英语听力：自然百科 探索世界奥秘之Alien Neighbors (寻访外星人) Unit04（在线收听）

...in Mars. Although it was completely wrong about Mars and everything he said about Mars practically was wrong. But, he built up interest in the planet which has carried over to this day. And, so, he was, he was quite useful to the Mars exploration program.'

Space exploration revealed Mars has little atmosphere and no liquid water on the surface. Yet, Its terrain appears to have been sculpted by water.

NASA geologist, Ken Herkenhoff, explains, 'It appears that there were great oceans on Mars sometime in the past and there is currently a debate raging as to exactly when these oceans occurred and how long they lasted and whether there were ice sheets that flowed on Mars.'

Today, the dry, cold Martian climate makes it inhospitable to life, but maybe not completely. An amazing survival story during the Apollo missions to our moon provided evidence that life can endure even harsher conditions.

Two and a half years after the SurveyorIII spacecraft was left on the moon, Apollo 12 astronauts returned, retrieved the lens of its TV-camera and with it a collection of what seemed to be alien bacteria. But they turned out to be stowaways, bacteria from earth, that had travelled on Surveyor to the moon and survived there for nearly three years, in an environment less hospitable than Mars. Here was the proof that life could endure, at least for a time, on a world other than our own.

What are the characteristics defining life as we know it? What else should seekers of life on other moons or planets be looking for? Living things move and grow. Life needs to take in nutrients and eliminate waste. And there is one other quality that is perhaps the most important indicator of all.

Oxford University biologist and author, Richard Dawkins: 'the most general characteristic's got to be reproduction, the property of heredity, everything else follows from that. Once you've got anywhere in the universe, heredity, the equivalent to DNA, doesn't have to be DNA, but the equivalent of something that makes copies of itself, genes in other words. Then, Darwinian Natural Selection, evolution and life will follow. Now , in practice, on this planet, life has all sorts of other properties like respiration and feeding and things like that, but the fundamental one is reproduction. '

Today, our planet is a haven for life, a far cry from its violent beginning. The young earth was radioactive, hot, volcanic with no oxygen. The atmosphere was cloudy, full of noxious gases, carbon dioxide, ammonia, methane. Red carpets of lava flowed over dark volcanic rock until water from planet's interior formed the vast oceans where life first appeared.

words in this passage

carry over:If something from one situation carries over or is carried over into another situation, it is allowed to exist in or affect the other situation: 延期至...

sculpt: to form into a particular shape雕刻, 造型

stowaway:a person who hides on a ship, aircraft or other vehicle偷渡者(here it means the bacteria from earth)

haven:a safe or peaceful place港口, 避难所,安全的地方

be a far cry from sth:to be completely different from something:大不相同的东西 (后接地点时也表示距离很远）

noxious gas: 有毒气体