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TOEFL listening: A university lecture by a professor on the possibility of life on Mars

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50 TOEFL lectures
50 TOEFL lectures
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    50 TOEFL lectures   50 TOEFL conversations

    TOEFL Listening Comprehension Transcript

    Listen to part of a university lecture by a professor on the possibility of life on Mars.

    Professor: The planet Mars has been in the news recently, because it is going to pass very close to us soon. So this might be a good time to talk about the Red Planet.

    The possibility of there being life on Mars has been a topic of speculation for more than a hundred and fifty years- ever since its "canals" were mapped by an Italian astronomer, Giovanni Schiparelli, back in 1877. He drew the first reasonably realistic map of Mars, and it included a system of "canali' across its surface. In Italian, "canali" just means "channels"- it doesn't imply artificial structures at all- but the idea caught on, and it was gradually developed, with a lot of help from fertile imaginations, into the concept of a complex, planet-wide irrigation system. Although most serious astronomers did not buy into this, the idea of an Earth-like planet- perhaps colder and dryer, and probably without any Martians- endured right up to the beginning of the Space Age, when Mars was still thought to have polar ice caps and a reasonable atmosphere. It also showed seasonal color changes that some thought could be some kind of primitive plant life blooming.

    But in the 1960s, NASA's Mariner missions sent back images of something very different, of a cratered, moon-like Mars. Both the polar caps and the atmosphere turned out to be almost pure CO2, and the density of its atmosphere was only one-hundredth of Earth's. And the "blooming plant life" turned out to be only a lot of dust, blown around by strong seasonal winds.

    In some ways, though, Mars became more interesting. It had giant volcanos. It had a vast maze of canyons. And it showed evidence of having had flowing water on its surface sometime in its distant past.

    And the possibility of living organisms on Mars could still not be ruled out. Now, you should realize that it is a lot easier to prove that something exists than it is to prove that something doesn't exist. Once you've discovered something, you've got it in the bag- but it's harder to prove that something's not there, because no matter how much you look without finding it, it could still be hiding under the next rock. So scientists continue to look under the Martian rocks.

    The Viking mission in 1976 included three biological experiments- the Labelled Release experiment, the Pyrolytic Release experiment, and the Gas Exchange experiment.

    The Labelled Release experiment mixed a Martian soil sample with water and Carbon-14 marked organic materials, and if any micro-organisms ate the materials, Carbon-14 would appear in any released gases. The Pyrolytic Release experiment simply incubated an unadulterated soil sample in a simulated Martian atmosphere containing Carbon-14 marked CO2. Then the sample was heated to break down- or pyrolytize- any organic material that'd been produced, and again the gases were tested for Carbon-14. And finally, the Gas Exchange experiment put a Martian soil sample into an organic "chicken soup" of marked chemicals, and if any of these were consumed by micro-organisms, the Carbon-14 would again be detected in the released gases.

    None of these experiments were successful. That is, none of them produced clear results detecting life forms. Most scientists now agree that the experiments were flawed- all of the results can be explained as purely chemical processes that do not require the presence of life. However, there is now evidence, as I said, that Mars once had significantly more water, and now scientists are considering the possibility that the planet once has life- but that it went extinct when conditions on Mars got worse.

    A meteorite called ALH84001- catchy name, eh?- was discovered in Antarctica in 1984, and it is one of a dozen meteorites that scientists believe, because of their age and composition, came from Mars. But ALH84001 is special- it carries with it three pieces of evidence for life on Mars. First, it carries polyclitic aromatic hydrocarbons, which is something that dead organisms often decompose into. And second, it has tiny carbonate globules that resemble mineral alterations that primitive Earth bacteria cause. And then third, it carries very tiny- 10- to 100-nanometer- ovoids that may actually be fossil bacteria. And all three of these pieces of evidence lie within a few micrometers of each other in a crack in the meteorite's surface. Together they are strong evidence for the existence of life in Mars's past.

    But the real research on this is just beginning. Maybe we'll learn more when we've heard back from NASA's Phoenix mission.

    50 TOEFL lectures
    50 TOEFL lectures
    A great variety of English listening comprehension tests that will help you increase your TOEFL test score.
  • based on TOEFL academic lectures
  • written and recorded by experienced US authors and voice-over specialists
  • 50 TOEFL lectures   50 TOEFL conversations

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