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  3. "Life Form Found"on Saturn’s Titan Scientists say they have discovered hi...A

"Life Form Found"on Saturn’s Titan Scientists say they have discovered hi...A

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问题 "Life Form Found"on Saturn’s Titan
Scientists say they have discovered hints of alien (外星球的)life on the Saturn’s moon. The discovery of a sort of life was annpunced after researchers at the US space agency, NASA, analyzed data from spacecraft Cassini, which pointed to the existence of methane (甲烷)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 . 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 methane-based 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 (乙炔)on Titan’s surface. But Cassini detected no acetylene onthesurface.
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."  
What do scientists claim to have found about Saturn?
A Water-based life on it.
B
选项 A new moon moving around it.
C Earthlike life on its biggest moon.
D Methane-based life on its biggest moon .
答案D
解析根据第一段最后一句：... data... Which pointed to the existence of methane (甲烷)- based form of life on Saturn’s biggest moon.
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