本篇文章共分 7 段，從月球形成的可能理論中，以巨型星體撞擊地球為出發點，逐步推導目前月球重要外貌特徵的可能成因。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Lunar History 月球歷史

1. 月球形成的主要理論

   Much is still unknown about the Moon's origin. Until recently, the most widely held hypothesis argued that the formation of the Moon paralleled that of Earth and the other planets. That is, the Moon formed from minute rock fragments and gases that composed a disk-shaped structure (the solar nebula) that orbited the early Sun. Debris from this disk collided and accumulated into larger masses that, in turn, accreted into planetary-sized bodies.

   關於月球的起源，仍有許多未知之處。直到最近，最廣泛的假設認為，月球的形成與地球和其他行星的形成相類似。也就是說，月球是由微小的岩石碎片和氣體形成的，這些岩石碎片和氣體組成了一個盤狀結構（太陽星雲），圍繞著早期太陽運行。來自這個圓盤的碎片碰撞並累積成較大的質量，進一步又累積成行星大小的物體。

2. 巨型星體撞擊論

   A new hypothesis, which has recently gained support from many scientists, suggests that a giant body collided with Earth to produce the Moon. The explosion caused by the impact of a Mars-sized body with a semi-molten Earth is thought to have ejected huge quantities of mantle rock from the primordial Earth. A portion of this ejected material remained in orbit around Earth, while the remainder either escaped or impacted upon Earth's surface. In a manner similar to that proposed in the earlier hypothesis, the material orbiting Earth then began to accumulate, eventually producing the Moon. Though the giant impact hypothesis provides a plausible mechanism for the Moon's formation, many questions must be answered before this proposal can be considered viable.

   一個最近獲得許多科學家支持的新假設指出，一個巨大的天體與地球相撞產生了月球。一個火星大小的天體與一個半熔化的地球相撞所引起的爆炸被認為是從原始地球上噴出了大量的地幔岩石。這些噴出的物質有一部分留在了地球周圍的軌道上，而其餘的要麼逃逸，要麼撞擊到了地球表面。以一種類似於先前假說中提出的方式，圍繞地球的物質開始積累，最終形成月球。儘管巨型撞擊假說為月球的形成提供了一個合理的機制，但在這個主張被認為是可行的之前，許多問題必須得到回答。

3. 隕石坑密度

   Despite the fact that the origin of the Moon is still debated, planetary geologists have been able to work out some of the basic details of the Moon's history, using among other things variations in crater density (quantity per unit area). Simply stated, the higher the crater density, the longer the topographic feature has existed. During its early history, the Moon was continually impacted as it swept up debris from the solar nebula. This continuous bombardment and perhaps radioactive decay generated enough heat to melt the Moon's outer shell and quite possibly the rest of the Moon as well.

   儘管對月球的起源仍有爭議，但行星地質學家已經能夠利用隕石坑密度（每單位面積的數量）的變化，找出月球歷史的一些基本細節。簡單地說，隕石坑密度越高，代表地形特徵存在的時間就越長。在其早期歷史中，月球在掃蕩太陽星雲的碎片時不斷受到衝擊。這種持續的轟擊和可能的放射性衰變產生了足夠的熱量來融化月球的外殼，也很可能融化月球的其他部分。

4. 月殼岩石成分

   When a large percentage of the debris had been gathered, the outer layer of the Moon began to cool and form a crystalline crust. From samples obtained by Apollo astronauts, the rocks of the primitive lunar crust are thought to be composed of a high percentage of a calcium-rich feldspar (anorthosite). This feldspar mineral crystallized early and, because it was less dense than the remaining melt, floated to the top and formed a surface scum. While this process was taking place, iron and other heavy metals probably sank to form a small central core. Even after the crust had solidified, its surface was continually bombarded. Remnants of the original crust occupy the densely cratered highlands, which have been estimated to be as much as 4.5 billion years old.

   當相當高比例的碎片被集中起來後，月球的外層開始冷卻並形成了結晶性的地殼。從阿波羅號太空人取得的樣本來看，原始月殼的岩石被認為是由高比例的一種富含鈣質的長石（正長石）所組成。這種長石礦物很早就結晶了，由於它的密度比剩餘的熔體小，所以漂浮在頂部，形成了表面浮渣。當這個過程發生時，鐵和其他重金屬可能下沉，形成一個小的中央核心。即使在地殼凝固之後，其表面也不斷受到轟炸。原始地殼的殘餘物佔據了密集的隕石坑高地，據估計，其年齡高達 45 億年。

5. 最後一波撞擊

   The last period of heavy bombardment recorded in the lunar highlands occurred almost 500 million years after the crust had formed. It is not known with certainty whether this final episode of bombardment was simply a clean-up phase where the remaining large particles in the Earth-Moon orbit were swept up or whether it was an influx of bodies from farther out in the solar system.

   在月球高原上記錄的最後一次猛烈撞擊發生在地殼形成後近 5 億年。目前還不確定的是，這最後一波撞擊只是一個清理階段，將地月軌道上剩餘的大顆粒掃除乾淨，還是來自太陽系更遠地方的天體湧入。

6. 海型盆地

   The next major event in the Moon's evolution was the formation of maria basins, which are large craters that filled with lava flowing up through cracks in the Moon's surface. The meteoroids that produced these huge pits ejected mountainous quantities of lunar rock into piles rising 5 kilometers or more. The Apennine mountain range, which typifies such an accumulation, was produced in conjunction with the formation of the Imbrium Basin, the site explored by the Apollo 15 astronauts. The crater density of the ejected material is greater than that of the surface of the associated basin, confirming that an appreciable time elapsed between the formation and filling of these basins. Radiometric dating of the maria basalts (a type of rock) puts their age between 3.2 and 3.8 billion years, somewhat younger than the initial crust. In places, the lava flows overlap the highlands, another testimonial to the lesser age of the maria deposits.

   月球演變過程中的下一個重大事件是馬里亞盆地 (海型盆地) 的形成，這是一個大坑，裡面充滿了從月球表面裂縫中流上來的熔岩。產生這些巨大坑洞的流星體將大量的月球岩石噴射出來，形成 5 公里或更高的堆積體。亞平寧山脈是這種堆積的典型代表，它是在形成英布留姆盆地的同時產生的，英布留姆盆地是阿波羅 15 號太空人探索的地點。噴出物質的隕石坑密度大於相關盆地的表面密度，證實了這些盆地的形成和充填之間經過了相當長的時間。對海相玄武岩（一種岩石）的放射性測定顯示，它們的年齡在 32 至 38 億年之間，比最初的地殼年齡要小一些。在一些地方，熔岩流與高地重疊，這也證明了海相沉積的年齡較小。

7. 射線型隕石坑

   The last prominent features to form on the lunar surface were the rayed craters as exemplified by the crater Copernicus. Rays of material ejected from these young depressions are clearly seen blanketing the surface of the maria and many older rayless craters. By contrast, the older craters have rounded rims, and their rays have been erased by the impact of small debris. However, even a relatively young crater like Copernicus must be millions of years old. Had it formed on Earth, erosional forces would have long since obliterated it.

   在月球表面最後形成的顯著特徵是帶有射線形式的隕石坑，其中哥白尼隕石坑就是一個例子。從這些年輕凹陷地區中噴出物質的輻射線型可以清楚地看到覆蓋在馬里亞和許多舊的無射線隕石坑的表面。然而，即使是像哥白尼這樣相對年輕的隕石坑，也一定有幾百萬年的歷史了。如果它是在地球上形成的話，那侵蝕力早就把它抹去了。