Science: Life on Mars: the debate still rages

Remember 'life on Mars'? The Nasa claim, not the David Bowie song. The latest research claims that Nasa was wrong, but British scientists say that those claims are, in turn, wrong. It looks like we will have to visit the Red Planet to decide. Charles Arthur, Science Editor, on the continuing debate.

New research which suggests that the "life on Mars" meteorite was actually just contaminated with terrestrial products drew an angry response from British scientists yesterday.

According to two separate papers published in Science magazine today, the organic material found in the ALH 84001 meteorite consisted of amino acids and compounds which entered the rock during the 13,000 years that it sat in the Antarctic wilderness before being discovered in 1984.

But Colin Pillinger, of the Open University, attacked both pieces of work as "naive", while Monica Grady of the Natural History Museum, who has studied the meteorite, commented: "The claims that they are making that the 'life on Mars' question is dead just aren't valid."

In August 1996 the US space agency Nasa claimed that combined studies of the potato-sized ALH 84001 showed that it had contained primordial life about 4 billion years ago, while still on Mars. Much of the evidence concerned fossilised remains of compounds called polycyclic aromatic hydrocarbons (PAHs).

The two papers in today's Science say ALH 84001 was contaminated by ice: one analyses amino acids in the rock, while the other looks at isotopes of various chemicals. The papers join a long list of others which have not backed up Nasa's claim. However, Professor Pillinger and colleagues produced work in 1996 claiming that they had also found evidence of Martian life in another meteorite.

Nasa claimed that primitive micro-organisms played a part in the production of carbonate found in the meteorite. Carbonate is formed when carbon dioxide, a well-known product of life, dissolves in water.

One of today's research papers denies this, saying analysis of carbon isotopes in the carbonate and organic molecules in the meteorite showed the two came from completely difference sources. Timothy Jull, who led the research at the University of Arizona at Tucson, said: "The organic material contains 14C [the carbon 14 isotope] and the carbonate doesn't, because the carbonate came from somewhere in space, presumably Mars."

Meanwhile Jeff Bada, a professor of marine chemistry at the University of California, San Diego, searched a sample of the meteorite for amino acids and found very low levels, the vast bulk of which was clearly terrestrial. Professor Bada said he could not rule out the possibility that minute amounts of some extra-terrestrial amino acids were preserved in the meteorite.

All the scientists involved now agree that only one solution will really answer the "life on Mars" question: a trip to the planet.