Sceptics have argued against the evidence produced by ALH84001 on three main grounds:

• that the rock could have been contaminated with carbon compounds from Earth;
  
  
• that the fossils are too small – more than 100 times smaller than any bacterial life found on earth – to be evidence of microscopic organisms;
  
  
• that PAHs are common in the universe and are not always associated with living material.

Regarding the circumstantial evidence:

• sceptics point out that while the layered rock formations seen on Mars may have formed as sediment settled on the bottom of seas or lakes, they may also have formed in another way. Even if they did form in the way the pro-life on Mars faction believe, the presence of water does not prove that life ever existed there.

All those involved in the debate about whether or not life could exist on Mars admit that a great deal more research needs to be done on the surface of the planet. A situation that is soon to be rectified by the Beagle 2 explorations. Speaking in 1996, when research on ALH84001 was made public, Daniel Goldin, NASA administrator, said: 'The evidence is exciting, even compelling, but not conclusive.' He also dashed hopes that signs of higher life forms were likely. 'We are not talking about little green men. There is no evidence or suggestion that any higher life form [than extremely small, single-cell structures] ever existed on Mars.'

Keep a look out for results from the Beagle 2 mission to see if he is right.

The case for | The case against

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