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.

答案详解：
1. C 短文的第一段提供了答案。该段告诉读者，科学家在土星的卫星土卫六（ Titan ）上发现了生命的迹象，这些生命的存在是以甲烷为基础的。
　　2.A根据第二段“Scientists have reportedly discovered clues showing primitive alien beings are
　　breathing’ in Titan’s dense atmosphere filled with hydrogen.”，说明土卫六（Titan）有生命迹象。因此A是答案。
　　3. B this form of life 指的是土卫六（ Titan ）上以甲烷为生命基础呼吸氢气的生物，即该文讨论的主题。
　　4.A科学家至今对土卫六是否存在生物有不同的观点和解释。Allen 说要做出土卫六上有生
　　物的结论，首先必须对Titan 上没有生物的观点做出回应；要排除土卫六上可能没有生物的观点，我们还须付出努力。所以只有A是正确选择。
　　5.D该短文的主题是，科学家在土卫六（Titan）上发现了一种与地球生物不同的生命形式的迹象，即以甲烷为生命基础呼吸氢气的生物，但科学家还未能最后验证这一发现。第四段中的一个句子能够揭示该短文的主题：“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.”