Water is one side of a "magic triangle" that comprises the main ingredients for making life or more exactly, life as we conceive of it.

After Earth, Mars is the only other planet of the solar system where there is a reasonably good chance of all the ingredients coming together.

So if something as humble as bacteria - even fossilised - is found on Mars, it implies we are not alone, for the universe with its billions of planets must be teeming with life.

David McKay an expert at NASA's Johnson Space Center has said there is already evidence of primitive bacterial life on Mars. He said it was in a meteorite found in Antarctica and believed to be from Mars.

The triangle's two other sides are an energy source and a small group of chemical elements to provide a starter kit for life.

Water nurtures life forms and enables them to evolve. The oceans that cover two thirds of the Earth's surface are believed to have been the cradle of life on this planet.

But not just any water will do.

Liquid water, rather than ice, is essential because it is the right temperature at which many elements dissolve in it and react. This does not happen with ice.

There is evidence that there is lots of ice on the surface at the Martian poles. And NASA's robotic rover Spirit has found a mineral, geothite, that is a strong hint that water existed on Mars.

But scientists are wondering what happened to the oceans that apparently ranged over the planet's surface. According to David Shuster of the California Institute of Technology and Benjamin Weiss of the Massachusetts Institute of Technology there has been no significant amounts of water on Mars for more than four billion years.

As for energy, Mars is well favoured in sunlight, the source that powers photosynthesis and thus, indirectly, all life on Earth.

True, as the fourth planet, and tens of millions of kilometres (miles) farther from the Sun than Earth, Mars is far colder and darker than here.

But compared with the outer planets, it is warm and light. At summer on the Martian equator, the temperature reaches a balmy 27 degrees Celsiusdegrees Fahrenheit).

And the planet turns quickly on its axis - a Martian day is only slightly longer than an Earth day.

This means that light and heat are distributed swiftly and evenly. That contrasts with planets where a "day" can last months in equivalent Earth terms, plunging half of the surface in prolonged frigidity.

Of the chemical building blocks, carbon, hydrogen, oxygen and nitrogen are the big ones because they can combine in thousands of ways to form compounds.

Carbon is especially important, for it can form long molecular chains that, on Earth, are the backbone of life here. Water will provide hydrogen and oxygen, its two elements.

Mars' exact geological composition is one of the big unknowns, since most evidence has been gleaned from visual and radar pictures, which give only indirect evidence.

The first mineral-tasting tests carried out by the US robotic explorer Spirit last week showed an abundance of olivine, a mineral with silicon, oxygen, iron and magnesium.

And the planet's ruddy surface is widely attributed to iron in rocks that, over billions of years, dissolved into the planet's surface water, oxidised and was then deposited by winds across the surface after the waters mysteriously disappeared.

But a caveat must be added.

Even if all three sides of the "magic triangle" exist on Mars, there is still no proof that life, even microscopic, exists there or indeed has ever existed.

And, almost as intriguingly, if any life there has vanished, what caused its extinction?

All rights reserved. � 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.

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