Official 30 Set 5

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  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
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Life on Mars

纠错
  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
What is the lecture mainly about?
  • A. Evidence proving that water was once present on Mars

  • B. Scientific analyses currently being done on mineral samples from Mars

  • C. Recent developments that could help determine whether life ever existed on Mars

  • D. An approach to determining whether amino acids on Earth originated on Mars

显示答案 正确答案: C

我的笔记 编辑笔记

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    NARRATOR:Listen to part of a lecture in an astronomy class.

    MALE PROFESSOR:There's been a lot of talk recently about life on Mars, at the level of microorganisms anyway, mainly because of a few important discoveries and inventions.For example, one major discovery was that at one point water was present on Mars.How do we know?Well, in 2004, an exploration robot discovered jarosite there.

    Jarosite is a yellowish-brown mineral with a crystalline structure that's also found on Earth.It contains iron, potassium, and hydroxide.The interesting thing is that on Earth at least, it needs highly acidic water to form.So we've got water... or had it at one point.And since most planetary scientists believe that water is essential to life, the presence of jarosite means that one prerequisite for life was once present on Mars.

    But there's another thing about jarosite: one step in its formation on Earth involves microorganisms.They actually speed up the formation of jarosite dramatically.Now, theoretically it is possible for jarosite to form without the help of biological life forms-but we don't really know for sure if this happens 'cause, well, because every corner of Earth has some form of biological life.But jarosite on Earth incorporates all kinds of microorganisms into its crystalline structure.So it's possible that if the jarosite on Mars was also formed with the help of microorganisms, we might be able to detect remnants of them in the samples we find.And we have instruments now that will enable us to try to do this.For example, there's a new instrument called the microfabricated organic analyzer, or M.O.A.

    The organic analyzer is an amazing tool.It will be able to collect soil samples and analyze them right there on Mars. Pure, untouched samples.It will let us eliminate the risk we would take of contaminating the samples if they were brought back to Earth.

    And what they'll look for specifically in the soil is amino acids.

    Amino acids, as you may know, are the building blocks of proteins.In fact, there are twenty standard amino acids involved in making proteins, and lots more that aren't.And here's the important thing: amino acids are what we call handed.They can exist in two forms, which are mirror images of each other. Like hands.Right and left hands have the same number of fingers in the same order, plus one thumb.But right and left hands are not the same; they're mirror images.Well, like hands, amino acids can be right- or left-handed.And the twenty that make up the proteins on Earth are all left-handed.

    Now, one reason the M.O.A., the organic analyzer, is so impressive is that it tests not just for the presence of amino acids, but also for the handedness of amino acids.If amino acids are found, it would be especially interesting if they show a prevalence of one type of handedness, either left, like amino acids on Earth, or right.See, other physical processes in space, processes that don't involve living organisms, can create amino acids.But the ones synthesized through abiotic processes, which is to say not involving microorganisms, occur in equal numbers of right-and left-handed.[Summarizing] So, a prevalence of left-handed amino acids would indicate they were biological in origin, which would be amazing.A prevalence of right-handed ones, well that would be really amazing.Because the organisms that created them would be unlike anything we have on Earth, which produce only left-handed ones.

  • 旁白:听一段天文学课程。

    教授:最近人们经常谈论火星上的生命(微生物层次),主要原因是一些重要的发现和发明。比如,一个重大发现是火星上有水。我们是怎么知道的?2004年,一个探索型机器人在火星上发现了黄钾铁矾。

    黄钾铁矾是一种黄棕色的矿物质,其晶体结构在地球上也有(相同的结构)。其中有铁、钾和氢氧化物。有趣的是,在地球上,需要酸性很强的水才能形成这种结构。所以我们有水,或一度曾有水。且由于大多数行星科学家都相信,水是生命之源,有黄钾铁矾说明火星曾具备生命的先决条件。

    但黄钾铁矾还有另一方面:在地球上,形成黄钾铁矾的其中一个步骤要有微生物参与。实际上,微生物能大幅加快黄钾铁矾的形成。理论上,在没有生物生命形式的帮助下,形成黄钾铁矾也是有可能的。但我们并不肯定这会不会发生,因为……因为地球上的每个角落都有某种形式的生物生命。但地球上的黄钾铁矾,其晶体结构中包含了各种各样的微生物。所以,假如火星上的黄钾铁矾也是在微生物的帮助下形成的,我们也许能在找到的样本中检测到残余物,这不是没有可能的事。我们现在有设备,能帮助我们做这件事。比如,现在有一种新型设备,叫微加工有机分析仪,又称M.O.A.

    有机分析仪是个神奇的工具。它能直接在火星上采集土壤样本并进行分析,样本是纯净的,原封未动的。分析仪能让我们排除风险,排除把样本带回地球,污染样本的风险。

    现在科学家特地要寻找的,就是土壤中的氨基酸。

    正如你们所知,氨基酸是蛋白质的基础材料。事实上,能形成蛋白质的标准氨基酸有20种,别的则不能。重要的是:氨基酸具有左右手性(特征)。氨基酸有两种存在形式,这两种形式是彼此的镜像,就像手一样。右手和左手都有相同数量的手指,顺序也一样,还有一个拇指。但右手和左手是不一样的;它们互为镜像。就像手一样,氨基酸也可以是“右撇子”或“左撇子”。地球上能形成蛋白质的20种氨基酸都是“左撇子”。

    M.O.A.,即有机分析仪如此让人惊叹的其中一个原因,是因为它不仅能测出有没有氨基酸,还能测出氨基酸的左右手性。假如真的发现有氨基酸,且在氨基酸中一种手性显得特别流行,左手性(就像地球上的氨基酸)或右手性,这就变得很有意思了。在太空中的其它物理过程,(即使是)没有生物体参与的过程,也能产生氨基酸。但是非生物过程合成的氨基酸,即没有微生物参与的氨基酸,会生成同样数量的“右撇子”和“左撇子”。那么,“左撇子”氨基酸占大多数的话,就说明了这些氨基酸根源上说是具有生物性的,这就太棒了。“右撇子”占大多数……这就更棒了!因为创造“右撇子”氨基酸的生物体和我们在地球上的所有生物体都不一样,地球上的生物体只会生成“左撇子”。

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  • 本题对应音频:
    3 感谢 不懂
    音频1
    解析

    题型分析:主旨题

    原文定位

    Theres been a lot of talk recently about life on Mars, at the level of microorganisms anyway, mainly because of a few important discoveries and inventions. For example, one major discovery was that at one point water was present on Mars. How do we know? Well, in 2004, an exploration robot discovered jarosite there.

    选项分析

    教授一开始there’s been a lot of talk 就引出关键信息 life on Mars,后面的 for example 举例里提到的水都是为了论证这一观点。

    C 选项与文意关键信息完全一致,是对全文主旨最准确的概括

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