Scientists say they have discovered hints of alien life on the Saturn’s moon. The discovery of a sort of life was announced after researchers at the US space agency,NASA,analyzed data from spacecraft Cassini,which pointed to,the existence of methane-based form of life on Saturn’s biggest moon.
　　 Scientists have reportedly discovered clues showing primitive alien beings are"breathing" in Titan’s dense atmosphere filled with hydrogen.
　　 They argue that hydrogen gets absorbed before hitting Titan’s planet-like surface covered with methane lakes and rivers. This,they say,points to the existence of some"bugs" consuming the hydrogen at the surface of the moon less than half the size of the Earth.
　　 "We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan,similar to the way we consume oxygen on Earth,"says NASA scientist Chris McKay."If these signs do turn out to be a sign of life,it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth."
　　 To date,scientists have not yet detected this form of life anywhere,though there are liquid- water-based microorganisms on Earth that grow well on methane or produce it as a waste product. On Titan, where temperatures are around 90 Kelvin(minus 290 degrees Farenheit),a methanebased organism would have to use a substance that is liquid as its medium for living processes, but not water itself. Water is frozen solid on Titan’s surface and much too cold to support life as we know it.
　　 Scientists had expected the Sun’s interactions with chemicals in the atmosphere to produce a coating of acetylene on Titan’s surface. But Cassini detected no acetylene on the surface.
　　 The absence of detectable acetylene on the Titan’s surface can very well have a non-biological explanation,said Mark Allen,a principal investigator of the NASA Titan team.
　　 "Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,"Allen said. "We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process,without biology,can explain these results."
　　1 .What have scientists found about Saturn?
　　 A They have found a new moon orbiting Saturn.
　　B They have found methane-based life on Saturn.
　　C They have found methane-based life on Titan.
　　D They have found earthlike life on a Saturn’s moon.
　　2. What do scientists say about Titan?
　　A There are life clues there.
　　B There is acetylene there.
　　C Water on Titan exists in the form of ice.
　　D Rivers and lakes there contain life formls.
　　3. To date,scientists have not yet detected this form of life.(paragraph 5)What does"this form
　　of life" refer to?
　　A Water-based life.
　　B Methane-based life.
　　C Liquid-water-based microorganisms.
　　D Gas-based life.
　　4. What can be inferred from what Allen said?
　　A Scientists have different arguments over whether there is life on Titan.
　　B Scientists all agree that there is life on Titan.
　　C Scientists all suggest that a biological explanation is reasonable.
　　D Scientists all agree that a non-biological chemical reaction is a possible explanation.
　　5. Which of the following can replace the title of this passage?
　　A Earthlike Living Beings Found on Titan.
　　B Finding of One More Moon of Saturn.
　　C Titan,a New Satellite Found.
　　D A different Life Form, a Possibility.

答案詳解：
1. C 短文的第一段提供了答案。該段告訴讀者，科學家在土星的衛星土衛六（ Titan ）上發現了生命的跡象，這些生命的存在是以甲烷為基礎的。
　　2.A根據第二段“Scientists have reportedly discovered clues showing primitive alien beings are
　　breathing’ in Titan’s dense atmosphere filled with hydrogen.”，說明土衛六（Titan）有生命跡象。因此A是答案。
　　3. B this form of life 指的是土衛六（ Titan ）上以甲烷為生命基礎呼吸氫氣的生物，即該文討論的主題。
　　4.A科學家至今對土衛六是否存在生物有不同的觀點和解釋。Allen 說要做出土衛六上有生
　　物的結論，首先必須對Titan 上沒有生物的觀點做出回應；要排除土衛六上可能沒有生物的觀點，我們還須付出努力。所以只有A是正確選擇。
　　5.D該短文的主題是，科學家在土衛六（Titan）上發現了一種與地球生物不同的生命形式的跡象，即以甲烷為生命基礎呼吸氫氣的生物，但科學家還未能最后驗證這一發現。第四段中的一個句子能夠揭示該短文的主題：“If these signs do turn out to be a sign of life, it would bedoubly exciting because it would represent a second form of life independent from water-based life on Earth.”