Official 22 Set 3

  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6

Faint Young Sun Paradox

  • Q1
  • Q2
  • Q3
  • Q4
  • Q5
  • Q6
What is the main purpose of the lecture?
  • A. To compare solutions to the greenhouse-gas problem

  • B. To examine methods used to study star formation in other solar systems

  • C. To discuss evidence for liquid water on young Earth and Mars

  • D. To discuss attempts to solve a puzzle related to the Sun

显示答案 正确答案: D

我的笔记 编辑笔记

  • 原文
  • 译文
  • 查看听力原文


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

    MALE PROFESSOR:Today I want to talk about a paradox that ties in with the topic we discussed last time.We were discussing the geological evidence of water- liquid water- on Earth and Mars three to four billion years ago.So what evidence of a liquid water environment did we find in rock samples taken from the oldest rocks on Earth?

    MALE STUDENT:Uh, like pebbles... fossilized algae...

    MALE PROFESSOR:Right. And on Mars?

    FEMALE STUDENT:Dry channels...

    MALE PROFESSOR:Good... all evidence of water in liquid form-large quantities of it.Now remember when we talked about star formation, we said that as a star ages, it becomes brighter.Right? Hydrogen turns into helium, which releases energy.So our standard model of star formation suggests that the Sun wasn't nearly as bright three to four billion years ago as it is today, which means that temperatures on Earth and Mars would have been lower... which, in turn, suggests...?

    MALE STUDENT:There would have been ice on Earth or Mars...?

    MALE PROFESSOR:Correct... if the young Sun was much fainter and cooler than the Sun today, liquid water couldn't have existed on either planet.Now, this apparent contradiction between geologic evidence and the stellar evolution model became known as "the faint young Sun paradox."

    Now, there have been several attempts to solve this paradox.First there was the greenhouse-gas solution.Well, you're probably familiar with the greenhouse gas effect, so I won't go into details now.The idea was that trapped greenhouse gases in the atmospheres of Earth and Mars might have caused temperatures to rise enough to compensate for the low heat the young Sun provided.And so it would have been warm enough on these planets for liquid water to exist.So what gas do you think was the first suspect in causing the greenhouse effect?

    FEMALE STUDENT:Um, carbon dioxide, I guess... like today?

    In fact, studies indicate that four billion years ago, carbon dioxide levels in the atmosphere were much higher than today's levels.But the studies also indicate that they weren't high enough to do the job-make up for a faint Sun.Well, then, some astronomers came up with the idea that atmospheric ammonia may have acted as a greenhouse gas, but ammonia would have been destroyed by the ultraviolet light coming from the Sun, and it had to be ruled out, too.

    Another solution, mm, which was proposed much later, was that perhaps the young Sun wasn't faint at all. Perhaps it was bright.So it's called the "bright young Sun" solution, according to which the Sun would have provided enough heat for the water on Earth and Mars to be liquid.But how could the early Sun be brighter and hotter than predicted by the standard model?Well, he answer is "mass."

    MALE STUDENT:You mean the Sun had more mass when it was young?

    MALE PROFESSOR:Well, if the young Sun was more massive than today's, it would have been hotter and brighter than the model predicts.But this would mean that it has lost mass over the course of four billion years.

    FEMALE STUDENT:Is that possible?

    Actually, the Sun is constantly losing mass through the solar wind... a stream of charged particles constantly blowing off the Sun.We know the Sun's current rate of mass loss... but if we assume that this rate has been steady over the last four billion years, the young Sun wouldn't have been massive enough to have warmed Earth, let alone Mars- not enough to have caused liquid water.

    MALE STUDENT:Maybe the solar wind was stronger then?

    There is evidence that the solar wind was more intense in the past, but we don't know for sure how much mass our Sun's lost over the last four billion years.Astronomers tried to estimate what solar mass could produce the required luminosity to explain liquid water on these planets.They also took into account that with a more massive young Sun, the planets would be closer to the Sun than they are today.And they found that about seven percent more mass would be required.

    FEMALE STUDENT:So the young Sun had seven percent more mass than our Sun?

    MALE PROFESSOR:Well, we don't know.According to observations of young Sun-like stars, our Sun may have lost as much as six percent of its initial mass, which doesn't quite make it.On the other hand, this estimate is based on a small sample, and the "bright young Sun" solution is appealing.We simply need more data to determine the mass-loss rate of stars.So there's reason to believe that we'll get an answer to that piece of the puzzle one day.

  • 旁白:听下面一段天文学课堂的讲解。









    学生:呃,我想是 二氧化碳,和今天一样?








    的确有证据显示太阳风在以前是更加强烈的,但是我们不能确切的知道在过去这四十亿年中太阳到底减少了多少质量。天文学家试图去推测,太阳质量是多少才能产生需要的发光度来解释这两个星球上的液体水。他们还考虑到如果早期的太阳质量更大,那么这两个行星会比目前更靠近太阳。他们发现太阳的质量需要大 7%才成。

    学生:所以说早期的太阳比现在的太阳质量大 7%?

    教授:呃,这个我们还不知道。根据对其它早期类太阳恒星的观察,推断出我们的太阳可能比原始的质量小了 6%,但这并不能说明问题。另一方面,这一推测仅仅是基于小部分的样本得出的,而且“明亮的早期太阳”解决方案也很有可能。我们只是需要更多的数据来确定恒星的质量减少率。所以我们有理由相信总有一天我们能得到这个谜的答案。

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    原文定位Professor: Today, I want to talk about a paradox the ties in with the topic we discuss last time. We were discussing the geological evidence of water, liquid water on Earth and Mars three to four billion years ago. So, what evidence of a liquid water environment did we find in rock samples taking from the oldest rocks on Earth?

    We simply need more data to determine the mass loss rate of stars. So there’s reason to believe that we will get an answer to that piece of the puzzle one day.


    章前面一开始教授就说到paradox 悖论, 然后分别详细讲解了star formation sun mass 以及solar wind 最后提到solve the puzzle one day.

    D选项即为文章核心名词paradox 的同义替换以及solve the puzzle 的原文定位,是对全文主旨最准确的概括





Faint Young Sun Paradox