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Last Updated: Saturday, 24 January, 2004, 18:14 GMT
Long history of water and Mars
By Dr David Whitehouse
BBC News Online science editor

The Hellas basin, Esa
A channel probably cut by water
The European Space Agency's Mars Express has returned the most detailed and spectacular views of valleys, canyons and craters yet seen.

They offer the prospect of great discoveries, but, despite what some headlines have suggested, the finding of water-ice at Mars' south pole and water vapour in the atmosphere are simply confirmations of what we already knew.

Mars Express has sent back yet more evidence that the Red Planet in the past was a wet world, with torrents of running water on its surface.

Looking at the spacecraft's images, it is easy to see why many now think there could be life there.

But a historical perspective is essential to understand what Mars Express is doing.

Great phantasm

Mars - its red colour associated with blood - has been known since prehistory.

It was only in 1659 that the first markings were seen on the planet, detected by Christiaan Huygens.

Mars south pole
The ice cap gleams brightly at Mars' south pole

A few years later, in 1666, Giovanni Cassini saw bright markings at Mars' poles; they grew and shrank with the Martian seasons.

William Herschel, between 1777 and 1783, was the first to suggest the polar caps were made of ice and snow that froze and melted.

But everyone knew Mars' polar regions were cold, too cold perhaps for ordinary ice to be responsible.

It was suggested in 1898 that the polar caps were solid carbon dioxide - so-called dry ice - which could explain the rapid melting at low temperatures (as carbon dioxide has a lower freezing point than water), but the idea was discarded.

The great phantasm that was the canals of Mars began about that time. Some saw intricate networks of canals that, they said, carried water from the poles to irrigate the arid equatorial regions. But it was all an illusion.

Fifty years later, science began to establish the basic parameters of the fourth planet from the Sun.

Spacecraft arrive

In 1947, the American astronomer Gerald Kuiper's telescopic observations found evidence of carbon dioxide on Mars, and in 1950 he showed that the polar caps had to be made of water-ice.

Infrared observations showed that water-ice and the polar caps appeared dark, whereas carbon dioxide ice appears bright at infrared wavelengths.

In 1963, Audouin Dollfus, at Paris Observatory, obtained results that indicated that there was a tiny amount of water vapour in Mars's atmosphere.

First ever close-up image of Mars
How it all began: Mariner 4's first ever close-up image of Mars
The US Mariner 4 was, in 1966, the first successful spacecraft to fly-by Mars, discovering craters like those seen on the Moon.

Its crude and grainy images were enough to dispel any idea of Mars having canals.

Although it arrived during a global dust storm that took some time to settle, Mariner 9 in 1971 gave the first clear indication that Mars, at one time, possessed substantial quantities of running water, posing the question, what happened to it all?

Some of it was frozen at the poles, some locked up in the subsurface and some was in the atmosphere as water vapour.

In 1976 the Viking spacecraft in orbit around Mars made extensive observations.

In particular, they showed that the northern polar cap had a surface temperature of about 200 Kelvin (minus 73 Celsius) indicating that it was too warm for it to be carbon dioxide - it had to be made of water-ice.

It is now known that the polar caps are made of water-ice, with a coating of carbon dioxide ice that disappears seasonally, though this does not happen to the southern polar cap, where the carbon dioxide never completely dissipates.

Water, water, everywhere

After a hiatus, spacecraft started returning to Mars in the late 1990s. Some spacecraft were lost to the "Mars jinx" but those that survived returned spectacular results.

Martian gullies
Gullies possibly cut by water stand out sharply on the Martian surface
The Mars Global Surveyor mapped the planet like never before detecting sharp gullies that must have been caused by running water in the recent geological past.

Mars Odyssey found water-ice in vast quantities just below the surface across great swathes of the planet away from the poles.

Technically, Mars Odyssey detected evidence of hydrogen beneath the surface, but almost everyone agrees that the hydrogen is associated with water.

Even before Mars Express sent back its remarkable images water and Mars were synonymous. The big question now is: could there be life?

The BBC's Fergus Walsh
"Never has the surface of Mars been shown in such spectacular detail"

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