Are we really alone in the universe? Mars might hold the key. Dreams of exploring the Red Planet are closely linked to our search for other forms of life. A mission to Mars may be the only way to answer this question. Beagle 2 is the next probe to land on Mars, specifically to search for signs of life.

Ever since HG Wells wrote The War of the Worlds, the possibility that Martians might inhabit the Red Planet has intrigued us. Being the most earth-like of the planets in our solar system, Mars is the obvious place to start the search for little green men or any other extra-terrestrials.

Early probes to Mars, such as the Mariner 4 mission in 1964, were disheartening. Mars was cold and barren, with an atmosphere consisting mainly of carbon dioxide. But more recent discoveries have encouraged scientists to believe that there may once have been life on Mars.

Signs of life

In 1996 a NASA-funded team of scientists announced a startling discovery. After a two-year investigation they claimed to have found organic molecules, thought to be from Mars, in a meteorite which had been discovered in Antarctica in 1984. The meteorite, which is 4.5 billion years old, is thought to have fallen to Earth about 13,000 years ago. The 1.89 kilogram (4.2lb), potato-shaped rock known as ALH84001, has been at the centre of the debate over whether or not life on Mars existed millions of years ago. The NASA scientists base their case on the following discoveries in the meteorite:

• Worm-like structures which appear to be microscopic fossils.
  
  
• A unique pattern of carbon compounds, which are the basis of life.
  
  
• Several unusual mineral phases - known products of primitive microscopic organisms on earth.
  
  
• The presence of polycylic aromatic hydrocarbons (PAHs) - the organic chemicals left behind when living material degrades.
  
  
• The meteorite matches the unique Martian chemistry measured by the Viking spacecraft which landed on Mars in 1976.
  
  
• Four years after the initial breakthrough further progress was announced. A group of NASA scientists at the Johnson Space Center, in Houston, Texas, said they had found tiny magnetite crystals, identical to those used by aqueous bacteria on earth as 'compasses' to find food and energy on ALH84001.

There is other circumstantial evidence for life on Mars:

• Layer upon layer of rock formations seen in pictures taken of Mars, from the Mariner 9 mission (1971) to more recent images from Mars Global Surveyor, support the view that Mars was once warm and wet, and therefore could have supported life.
  
  
• Pictures taken by Mars Global Surveyor in 1999 showed gullies that indicated water had been on the Martian surface in 'recent' times - that is, within the past million years.

The case for | The case against

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