英语听力：自然百科 探索世界奥秘之Life On Mars(火星生命)Unit08（在线收听）

But even if there had been water, life doesn't start overnight. Even a simple microbe is the product of complex chemical systems that take millions of years to evolve. Had the water lasted long enough for life to form?

Well, when we look at the history of life on Earth, it appears to start very quickly, maybe a hundred million years. Now that seems like a long time, but for a planet that's short. So I would guess that if there was water on Mars for a couple of hundred million years, then life had a good shot at getting started there.

And soon they found evidence that water had been there for millions of years. It was all because of one very important picture. Sent back in December 2000, it was a vast formations of sedimentary rock. Sediment is basically made of sand, and could only have been deposited over millions of years by a huge body of water, like a lake or an ocean. It showed that there had not only been masses of water, but that it had been around easily long enough for life to form. The sediments on the surface are now dry and exposed. If they ever had contained life, it cannot possibly have survived. However, scientists hope that microbes deeper down might still be alive, frozen in the ground.

To find the actual organic remains of a Martian organism, we are gonna need to go to the frozen material. In the ice life might have been preserved, frozen in a state of suspended animation.

No one at this stage could know if microbes, formed millions of years ago, could have survived on Mars. But there are indications that this really is possible.

Antarctica, the closest place on Earth to conditions on Mars. More extreme even than Iceland, it's the coldest place on our planet. Until a few years ago, no one thought that anything could survive being deep frozen for millions of years. But research here has helped change that.

sedimentary rock: Rock formed at or near the Earth's surface by the accumulation and lithification of fragments of preexisting rocks or by precipitation from solution at normal surface temperatures. Sedimentary rocks can be formed only where sediments are deposited long enough to become compacted and cemented into hard beds or strata. They are the most common rocks exposed on the Earth's surface but are only a minor constituent of the entire crust. Their defining characteristic is that they are formed in layers. Each layer has features that reflect the conditions during deposition, the nature of the source material (and, often, the organisms present), and the means of transport