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Last Updated: Wednesday, 6 February 2008, 12:47 GMT
The long quest for life on Mars
By Dr Chris Lintott
Co-Presenter, BBC Sky At Night

Look at those cavemen go? Some bloggers saw a Bigfoot-like figure in this image from Mars

"Life on Mars!" declared the headlines: an image showing a tantalisingly blurred Bigfoot-like figure, captured in a panorama by Nasa's robot geologist Spirit, spread round the world last month.

First identified by a Japanese blogger, it didn't take long for the image to reach thousands upon thousands of people, appearing in newspapers and becoming one of the most emailed stories on the BBC News website.

It is a rock, just a couple of centimetres tall, but it is easy to understand why some envisaged a humanoid figure there.

The fact is that we humans have evolved to err on the side of caution when spotting figures or faces, something that becomes apparent whenever the face of Jesus is found on a piece of toast.

Desert mirages

But there is something about Mars, in particular, that has always captured the imagination.

Through a small telescope, dark patches can be seen against the Red Planet's disc. Mars is a dusty world, and right up until the start of the space age it was believed that these were patches of vegetation.

Canals were once though to exist on the surface of Mars

In the 1960s, the astronomer Ernst Opik (Lembit's grandfather) argued that they had to be growing plants; how else would the same dark patches appear after each of the dust storms that engulf the planet?

If plants grew on the surface, could there be intelligent Martians too? The great Italian observer, Giovanni Schiaparelli, wrote of "canali" that he could see on the surface.

Translated into English as "canals", it prompted ideas of a Martian civilization eking out an existence in their dusty desert by transporting water down from the polar ice caps (which, just as had been predicted, consist of a mixture of water and carbon dioxide).

Just as web surfers in 2008 see Bigfoot in an image sent back by one of Nasa's robot rovers, many otherwise competent astronomers saw what they wanted to see, drawing ever more complex networks of canals on their maps of Mars.

European craft revealed The Face to be a trick of the shadows

The idea of a Martian canal system was so alluring that when the Guzman prize was offered in France in 1900, guaranteeing 100,000 francs then a fortune to the first person to make contact with aliens, Martians were excluded because they would be too easy.

The canals of Mars, along with Opik's plants, do not exist.

The first robotic missions to the Red Planet revealed a barren, rusty desert world.

The Viking missions of the mid-1970s carried experiments to test for life in the Martian soil, and no unambiguous sign was found.

The surface is just too cold, and the atmosphere too thin to support liquid water - the essential ingredient for life as we know it. Consequently, Nasa turned its attention to the outer Solar System and Mars was written off as a dead world.

Watery hopes

For those who wanted to believe in the missing Martians, though, the most famous of all images from Nasa's Viking spacecraft - launched to Mars in the 1970s - kept hope alive.

Surface with a smile: Crater Galle

A chance alignment of spacecraft, sunlight and hillside made an otherwise unremarkable patch of the surface, known as Cydonia, appear like face on Mars.

To conspiracy theorists everywhere, this seemed unshakable evidence that Nasa was concealing evidence for an ancient Martian civilization.

Almost thirty years later, Mars Global Surveyor flew over the area and revealed the face to have been just a trick of the shadows.

Mars had the last laugh, though, as Mars Express images of crater Galle revealed a smiley face apparently drawn onto its dust floor.

Putative Martian fossil in ALH84001 (Nasa)
Evidence for fossils in ALH84001 is disputed
This seems to have generated little talk of conspiracy; perhaps the smiling crater is somehow less serious than the sphinx-like face of Cydonia.

Ironically, Mars Express and the fleet of American spacecraft that have visited Mars in the last decade are there because they too have come to believe once more in the chance of life on Mars.

Spirit and Opportunity, the two rovers now entering their fifth (Earth) year on the surface, have between them provided strong evidence that Mars used to be a wet world.

Phoenix (Nasa)
Nasa's Phoenix probe will land on Mars later this year
In 1996, researchers also claimed to have found traces of fossil life in a Martian meteorite, known as ALH84001, that fell to Antarctica. However, the evidence has since been disputed.

Radar experiments have revealed that large quantities of what is probably water ice exist below the surface.

In May, the US Phoenix lander will touch down near the planet's north pole, and dig below the surface in order to investigate these icy deposits.

Further missions will follow, and it may not be too long until the headline "life on Mars" refers to the biggest astronomical discovery of all time, and not to a lost little Bigfoot.



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