The unusual balance of various elements in the meteorite (called ALH84001) indicates that it was not formed on Earth. Also, it must have spent some time in space because it contains radioactive versions of common elements not found on Earth.
When did it arrive on Earth?
About 13,000 years ago, after being thrown into space from Mars by a major asteroid impact about 15 million years ago.
What exactly have the Nasa scientists found?
The rock contains tiny fractures, thought to have occurred when it was on Mars. These fractures contain clear signs of molecules known as PAHs (polycyclic aromatic hydrocarbons) - the first such molecules ever seen in a Martian rock. Scientists reckon that PAHs are formed either inside stars or by living organisms such as bacteria. Many have been found in Earth sediments.
Why weren't these signs found earlier?
ALH84001 was discovered in 1984, but not recognised as being from Mars until 1994. It is one of only 12 Martian meteorites known. "Three years isn't a long time to find something like this," said Jamie Gilmour, research fellow in earth sciences at Manchester University.
Is everyone convinced this is evidence of life on Mars?
Many are holding back until they know more. The scientific paper is not due to be published until next week. The US astronomer Carl Sagan said the findings "are not evidence of life".
John Kerridge, a planetary scientist at the University of California, San Diego, said "The conclusion is at best premature and more probably wrong. The PAHs are just not a reliable biomarker."
Could the "Mars life" actually have come from Earth?
The possibility that the meteorite was contaminated with Earth organisms was a major question for the Nasa scientists. They provide a number of reasons refuting it. The key one is that the PAHs were more numerous towards the centre of the meteorite than outside it.
Could this just be a publicity drive by Nasa to attract funding for Mars missions?
Possibly, but it's unlikely. Nasa said recently that it lacks the funds to realise plans to launch a spacecraft to Mars every two years. Before the latest results were publicised, an independent committee warned that the hunt for fossil life on the planet would require extra funding. And Nasa administrator Daniel Goldin has called on Nasa to bring back a sample from Mars by 2003.
But while the new research will make it hard for politicians to refuse requests for exploration funding, two factors suggest the findings are credible.
First, the scientists involved would face ridicule if their case is overstated. Secondly, Science magazine - in which the work will be published - has a rigorous checking system by which independent scientists review the work. If they thought it was faked, they would have rejected the paper.
What life might exist now on Mars?
Any life now on Mars is probably in suspended animation - frozen in the polar regions (of carbon dioxide) or in underground, frozen water. Possibly there is something still alive closer to the core of Mars - but that is only conjecture.Reuse content