In all the universe, we know of just one world that hosts life: our own Earth. Yet right here in the solar system, several worlds seem to have the right ingredients for life. But it's likely to take decades to find out if anything inhabits those worlds.
The world that's intrigued us the longest, of course, is Mars. There's abundant evidence that it was once much warmer and wetter than it is today. Some of its water likely escaped into space, but a good bit of it is still around. It's frozen in the polar ice caps or in permafrost, or it's locked up in underground reservoirs.
It would have taken a long time for Mars to cool off and for its water to freeze or escape into space, though. So if life did arise on the planet, it would have had time to adapt to the changing conditions -- and it could still be around today. Microbes could live below the surface, away from the Sun's harmful rays. And larger organisms could live in the underground reservoirs.
No one knows what the odds of finding life on Mars really are. Most biologists are pessimistic. But here on Earth, life can exist just about anywhere -- from the Antarctic ice to scalding vents at the bottom of the oceans to fissures in the rocks deep below the surface. So until we've had a chance to explore Mars in greater detail, few scientists are quite ready to give up on the possibility of life on the Red Planet.
More about life in the solar system tomorrow.
Script by Damond Benningfield, Copyright 2010