Are we alone in the Universe? Or are there aliens somewhere in space? New evidence suggests not only might other life-forms be out there, they may even be living on the planet right next door to us - Mars.
Recent discoveries have shown that Mars has all the ingredients for life, including water. Now the Mars Odyssey probe, launched in April 2001, has detected huge frozen areas of permafrost, just like that found in the Antarctic on Earth.
According to astronomers, the position of this frozen slush could hold the key to Mars' mysterious water cycle. And the surface ice may hide something even more exciting below.
Today, Mars is a frozen desert - the average temperature is -70ÂºC. But long ago Mars was very different. The first clues came from Mariner 9, which sent back fuzzy images of the surface of Mars, revealing volcanoes, canyons and meandering valleys that looked like ancient rivers. Rivers form from rain water run-off, rain comes from clouds, and clouds mean an atmosphere. If there had been rivers on Mars, it meant the planet had once been warm and wet, like the Earth - the perfect conditions for life to evolve.
For thirty years scientists like Mike Carr have been poring over more and more detailed images of Mars, trying to piece together the planet's history. When the latest probe, Mars Global Surveyor, was launched three years ago, everyone hoped its high resolution electronic cameras would settle the debate about the ancient rivers.
Thousands of close up images were beamed back, revealing sections of the valleys in fantastic detail, but none of them were any use. Over the billions of years, the valleys had all been eroded and filled with sand. It was impossible to say how they had been formed. Then finally, the Mars Global Surveyor team noticed a tiny feature in one image, which convinced them that billions of years ago there were rivers and lakes on Mars.
But did these warm, wet conditions last long enough for life to evolve? An unusual lake in Turkey has convinced Scottish geologist, Mike Russell, that Martian lakes were once teeming with primitive bacteria. So if life did evolve, is it possible that something is still living on Mars now? It would have to survive millions of years deep frozen in the Martian permafrost.
Russian biologist David Gilichinsky has been studying how long microscopic life on Earth can survive frozen in permafrost. Each summer he travels to Siberia to drill into the permafrost.
So far he has managed to resuscitate bacteria and algae which have been frozen for three million years at temperatures below -10ÂºC. His latest results from Antarctica suggest bacteria may be able to survive even longer at even colder temperatures - perhaps thirty million years, at -20ÂºC. Somehow bacteria live off the tiny amounts of liquid water that exist even in permafrost this cold. So perhaps they could still be clinging on, below the surface of Mars.
Astronomer Bill Hartmann, believes that there may be far more going on beneath the surface of Mars than anyone suspected. Last year, Global Surveyor sent back astonishing images of gullies running down steep slopes on Mars. Bill is convinced these were carved by water erupting from underground aquifers, heated by volcanic activity deep below.
If he's right, then perhaps there has been life on Mars for billions of years, surviving undergound when the surface of the planet became the freeze dried desert it is now.
The search for life on Mars is moving underground. Over the next few years a series of missions to the Red Planet are planned, but any signs of life will be hidden away - buried deep, and hard to find. When will Mars give up its greatest secret?
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