共用题干 第三篇“Life Form Found" on Saturn ' s TitanScientists say they have discovered hints of alien life on the Saturn'5 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,wheretemperatures are around minus 17 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'5 surface and much too cold to support life as we know it.Scientists had expected the Sun'5 interactions with chemicals in the atmosphere to produce a coating of acetylene on Titan'5 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 explana-tion,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-bio- logical 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.Methane-based life on its biggest moon. C.A new moon moving around it. D.Earthlike life on its biggest moon.正确答案B
