，月球學家可能需要等待很長一段時間才能夠真正測試徹奇希-阿薩福格模型。

　　Lunar history

　　月球的歷史

　　EARTHS moon has a split personality. One half of its surfacethe half which, thanks to thevagaries of orbital mechanics, always faces Earthis dominated by dark, smooth expanses ofancient, frozen lava known as maria . The contrast between the darkness of themaria and the brightness of the surrounding highlands forms a pattern popularly known,depending on the culture of the observer, as the man in the moon, the rabbit on the moon orone of many other optical illusions.

　　地球的衛(wèi)星---月球有著分裂的人格。其表面的一半 ---由于軌道力學的變幻莫測，這一半總是面對地球---一直處于黑暗之中。這片黑暗的地方被稱為月海，不僅平坦廣闊，而且由古老而凍結的火山巖形成。根據(jù)觀察者不同的文化，月海的黑暗和周圍群山的明亮對比形成了一種眾所周知的模式，比如嫦娥，月兔或許多其它視錯覺之一。

　　When astronomers got their first glimpses of the moons far side, however, they saw astrikingly different landscape. Early lunar probes revealed a surface that was mountainous,rugged, heavily cratered and virtually devoid of maria. To quote Bill Anders, one of theastronauts on Apollo 8 and thus one of the first three people to see the far side of the moondirectly, it looks like a sand pile my kids have been playing in...all beat up, no definition, just alot of bumps and holes.

　　然而，當天文學家首次瞥見月球較遠的那側時，他們看到了一處截然不同的景觀。早期月球探測器顯示了這塊多山，崎嶇不平，擁有大量隕石坑以及幾乎毫無大海的地域的表面。引用阿波羅8號宇航員之一，也是首次直接看到月球較遠那側的三個人之一的比爾?安德斯的話說，那塊地方就像孩子們玩的沙堆一樣到處都是破破爛爛的，無法解釋，就是很多坑坑洼洼。

　　Selenologists, as those who study the moon callthemselves, have kicked around many theories toexplain the differences between the moons twofaces: Earth partly shielding the moon frommeteoritic impacts; uneven heating from beneath;the collision that excavated a 2,500km-wide craterat the lunar south pole, one of the biggest in thesolar system. None seems satisfactory. But in apaper just published in Nature Martin Jutzi and ErikAsphaug of the University of California, Santa Cruz,examine yet another idea. This is that the mountainsof the lunar far side may be debris left over from an even bigger collision than the one at thesouth polea collision that left lumps on the lunar surface rather than digging holes in it.

　　作為研究月球而稱他們自己為月球學家的那些人已經非正式地討論了許多理論來解釋月球的兩種截然不同表面的區(qū)別：地球部分地保護了月球免受隕石的碰撞;月球內部不均勻的加熱;在月球南極空砸出一個2500公里寬的隕石坑---太陽系最大的隕石坑之一---的那次碰撞。好像沒人對些理論滿意。但是，加州大學圣克魯斯分校的馬丁?徹奇希和埃里克?阿薩福格剛剛在《自然雜志》發(fā)表的一篇論文檢驗了迄今為止的另一個猜想。這個猜想就是月球較遠那側的群山甚至可能是比月球南極那次碰撞更大的碰撞所留下的巖屑---這次更大碰撞在月球表面留下了隆起，而非砸坑。

　　Moonstruck

　　沉迷于幻想

　　The most widely accepted theory of how the moon formed suggests it is the result of a cosmicsmash-up between Earth and an object roughly the size of Mars that happened about 4.5 billionyears ago during the solar systems turbulent youth. This impact would have sprayed hugequantities of material into space, forming a ring around Earth similar to, but much thicker than,those that now adorn Saturn.

　　月球如何形成的最廣泛接受的理論表明，大約發(fā)生在45億年前的年輕的太陽系動蕩期間，地球和一個大約火星大小的物體間的宇宙大碰撞的結果而產生了月球。這次碰撞已將大量的物質散發(fā)到了太空，從而形成了環(huán)繞地球的光環(huán)，類似于那些現(xiàn)在裝飾土星的光環(huán)，但是要厚得多。

　　The moon is thought to have condensed rapidly from this ring. Computer simulation of theprocess suggests, however, that several smaller moons may have formed at the same time.Some would then have collided with Earth or the moon, while others would have been ejectedfrom the system altogether. But one or more of them often end up, according to thesimulations, in what are known as Trojan orbits, where they followed the proto-moon endlesslyaround Earth.

　　月球被認為是從這個光環(huán)的氣體凝聚而成。然而，計算機模擬的過程表明，這次大碰撞可能在同一時間形成了幾個更小的衛(wèi)星。然后，有些衛(wèi)星就與地球或月球相撞，而其它衛(wèi)星則完全被太陽系噴射出來。但，根據(jù)計算機多次模擬，其中的一個或多個衛(wèi)星往往停留在被稱為特洛伊星座的軌道上而終結。這些衛(wèi)星在那兒伴隨著永無休止地圍繞地球的原月。

　　Or, rather, almost endlessly. For the hypothesis of Dr Jutzi and Dr Asphaug is that a largemoonlet did eventually catch up and collide with the proto-moonat a relatively low speedgiven that they shared an orbitand produced the highlands on its far side as a result.

　　或者，更確切些，幾乎是沒有休止的。因為徹奇希博士和阿薩福格博士的假說就是一個大的小衛(wèi)星最終趕了上來并與這個原月發(fā)生了碰撞---考慮到它們共享一條軌道，是以相對低的速度碰撞---結果，就在月球較遠的那側產生了那些群山。

　　To test their theory, the two researchers simulated such a collision on a computer. They foundthat, assuming the second moon was indeed smaller than the main one, and was made ofsimilar stuff , then a comparatively low-speed collision at around 2.4 kilometres asecond would mean that much more material from the moonlet was deposited on the lunarsurface than was blasted off by the impact. Most of the excess rock, moreover, would havestayed near the point of impact, covering about half of the moon with a thicker,mountainous layer of crustjust what is seen on the real moon.

　　為了測試他們的理論，這兩位研究者在計算機上模擬了這樣一次大碰撞。他們發(fā)現(xiàn)，假設第二個月球確實比最大的月球要小，并且由相似的物質組成。然后，以大約每秒2.4公里的相對低速碰撞意味著，小衛(wèi)星上更多的物質被沉積在月球表面而不是因碰撞而噴射出來。此外，過多的巖石將留落在碰撞點附近，且以更厚、多山的外殼層占了月球約為一半的面積 ---正如真正月球上所見到的一樣。

　　Of course, computer simulations can say only whatcould have happened, not what did. For that,physical evidence is required. Some suggestiveevidence does exist, though. For a start, the cruston the far side of the moon is indeed much thickerthan that on the nearside. Also, because the secondmoon would have been smaller than the main one, itwould have cooled and solidified more quickly,meaning that its rocks would be older. Sure enough,different parts of the present moons crust,recovered by various missions to the lunar surface,differ in age by up to 200m years, a fact that has long puzzled selenologists but which theimpact theory neatly explains. Moreover, such a huge collision on the far side would haveshifted much of the magma ocean that then underlay the moons surface to the near side,which led to the formation of the maria.

　　當然，計算機模擬只能估計已經發(fā)生的情形，而不是確實發(fā)生的情況。為此，實物證據(jù)是需要的。雖然，某些提示性的證據(jù)確實存在。因為從一開始，較遠那側的月球外殼確實比較近側的外殼要厚實的多。此外，因為這第二個月球應該比最大的月球要小的多，所以這個月球應該冷卻、凝固要快得多。這也意味著這個月球的巖石要年久的多。果真如此，現(xiàn)在月球外殼的不同部分，各次去月球表面的探測任務發(fā)現(xiàn)，這些巖石年代的不同達到 2億年之久。一個長期困擾月球學家的客觀事實被碰撞理論巧妙地解釋了。此外，在月球較遠那側如此巨大的一次碰撞應該迅速移動很多巖漿洋，然后在月球表面一直鋪到近地側，這就導致了月海的形成。

　　More evidence may surface when the Gravity Recovery and Interior Laboratory mission, anAmerican spacecraft designed to map the moons gravitational field, arrives in lunar orbit inaround six months time. That should yield a detailed map of the lunar crust, and how itsthickness varies from place to place. But the best way to check the new hypothesis would befor selenologists to get their hands on rocks from the far side.

　　當引力恢復和室內實驗任務時，更多的證據(jù)可能會出現(xiàn)。一架設計用來勘測月球引力場的美國航天器，大約會在六個月內到達月球軌道。這次探測任務應該會產生出一份詳細的月球外殼圖，以及各個地方的厚度如何變化。但是，對月球學家來說，檢查這個新假說的最好方式就是找到月球較遠那側的巖石。

　　Sadly, that is unlikely to happen soon. Neither America nor Russia has any sample-recoverymissions planned for the moon. China does . But, like the American andSoviet missions of the 1960s and 1970s, the Chinese are expected to opt for the safety of alanding on the near side. Selenologists may therefore have to wait a long time before they cantest the Jutzi-Asphaug model properly.

　　可嘆的是，這種情況不可能很快發(fā)生。無論是美國還是俄羅斯都沒有任何計劃去月球恢復取樣的任務。中國倒有一個。但是，像20世紀60年代和70年代的美國和蘇聯(lián)的任務一樣，中國人選擇在近地側安全著陸。因此，月球學家可能需要等待很長一段時間才能夠真正測試徹奇希-阿薩福格模型。