[ 2204听力 ] 三

Posted 2023-03-03 we1less

[ 第三次课 天文学 ]
Listen to part of a lecture in an Astronomy class 

(male professor) The surface of Mars today is mostly very cold, certainly too cold for there to be liquid water present. Yet, scientists have speculated for decades that in its early history, about four billion years ago, Mars was much warmer and wetter than it is today. Observations from Earth and from space have detected features in the Martian landscape indicating that Mars once had liquid water on its surface, features like interconnected valleys that appear to have been formed by rivers and also depressions, low lying areas filled with dust and sediment that look like ancient lake beds.

        wetter  adj. 潮的;湿的;潮湿的;有雨的;

        Observations  观察;观测;观测量;观察结果;观察报告

        Martian  火星的;来自火星的

        interconnected  v. (使类似的事物)相联系，相互联系，相互连接

        appear  v. 显得，看来，似乎;出现，呈现，显现;

        formed  v. (使)出现，产生;(使)形成;

        depression  n. 抑郁症;精神忧郁;抑郁;沮丧;消沉;萧条期;经济衰退;不景气;洼地;低气压

So, what might account for this warm wet Mars? Well, one hypothesis suggests that Mars’ atmosphere once had very high levels of carbon dioxide. OK, let’s take a step back for a minute. We know that carbon dioxide is a greenhouse gas and that greenhouse gases capture solar energy that would otherwise radiate back into space from a planet’s surface. So, these greenhouse gases trap that energy within the planet’s lower atmosphere and keep the surface of the planet warm. Perhaps warm enough for water to exist there in liquid form like it does on Earth. 

        hypothesis  n. 假设;(有少量事实依据但未被证实的)假说;(凭空的)猜想;猜测

        otherwise  adv. 否则;不然;除此以外;在其他方面;另;亦

        radiate  v. 辐射;(使品质或情感)显出;流露;(使热、光、能量)放射;

But some astronomers remained skeptical. You see another thing about carbon dioxide is that it is easily absorbed into water and eventually the carbon dioxide in the water may combine with other elements in the water to form minerals called carbonates. Carbonates then settle to the bottom of these bodies of water and form deposits there, chalk-like layers that can remain long after the water itself is gone. So, if the hypothesis is correct, if the atmosphere of Mars was once rich in carbon dioxide, there should be now huge deposits of carbonate on the planet. But nobody was able to detect that much carbonate there using the standard method for studying the chemical composition of a planet spectroscopy.

Astronomers use spectroscopy to study the color spectrum of light, in this case, the light being reflected from planets. By analyzing the reflected light, researchers can determine, among other things, the chemical composition of a planet. And a detailed spectroscopic analysis of light reflected from the surface of Mars did not detect huge amounts of carbonate. It found less than one-third of the carbonate expected to be present, if a carbon dioxide-rich atmosphere had, in fact, once existed there.

So, if we are not seeing lots of carbonate, would we be right to reject this hypothesis? Or could there be other explanations? Well, maybe it’s just much less simple than we thought. For example, we’ve detected a lot of sulfates on Mars. This means the planet’s atmosphere once contained a great deal of sulfur in various forms. And we know that atmospheric sulfur and carbon dioxide interact in all sorts of complex ways. So that may help explain why we’ve detected so much less carbonate than anticipated.

And there’s another issue. Spectroscopic analysis of reflected light only detects materials on the planet’s surface. But what if the carbonates are mixed together with dust and other sediments to form much less concentrated deposits? Or covered over and buried just under the surface. Spectroscopic analysis wouldn’t detect those carbonates. So, we need to gather evidence in other ways like from spacecraft that have actually landed on Mars. And, in fact, instruments on a spacecraft that landed near Mars’ North Pole confirmed that carbonates are present just beneath the surface there, which seems to support the idea of early carbonate formation on Mars.

The problem here, though, is that the instruments couldn’t measure the amount of carbonate present in the rocks. However, a recent analysis of data from one of the Mars rovers did result in an accurate measurement of the amount of carbonate in some rocks jutting up from the surface of Mars in an area called the Columbia Hills. The rover determined that the rocks average around twenty five percent carbonate. If such a high carbonate content were to prove common elsewhere on Mars that’ll be strong support for high levels of atmospheric carbon dioxide in Mars’ distant past. If not, if that rock was just an isolated occurrence, well, the search and the debate continue.

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Carbonates  碳酸盐;碳酸;碳酸酯;碳酸岩;碳酸盐类
Spectroscopy  光谱学;光谱;波谱学;光谱法

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1. What is the lecture mainly about?

A) A survey of various types of rock found on the surface of Mars

B) Recent improvements in analyzing the composition of planetary rocks

C) Measuring the water content of the atmosphere of Mars

D) Conflicting evidence about the early atmosphere of Mars

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2. According to the professor, what landscape features on the surface of Mars indicate that liquid water was probably once present there? [choose two answers]

A) A network of valleys resembling those formed by rivers

B) Rock formations created by floodwaters

C) Low areas filled with dust and sediment

D) Evidence of erosion on mountainsides caused by melting ice

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3. According to the professor, why did some astronomers question the hypothesis that there were high levels of greenhouse gases in Mars’ ancient atmosphere?

A) Spectroscopic analysis did not detect large amounts of carbonate on Mars’ surface.

B) There is no evidence that there were high levels of oxygen in Mars’ ancient atmosphere.

C) The relationship between warm temperatures and carbonate formation is unproven.

D) Spectroscopic analysis of other planets has not detected greenhouse gases.

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4. What does the professor say about the sulfur in Mars’ ancient atmosphere?\\

A) It may have been present in greater quantities than carbon dioxide

B) It may have been largely absorbed by liquid water on Mars’ surface.

C) It may have contributed to the ultimate cooling of Mars’ climate.

D) It may have affected the amounts of carbonate formed on Mars.

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5. What does the professor imply about the spectroscopic analysis of Mars?

A) It highlights the value of spectroscopy in astronomy.

B) It may be misleading because of the limits of spectroscopy.

C) Mars’ atmosphere may have interfered with accurate measurement for the study.

D) Mistakes were probably made in interpreting the data collected during the study.

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6. What does the professor imply about the information gathered by the Mars rover at the Columbia Hills?

A) It helps confirm the results of previous explorations.

B) It supports the hypothesis discussed by the professor.

C) It casts doubt on the claim that carbonate formation is related to temperature.

D) It will not be duplicated in the foreseeable future.