Sometime after midnight on February 8,1969, a large, bright meteor entered Earth's atmosphere and broke into thousands of pieces, plummeted to the ground, and scattered over an area 50 miles long and 10 miles wide in the state of Chihuahua in Mexico. The first meteorite from this fall was found in the village of Pueblito de Allende. Altogether, roughly two tons of meteorite fragments were recovered, all of which bear the name Allende for the location of the first discovery.
Individual specimens of Allende are covered with a black, glassy crust that formed when their exteriors melted as they were slowed by Earth's atmosphere. When broken open, Allende stones are revealed to contain an assortment of small, distinctive objects, spherical or irregular in shape and embedded in a dark gray matrix (binding material), which were once constituents of the solar nebula—the interstellar cloud of gas and dust out of which our solar system was formed.
The Allende meteorite is classified as a chondrite. Chondrites take their name from the Greek word chondros—meaning "seed"—an allusion to their appearance as rocks containing tiny seeds. These seeds are actually chondrules: millimeter-sized melted droplets of silicate material that were cooled into spheres of glass and crystal. A few chondrules contain grains that survived the melting event, so these enigmatic chondrules must have formed when compact masses of nebular dust were fused at high temperatures—approaching 1,700 degrees Celsius—and then cooled before these surviving grains could melt. Study of the textures of chondrules confirms that they cooled rather quickly, in times measured in minutes or hours, so the heating events that formed them must have been localized. It seems very unlikely that large portions of the nebula were heated to such extreme temperatures, and huge nebula areas could not possibly have lost heat so fast. Chondrules must have been melted in small pockets of the nebula that were able to lose heat rapidly. The origin of these peculiar glassy spheres remains an enigma.
Equally perplexing constituents of Allende are the refractory inclusions: irregular white masses that tend to be larger than chondrules. They are composed of minerals uncommon on Earth, all rich in calcium, aluminum, and titanium, the most refractory (resistant to melting) of the major elements in the nebula. The same minerals that occur in refractory inclusions are believed to be the earliest-formed substances to have condensed out of the solar nebula. However, studies of the textures of inclusions reveal that the order in which the minerals appeared in the inclusions varies from inclusion to inclusion, and often does not match the theoretical condensation sequence for those metals.
Chondrules and inclusions in Allende are held together by the chondrite matrix, a mixture of fine-grained, mostly silicate minerals that also includes grains of iron metal and iron sulfide. At one time it was thought that these matrix grains might be pristine nebular dust, the sort of stuff from which chondrules and inclusions were made. However, detailed studies of the chondrite matrix suggest that much of it, too, has been formed by condensation or melting in the nebula, although minute amounts of surviving interstellar dust are mixed with the processed materials.
All these diverse constituents are aggregated together to form chondritic meteorites, like Allende, that have chemical compositions much like that of the Sun. To compare the compositions of a meteorite and the Sun, it is necessary that we use ratios of elements rather than simply the abundances of atoms. After all, the Sun has many more atoms of any element, say iron, than does a meteorite specimen, but the ratios of iron to silicon in the two kinds of matter might be comparable. The compositional similarity is striking. The major difference is that Allende is depleted in the most volatile elements, like hydrogen, carbon, oxygen, nitrogen, and the noble gases, relative to the Sun. These are the elements that tend to form gases even at very low temperatures. We might think of chondrites as samples of distilled Sun, a sort of solar sludge from which only gases have been removed. Since practically all the solar system's mass resides in the Sun, this similarity in chemistry means that chondrites have average solar system composition, except for the most volatile elements; they are truly lumps of nebular matter, probably similar in composition to the matter from which planets were assembled.
Paragraph 1: Sometime after midnight on February 8,1969, a large, bright meteor entered Earth's atmosphere and broke into thousands of pieces, plummeted to the ground, and scattered over an area 50 miles long and 10 miles wide in the state of Chihuahua in Mexico. The first meteorite from this fall was found in the village of Pueblito de Allende. Altogether, roughly two tons of meteorite fragments were recovered, all of which bear the name Allende for the location of the first discovery.
1.The word "location' in the passage is closest in meaning to
O sight
O sake
O success
O place
2.Which of the following can be inferred from paragraph 1 about the large meteor that entered Earths atmosphere on February 8, 1969?
O It was almost ten miles wide.
O It was the biggest meteor ever to hit Mexico.
O It weighed more than two tons.
O It broke into more pieces than most meteors do.
Paragraph 2: Individual specimens of Allende are covered with a black, glassy crust that formed when their exteriors melted as they were slowed by Earth's atmosphere. When broken open, Allende stones are revealed to contain an assortment of small, distinctive objects, spherical or irregular in shape and embedded in a dark gray matrix (binding material), which were once constituents of the solar nebula—the interstellar cloud of gas and dust out of which our solar system was formed.
3.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
O Allende meteorites were formed when constituents of the interstellar cloud of gas and dust got trapped inside small, roughly spherical objects and these objects became bound together in a dark gray matrix.
O Inside Allende meteorites is a dark gray matrix that binds together small spherical or irregular objects formed from the interstellar cloud of gas and dust out of which the solar system was made.
O By breaking open Allende meteorites, scientists were able to find out what the solar nebula was made of.
O Allende meteorites were filled with material formed almost entirelyfrom interstellar gas and dust.
Paragraph 3: The Allende meteorite is classified as a chondrite. Chondrites take their name from the Greek word chondros—meaning "seed"—an allusion to their appearance as rocks containing tiny seeds. These seeds are actually chondrules: millimeter-sized melted droplets of silicate material that were cooled into spheres of glass and crystal. A few chondrules contain grains that survived the melting event, so these enigmatic chondrules must have formed when compact masses of nebular dust were fused at high temperatures—approaching 1,700 degrees Celsius—and then cooled before these surviving grains could melt. Study of the textures of chondrules confirms that they cooled rather quickly, in times measured in minutes or hours, so the heating events that formed them must have been localized. It seems very unlikely that large portions of the nebula were heated to such extreme temperatures, and huge nebula areas could not possibly have lost heat so fast. Chondrules must have been melted in small pockets of the nebula that were able to lose heat rapidly. The origin of these peculiar glassy spheres remains an enigma.
4.The word "allusion" in the passage is closest in meaning to
O addition
O modification
O resemblance
O reference
5.The word "enigmatic" in the passage is closest in meaning to
O dangerous
O mysterious
O interesting
O surprising
6.According to paragraph 3, what does the presence of grains inside some of the chondrules indicate?
O The chondrules were formed of silicate material.
O The chondrules were formed at high temperatures and then cooled rapidly.
O The grains were formed in huge areas of the solar nebula
O The grains were formed after the chondrules were fused together into chondrites.
Paragraph 4: Equally perplexing constituents of Allende are the refractory inclusions: irregular white masses that tend to be larger than chondrules. They are composed of minerals uncommon on Earth, all rich in calcium, aluminum, and titanium, the most refractory (resistant to melting) of the major elements in the nebula. The same minerals that occur in refractory inclusions are believed to be the earliest-formed substances to have condensed out of the solar nebula. However, studies of the textures of inclusions reveal that the order in which the minerals appeared in the inclusions varies from inclusion to inclusion, and often does not match the theoretical condensation sequence for those metals.
7.According to paragraph 4, all of the following are true about the minerals found in the refractory inclusions EXCEPT:
O These minerals are among the most resistant to melting of all the major elements in the solar nebula.
O These minerals are believed to be some of the first elements to have condensed out of the solar nebula.
O These minerals are among the least commonly found elements on Earth.
O These elements occur in the order that scientists would have predicted.
Paragraph 5: Chondrules and inclusions in Allende are held together by the chondrite matrix, a mixture of fine-grained, mostly silicate minerals that also includes grains of iron metal and iron sulfide. At one time it was thought that these matrix grains might be pristine nebular dust, the sort of stuff from which chondrules and inclusions were made. However, detailed studies of the chondrite matrix suggest that much of it, too, has been formed by condensation or melting in the nebula, although minute amounts of surviving interstellar dust are mixed with the processed materials.
8.The word "pristine" in the passage is closest in meaning to
O pure
O solid
O ordinary
O trapped
9.According to paragraph 5, which of the following is indicated by studies of the mixture holding the inclusions together?
O Large amounts of this material were formed by condensation or melting in the nebula.
O This material contains more iron and iron sulfide than had previously been thought.
O This material is very similar to the material from which the refractory inclusions are made
O The grains in this material are made from the same elements as chondrules are.
Paragraph 6: All these diverse constituents are aggregated together to form chondritic meteorites, like Allende, that have chemical compositions much like that of the Sun. To compare the compositions of a meteorite and the Sun, it is necessary that we use ratios of elements rather than simply the abundances of atoms. After all, the Sun has many more atoms of any element, say iron, than does a meteorite specimen, but the ratios of iron to silicon in the two kinds of matter might be comparable. The compositional similarity is striking. The major difference is that Allende is depleted in the most volatile elements, like hydrogen, carbon, oxygen, nitrogen, and the noble gases, relative to the Sun. These are the elements that tend to form gases even at very low temperatures. We might think of chondrites as samples of distilled Sun, a sort of solar sludge from which only gases have been removed. Since practically all the solar system's mass resides in the Sun, this similarity in chemistry means that chondrites have average solar system composition, except for the most volatile elements; they are truly lumps of nebular matter, probably similar in composition to the matter from which planets were assembled.
10.In paragraph 6, why does the author mention that "the Sun has many more atoms of any element, say iron, than does a meteorite specimen"?
O To show how difficult it is to compare the composition of a meteorite with that of the Sun
O To explain why a comparison of the compositions of a meteorite and of the Sun has to be done in terms of ratios of elements
O To identify the most common element in both the Sun and meteorite specimens
O To emphasize how much largerthe Sun is than any meteorite specimen is
11.According to paragraph 6, the composition of chondritic meteorites differs from the composition of the Sun primarily in
O containing nebular matter
O containing many fewer atoms of iron
O the relative amount of volatile elements
O the ratio of iron to silicon
12.According to paragraph 6, what is the significance of the similarity in composition between chondrites and the Sun?
O It indicates what the matter from which planets were formed was probably like.
O It may explain howthe Sun originally developed.
O It helps scientists estimate the variations in the chemical composition of different meteors.
O It suggests that most meteorites may contain large quantities of volatile elements.
Equally perplexing constituents of Allende are the refractory inclusions: irregular white masses that tend to be larger than chondrules. ■ They are composed of minerals uncommon on Earth, all rich in calcium, aluminum, and titanium, the most refractory (resistant to melting) of the major elements in the nebula. ■ The same minerals that occur in refractory inclusions are believed to be the earliest-formed substances to have condensed out of the solar nebula. ■ However, studies of the textures of inclusions reveal that the order in which the minerals appeared in the inclusions varies from inclusion to inclusion, and often does not match the theoretical condensation sequence for those metals. ■
13.Look at the four squares [■] that indicate where the following sentence could be added to the passage
It is therefore still unclear if all inclusions were formed in the same way.
Where would the sentence best fit?
14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Studies of the Allende meteorite provided information about the composition of chondritic meteorites and their possible origin.
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Answer Choices
O When Allende entered Earth's atmosphere, it broke into thousands of pieces called chondrites because they look like glassy, black seeds.
O The mineral content of chondrules suggests that they were probably formed in isolated regions of the nebula that remained much hotter than the rest.
O Chondrules are tiny, millimeter-sized drops of silicate materials that probably formed when lumps of nebular dust were fused at extremely high temperatures and then quickly cooled.
O Irregularly shaped inclusions in Allende are composed of minerals that are resistant to melting and are believed to be the earlest minerals to have condensed out of the nebula.
O The matrix that holds the chondrules and inclusions together in Allende consists mainly of grains of nebular dust that were trapped inside the meteor before they could be melted.
O Except for being depleted in volatile elements, chondritic meteorites are probably very similar in composition to the matter from which planets were assembled.
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在1969年2月8日子夜后的某一時(shí)刻，一顆巨大明亮的流星進(jìn)入地球大氣層，碎成無數(shù)的碎塊，墜落到地面，散布在墨西哥奇瓦瓦州境內(nèi)50英里長(zhǎng)10英里寬的區(qū)域內(nèi)。在皮柏里托•德•阿倫德村發(fā)現(xiàn)了這次墜落的第一塊隕石�？偣泊蠹s找到了2噸的隕星碎片，所有的碎片都是以首次發(fā)現(xiàn)的所在地阿倫德命名。
每塊阿倫德碎片樣本都覆蓋著一層黑色的、玻璃樣的熔殼，這層熔殼是在它們的外表面與地球大氣層摩擦減速中熔化形成的。把阿倫德隕石破開，發(fā)現(xiàn)里面含有各種各樣細(xì)小的、獨(dú)特的物體，這些物體呈球狀或者不規(guī)則狀，嵌在深灰色的基質(zhì)（結(jié)合物質(zhì)）中，它們?cè)翘栃窃?/span>——形成太陽系的由氣體和塵埃組成的星際云團(tuán)的一部分。
阿倫德隕星屬于球粒隕石。球粒隕石的名字是源于希臘語中的單詞“chondros”，意思是種子，這是指它們的外觀看起來仿佛是鑲嵌著細(xì)小的種子的巖石。這些種子實(shí)際上是隕石球粒：被冷卻成玻璃球和水晶球的硅酸鹽物質(zhì)的毫米大小的熔融液滴。少數(shù)隕石球粒含有未遭熔化的顆粒，所以這些神秘的隕石球�？隙ㄊ窃诮咏�1700攝氏度的高溫下熔化的星云塵埃致密團(tuán)塊中形成的，隨后這些團(tuán)塊在幸存的顆粒尚未熔化之前就冷卻了。對(duì)隕石球粒質(zhì)地的研究確認(rèn)它們的確是以極快的速度冷卻的，短則幾分鐘，長(zhǎng)則數(shù)小時(shí)，所以形成隕石球粒的高溫事件肯定是限于局部的。大塊的星云升到極端高的溫度是不大可能的，而且大塊星云不可能散熱這么快。隕石球粒肯定是在星云內(nèi)部能夠快速散熱的小型袋狀結(jié)構(gòu)處被熔化的。這些奇特的玻璃球的來源目前仍是未解之謎。
同樣令人困惑的是阿倫德隕星的成分是耐高溫的內(nèi)含物：比隕石球粒要大些的不規(guī)則的白色團(tuán)塊。它們是由地球上罕見的礦物質(zhì)組成的，富含鈣、鋁以及在星云中最耐高溫的（耐熔化的）主要元素鈦。這些出現(xiàn)在耐高溫內(nèi)含物中相同的礦物質(zhì)被認(rèn)為是在太陽星云中最早凝結(jié)而成的物質(zhì)。然而，對(duì)內(nèi)含物質(zhì)地的研究發(fā)現(xiàn)不同的內(nèi)含物中礦物質(zhì)出現(xiàn)的順序并不相同，往往與理論上這些金屬的凝結(jié)序列不一致。
阿倫德隕星里的隕石球粒和內(nèi)含物是由球粒隕石基質(zhì)結(jié)合到一起的，這是一種細(xì)�；旌衔�，主要是包括鐵顆粒和硫化鐵的硅酸鹽礦物。人們一度認(rèn)為這些基質(zhì)顆�？赡苁窃嫉男窃茐m埃，也就是形成隕石球粒和內(nèi)含物的物質(zhì)。不過對(duì)球粒隕石基質(zhì)的詳細(xì)研究表明多數(shù)基質(zhì)的確是由星云的凝結(jié)和熔融形成的，盡管在這些被處理過的物質(zhì)中還混有小部分殘留的星際塵埃。
所有這些各種各樣的組分被凝聚到一起形成了球粒隕石，就像與太陽具有很多相似化學(xué)組分的阿倫德隕星。為了比較隕星和太陽的組分，我們需要比較元素的比率，而不是簡(jiǎn)單地比較原子的豐度。畢竟，太陽含有的任何一種元素的原子數(shù)都要比一塊隕星樣品含有的多，但是兩者間的鐵和硅的比率可能是具有可比性的。結(jié)果發(fā)現(xiàn)它們?cè)诮M成上具有驚人的相似性。主要的差別是相比太陽，阿倫德隕星失去了大部分的揮發(fā)性元素，例如氫、碳、氧、氮以及惰性氣體。這些元素即使是在很低的氣溫下都是氣體狀態(tài)。我們或許會(huì)認(rèn)為球粒隕石是“蒸餾后的太陽”的樣本，一種除去氣體后的太陽沉淀物。由于太陽系的質(zhì)量幾乎都集中于太陽，這種相似的化學(xué)組成意味著除了大部分的揮發(fā)性元素外，球粒隕石具有正常的太陽系組成；它們是真正星云物質(zhì)的團(tuán)塊，很可能與形成行星的物質(zhì)具有相似的組成。
