英语听力：自然百科 探索世界奥秘之Life On Mars(火星生命)Unit05（在线收听）

William Boynton is professor of planetary sciences at the University of Arizona. And he's dedicated his life to looking for water on Mars.

The link between life and water is very strong. Even here in the desert, there's life all around us. Now when looking for life on Mars, we use our basis of knowledge on Earth. And on Earth we know life requires water. So on Mars, we're gonna be looking for the places where we can find water.

Just like the desert, Mars may look dry but there are signs of water there. It's been known for almost a century that there are icecaps at the Martian poles.

When we look up at Mars, we see a dry, dusty planet. It doesn't look like a place for life. But there's tiny amounts of water in the atmosphere. And we know there are ice concentrations in the poles. So we're actually very optimistic that because there is at least, a speck of water on Mars, there might be a speck of life.

However, while the poles contain water ice, temperatures there can reach a staggering minus 150 degrees centigrade which meant these were not good places to search for life.

The conditions there are way too extreme. It's really just too cold. We really need liquid water for life. What we want to find on Mars is some place where we can find water in the more temperate regions. What we ultimately need to find is liquid water.

But the first indications were that if life needed liquid water, then Mars was out of luck. Conditions all over the planet seemed too cold, there appeared to be no prospect of water anywhere. These photographs changed everything. Taken by the Viking Space Probe in 1976, they showed what looked like dried-up river valleys.

You can see one here. You can see there's a, a valley through here, you can see it branches, the tributaries. Here's one branch going off here with tributaries. So this looks very much like, er, a terrestrial river system.

If these were dried-up riverbeds, it meant that Mars must once have had the perfect conditions for life.

ice concentration: the ratio expressed in tenths describing the area of the water surface covered by ice as a fraction of the whole area