Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, where temperatures are close to absolute zero—must be pristine relics of the material that formed the outer planets. The conceptual shift away from seeing comets as pristine relics began in the 1970s, when laboratory simulations revealed there was sufficient ultraviolet radiation reaching comets to darken their surfaces and there were sufficient cosmic rays to alter chemical bonds or even molecular structure near the surface. Nevertheless, astronomers still believed that when a comet approached the Sun— where they could study it—the Sun’s intense heat would remove the corrupted surface layer, exposing the interior. About the same time, though, scientists realized comets might contain decaying radioactive isotopes that could have warmed cometary interiors to temperatures that caused the interiors to evolve.
Consider each of the choices separately and select all that apply.
1. According to the passage, astronomers recognize which of the following as being liable to cause changes to comets?
A. cosmic rays
B. radioactive decay
C. ultraviolet radiation
答案：ABC
Consider each of the choices separately and select all that apply.
2. According to the passage, astronomers’ belief that comets are pristine relics was
A. overturned by analysis of what happens when comets approach the Sun
B. supported by what observations revealed about the composition of the outer planets
C. based on consideration of the conditions that prevail where comets are located
答案：C
解析見下。
Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, where temperatures are close to absolute zero—must be pristine relics of the material that formed the outer planets.
第1句：研究行星形成的天文學家曾經相信彗星——因為它們大部分留在遙遠的奧爾特雲，在那裡溫度接近絕對零度——一定是構成外層行星的材料的原始殘留物。
結合後文能看出，這裡天文學家覺得彗星是【原始殘留物】的原因在於：它們所處的地方（奧爾特雲）遙遠並且溫度低，所以，天文學家們推測奧爾特雲那地方變化不大，沒有受到什麼外界干擾。因此，那裡的彗星就能夠保留原始形態，不怎麼被外界條件改變。既然大部分彗星都在那地方，所以基本上大部分彗星都可以看作維持了原始形態，故而可以叫做“原始殘留物”。
這只是天文學家的推測，不一定符合事實。
The conceptual shift away from seeing comets as pristine relics began in the 1970s, when laboratory simulations revealed there was sufficient ultraviolet radiation reaching comets to darken their surfaces and there were sufficient cosmic rays to alter chemical bonds or even molecular structure near the surface.
第2句：從二十世紀七十年代開始，從實驗室模擬發現有足夠的紫外線輻射到達彗星使它們的表面變暗，並且有足夠的宇宙射線來改變（彗星）表面附近的化學鍵甚至分子結構，將彗星看作原始殘留物的概念開始轉變。
從二十世紀七十年代開始，老觀點被動搖了。因為發現了新證據：
1、實驗室模擬發現，有足夠的紫外線輻射到達彗星使它們的表面變暗——換句話說，彗星並不是免受外界干擾摧殘的，因此不能看成保留了原始狀態的【原始殘留物】；
2、有足夠的宇宙射線來改變（彗星）表面附近的化學鍵甚至分子結構——和第一點說明的問題一樣，還能進一步說明彗星被外界改變的程度更加加深了。第一點只說明表面色澤被改變（變暗），第二點說明連表面的化學鍵甚至分子結構都能被改變。
Nevertheless, astronomers still believed that when a comet approached the Sun— where they could study it—the Sun’s intense heat would remove the corrupted surface layer, exposing the interior.
第3句：儘管如此，天文學家仍然認為，當一顆彗星靠近太陽時——在那兒他們才可以研究它——太陽強烈的熱量會去除腐蝕的表層，從而暴露出內部。
天文學家一般等到彗星到哪兒才能研究彗星？——當它靠近太陽時。
上一句說明，起碼彗星表面不能當作【原始殘留物】看待：表面已經被宇宙射線、紫外線腐蝕了。
這一句，科學家又給彗星找回了點被研究的價值：雖然表層被腐蝕了，但內部可能還是完好的嘛，那內部還是可以當作【原始殘留物】來研究的。
什麼時候彗星會露出內部呢？
當它靠近太陽時，太陽強烈的熱量會去除腐蝕的表層，從而暴露出內部——這就是研究的好契機。
About the same time, though, scientists realized comets might contain decaying radioactive isotopes that could have warmed cometary interiors to temperatures that caused the interiors to evolve.
第4句：然而與此同時，科學家們認識到彗星可能含有會衰退的放射性同位素，這些放射性同位素可能會將彗星的內部溫度升高到導致內部進化的溫度。
然而，又反轉了。這次科學家們又發現，內部也不一定是原始狀態。
彗星可能含有會衰退的放射性同位素——這些放射性同位素可能會將彗星的內部溫度升高——升高到導致彗星內部進化的溫度——彗星的內部可能也改變了。
得，想把彗星作為【原始殘留物】來研究看來是沒戲了。
Consider each of the choices separately and select all that apply.
分別考慮每個選項並選擇所有適用的選項。
1. According to the passage, astronomers recognize which of the following as being liable to cause changes to comets?
根據這篇文章，天文學家承認以下哪一項可能導致彗星變化？
A. cosmic rays
A.宇宙射線
第2句，改變表面。A對。
B. radioactive decay
B.放射性衰變
第4句，decaying radioactive isotopes衰退的放射性同位素，就是改變彗星內部的【放射性衰變】。B對。
C. ultraviolet radiation
C.紫外線輻射
第2句，改變表面。C對。
第二題解析略。需要請留言。
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