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　　Scientists say they have discovered hints of alien life on the Saturns 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 Saturns biggest moon.

　　Scientists have reportedly discovered clues showing primitive alien beings arebreathing in Titans dense atmosphere filled with hydrogen.

　　They argue that hydrogen gets absorbed before hitting Titans planet-like surface covered with methane lakes and rivers. This,they say,points to the existence of somebugs consuming the hydrogen at the surface of the moon less than half the size of the Earth.

　　We suggested hydrogen consumption because its 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,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 Titans surface and much too cold to support life as we know it.

　　Scientists had expected the Suns interactions with chemicals in the atmosphere to produce a coating of acetylene on Titans surface. But Cassini detected no acetylene on the surface.

　　The absence of detectable acetylene on the Titans 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 Saturns 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.What doesthis 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.

　　Scientists say they have discovered hints of alien life on the Saturns 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 Saturns biggest moon.

　　Scientists have reportedly discovered clues showing primitive alien beings arebreathing in Titans dense atmosphere filled with hydrogen.

　　They argue that hydrogen gets absorbed before hitting Titans planet-like surface covered with methane lakes and rivers. This,they say,points to the existence of somebugs consuming the hydrogen at the surface of the moon less than half the size of the Earth.

　　We suggested hydrogen consumption because its 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,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 Titans surface and much too cold to support life as we know it.

　　Scientists had expected the Suns interactions with chemicals in the atmosphere to produce a coating of acetylene on Titans surface. But Cassini detected no acetylene on the surface.

　　The absence of detectable acetylene on the Titans 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 Saturns 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.What doesthis 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.