Scientists believe that conditions on Mars around 3.8 billion years ago were very similar to those of the early earth, when primitive organisms were spreading through our oceans. At that time, Mars would have been much warmer and wetter than it is today, with an atmosphere composed mainly of carbon dioxide, just like that of the earth at the time. Under these conditions, it is highly probable that life may have arisen on Mars as well.
But, even if life did not arise naturally on Mars, it does not mean that it could not have existed there. According to Professor Paul Davies of the University of Adelaide, Australia, life forms could have been transferred between the earth and Mars in wreckage (broke pieces) created by the impact of comet(彗星) and small planets on the surface of the two planets.
Even today, about 500 tons of material from Mars lands on earth every year. It is mainly in the form of the dust but occasionally a larger chunk(厚片，大块) strikes the earth. In 1911, a piece of Martian rock crashed in Egypt, killing a dog. According to Professor Davies, it is in these chunks of rock, which were much larger and more frequent in the past that life forms could have been transported from planet to planet.
“But how could these life forms have survived their journey through space?” says Paul Davies, “The difficulty in believing this theory is that a bacterium on its own in space has to struggle itself not only against cold but also against deadly cosmic (宇宙的) radiation. But wrapped in a rock the situation is different. A rock ten meters across would shield life inside it from a lot of radiation and the temperature might only be minus 10 or 20 degrees, the sort of thing we have on earth.”
Where do some scientists suppose life probably come from? (2’) (No more than 6 words)
What does Professor Paul Davies believe? (2’) (No more than 6 words)
Why could life survive when transferred from one planet to another? (3’) (No more than 12 words)
List at least two differences about Mars between the past and now. (3’) (No more than 16 words)