Friday, August 27, 1999 Published at 10:38 GMT 11:38 UK
Life on Mars - new claims
Is this a fossilised microbe on the Nakhla meteorite?
By BBC News Online Science Editor Dr David Whitehouse
Sensational new claims about life on Mars are about to be made by US scientists.
Some of the researchers who claimed in 1996 to have found evidence for past life in a Martian meteorite now say they have further evidence to support their theories in one, possibly two, other rocks.
They will reveal their findings at a forthcoming conference in America. The announcement will once again arouse great controversy in the scientific community, which was far from convinced by the 1996 evidence.
The new evidence comes from a study of the so-called Nakhla meteorite that fell at Nakhla, Egypt, in 1911.
It broke up into many pieces. Years later, a detailed analysis of the rock revealed it to be one of only 13 known meteorites from the planet Mars.
It is estimated to be about 1.37 billion years old and was thrown into space when a giant asteroid slammed into Mars hundreds of millions of years ago.
After millions of years in space, it fell to Earth in 1911.
Examination of the Nakhla meteorite, using an optical and a more powerful scanning electron microscope (SEM), by a team from NASA's Johnson Space Center led by Dr David McKay, has revealed rounded particles of a limited size range.
The researchers suggest that these structures are the mineralised remnants of bacteria that once lived on Mars. They say that their size is similar to bacteria found on Earth.
They even go onto to say that they believe the Nakhla meteorite may have been colonised by two generations of bacteria.
They add that another of the meteorites from Mars, a rock called Shergotty, may also contain the bacterial fossils.
For most scientists, though, curious and minute shapes in meteorites are not enough to make them believe that bacteria once lived on Mars.
They say it is all too easy to be fooled by the shapes of mineral grains, especially if viewed with an eye looking for organic shapes.