NARRATOR But what about Mars?

Organic compounds have yet to be found here, but scientists are searching the planet for the other preconditions of life.

There have been many missions to Mars and nearly all suggest that water once flowed on the surface. These detailed images from satellites orbiting Mars reveal vast canyons blasted out by epic floods and valleys carved by raging rivers. But the evidence indicates that all this water disappeared from the surface billions of years ago, as Mars cooled down and lost its atmosphere.

But on May 25, 2008, a spacecraft called Phoenix touches down near Mars' north pole. Digging a few inches down, it exposes a white material that vaporizes after a few days.

Soil analysis reveals it is water ice.

NARRATOR Satellites analyze radar waves bouncing back from both polar caps. They reveal that beneath a layer of frozen carbon dioxide there is a lot of water ice. If it all melted, it would cover the whole planet in an ocean more than 80 feet deep.

NARRATOR The same satellites orbiting Mars are discovering that buried ice is also widespread beneath the desert floors.

CHRIS MCKAY (ASTROBIOLOGIST, NASA AMES RESEARCH CENTER) When we look at Mars, we see what looks like a desert world, with no water, but, in fact, Mars has lots of water. It's ice; Mars is an ice cube covered with a layer of dirt.

NARRATOR But this doesn't mean that finding life here is imminent. Ice doesn't melt the same way on Mars as it does on Earth. The atmospheric pressure here is 150 times lower than ours. It's impossible for water to exist as a liquid at the surface.

CHRIS MCKAY Ice on Mars behaves like dry ice does on Earth. A piece of dry ice, on Earth, goes directly from the solid ice to vapor. It doesn't form a liquid. That's why we call it dry ice.

On Mars, the pressure is so low that water ice does the same thing.

NARRATOR No liquid water on the surface of Mars today means that vital chemical reactions cannot take place. It seems impossible that life could exist there. But could it exist in the buried ice itself?

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