本篇文章共分 8 段，從月球上無數的環形山結構早期的火山形成論為主流，到近代才因為新的實驗與實地觀察證明其應該是撞擊所產生的科學發展過程，詳細說明月球上的坑洞是為何是流星所造成的隕石坑，而非火山口。

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。

The Impact Origin of Lunar Craters 月球隕石坑的撞擊起源

1. 月球坑洞起源爭議

   From their discovery in 1609 until recently, lunar craters were almost unanimously thought to have volcanic origins, even though the topography of the Moon and its craters had been studied minutely by generations of observers. Dissenting voices, such as that of Robert Hooke, who postulated in 1665 that they were formed by the impact of meteors, did little to sway the majority opinion. The most serious challenge to the volcanic-formation hypothesis was made in 1893 by the geologist G. K. Gilbert. After analyzing the depth-to-diameter ratios of craters, Gilbert suggested that the craters could have been formed only by the impact of meteors. The central peaks that were commonly observed resulted from rebound of rock, in a similar manner to liquids (think of drops of water hitting a puddle). Bright rays surrounded many craters that were formed from material flung out during the impact—impact ejecta.

   從 1609 年被發現到最近，人們幾乎一致認為月球表面坑洞是由火山造成的，儘管好幾個世代的觀察者對月球的地形和坑洞進行了細緻的研究。但是不同的主張，例如羅伯特．胡克在 1665 年認為這些是由流星的撞擊形成的，並沒有動搖大多數人的意見。1893 年，地質學家 G. K. 吉爾伯特對火山形成的假說提出了最重要的挑戰。在分析了坑洞的深度對直徑比之後，吉爾伯特提出，坑洞只可能由流星的撞擊形成。通常觀察到的中央山峰是岩石反彈的結果，其方式與液體相似（想想水滴撞擊水坑的情形）。明亮的線條圍繞著許多隕石坑，這些線條是由撞擊時甩出的物質所形成—即撞擊噴出物。

2. 撞擊說與早期實驗不合

   Although impacts with a surface can occur at all angles, from less than 5° to 90°, early experiments suggested that only vertical (90°) impacts were capable of producing circular craters. So the main argument against an impact mechanism for lunar crater formation was that only circular craters could be seen on the Moon. However, Gilbert himself showed that the average impact angle was 45° — in fact, very few impacts were close to vertical.

   雖然與地表的撞擊可以發生在各種角度，從小於 5° 到 90°，但早期的實驗顯示，只有垂直（90°）的撞擊才能夠產生圓形的隕石坑。因此，反對月球坑洞形成的撞擊機制的主要論點是，在月球上只能看到圓形坑洞。但是，就連吉爾伯特本人亦指出，平均撞擊角度為 45°—事實上，很少有接近垂直的撞擊。

3. 撞擊理論開始受接納

   In 1916 E J. Öpik published work that recognized that the impact of meteors at very high velocities would be fundamentally different from the impact at low velocities created in early laboratory experiments. Such high-velocity impacts would have a similar effect to an explosion and craters would be circular even at low impact angles. Nevertheless, the volcanic hypothesis remained intact, and there was significant unwillingness amongst the scientific community about accepting impact cratering. This began to change in the 1960s with the work of the American geologist Gene Shoemaker, who made a lifetime study of cratering.

   1916 年，E. J. Öpik 發表的研究認為，流星以非常高的速度撞擊將與早期實驗室實驗中產生的低速度撞擊有基本的不同。這種高速撞擊會產生類似於爆炸的效果，即使在低角度的撞擊下，隕石坑也是圓形的。儘管如此，火山假說仍然沒有改變，科學界非常不願意接受撞擊形成隕石坑的主張。這種情況在 20 世紀 60 年代隨著一生都在研究隕石坑的美國地質學家吉殷．舒梅克的研究成果而開始改變。

4. 地球隕石坑的特徵

   Of the 170 known impact craters on the Earth, Meteor Crater in Arizona was one of the first to be recognized as being formed from an impact. In 1906 D. M. Barringer had provided good evidence for impact formation. However, Meteor Crater was not universally accepted as an impact crater until the 1960s. lts impact origin was finally demonstrated in papers published in 1960 and1963 by Shoemaker. Three lines of evidence show that an impact, and not a volcanic eruption, formed Meteor Crater.

   在地球上已知的 170 個撞擊坑中，亞利桑那州的流星隕石坑是最早被認為是由撞擊形成的坑洞之一。1906 年，D. M. 巴林傑為撞擊形成提供了有效的證據。然而，直到 20 世紀 60 年代，流星隕石坑才被普遍接受為撞擊坑。1960 年和 1963 年舒梅克發表的論文最終證明了它的撞擊起源。三項證據顯示，流星隕石坑是由撞擊而非火山爆發形成的。

5. 撞擊遺留的碎片

   First, as Barringer had recognized, many large fragments from an impact have been found on the desert plains surrounding Meteor Crater. Impact structures that are larger than Meteor Crater do not usually preserve fragments of the original projectiles because they vaporize on impact.

   首先，正如巴林傑所認為的，在流星隕石坑周圍的沙漠平原上已經發現了許多來自撞擊的大型碎片。比流星隕石坑更大的撞擊結構通常不會留下原始射彈的碎片，因為它們在撞擊時被蒸發了。

6. 撞擊造成的結構組成

   In addition, most volcanic craters are formed by sustained compression of volcanic gases which blast out large volumes of fragments. Volcanic ejecta form simple aprons around craters, the first-erupted materials at the bottom and the last at the top. Shoemakers field studies showed that a completely different situation exists at Meteor Crater, where the ejecta form an inverted flap flung out and overturned in a single blast. Thus, a drill hole through the ejecta on the rim would pass through layers of the same material twice. In Shoemaker’s words, the strata appeared to “have been peeled back from the area of the crater,” somewhat like petals of a flower blossoming. Similar structures are observed in laboratory experiments involving hypervelocity impacts.

   此外，大多數火山口是由火山氣體的持續壓縮形成的，這些氣體會噴出大量的碎片。火山噴出物在火山口周圍形成簡單的圍裙結構，最先噴出的物質在底部，最後的物質在頂部。舒梅克的田野研究顯示，在流星隕石坑存在著完全不同的情況，噴出物形成了一個倒置的封蓋，在單一爆炸中被甩了出去並翻了過來。因此，一個穿過邊緣噴出物的鑽孔會穿過同樣材料的結構層兩次。用舒梅克的話說，這些地層似乎是「從坑洞區域被剝離出來的」，有點像花瓣綻放。在涉及超高速撞擊的實驗室實驗中也觀察到類似的結構。

7. 撞擊壓力

   The final piece of evidence comes from an investigation of the effects of the intense pressures involved in an impact. At Meteor Crater, shattered target rocks are cemented together by glass at depths of 200-400 meters. The glass is formed by melting of the target during the impact. Melting occurs principally as a result of the very high pressures involved during an impact, and because the pressure is applied almost instantaneously, it is referred to as shock pressure. Unusual high pressure minerals are often found in the rocks affected by impact. These minerals are produced by shock pressure during the impact.

   最後一項證據來自於對撞擊所涉及的巨大壓力影響研究。在流星隕石坑，破碎的目標岩石在 200-400 米的深度被玻璃粘合在一起。這些玻璃是由撞擊過程中目標的熔化所形成。熔化的發生主要是由於撞擊過程中所涉及的超高壓力，由於壓力幾乎是在瞬間所施加，所以被稱為衝擊壓力。在受撞擊影響的岩石中經常發現不尋常的高壓礦物。這些礦物是由撞擊過程中的衝擊壓力所產生。

8. 月球隕石坑撞擊論成主流

   In 1962 Shoemaker made a case for the impact formation of Copernicus Crater, one of the most prominent craters on the near side of the Moon. Before the decade was out, most scientists had come to accept that lunar craters were formed by impact rather than volcanism.

   1962 年，舒梅克為哥白尼隕石坑的撞擊形成提出了理由，哥白尼隕石坑是月球正面最突出的隕石坑之一。在 60 年代結束之前，大多數科學家已經開始接受月球隕石坑是由撞擊而不是由火山形成的主張。