Four and a half billion years ago, a rock was formed on Mars by volcanic processes.
Half a billion years later, this rock was broken into many smaller pieces by a collision with a nearby meteorite.
Some groundwater has also seeped into the rock.
16 million years ago, an asteroid hit Mars somewhere near where this rock is.
The impact hurled chunks of rock into space.
A 2 kg piece of rock orbited the Sun until 13,000 years ago, when it came close to Earth.
This piece fell onto a glacier in Antarctica.
In 13,000 years, it reached the Allan Hills region of Antarctica, buried in ice.
In 1984, this meteorite was discovered and named ALH84001.
Many people created the meteorite story we just told.
This year, a team led by David McKay of the US space organization NASA said that there appear to be signs that life may have existed on this rock since ancient times: The meteorite contains organic molecules, in the same family as naphthalene (used in mothballs).
When bacteria decompose, such compounds are produced.
Many meteorites contain such compounds.
Meteorites contain iron oxide (magnetite) of the type secreted by some bacteria on Earth.
It contains iron sulfide, which is produced by some anaerobic bacteria (those that do not use oxygen).
Meteorites contain spheres of carbonate material, possibly formed by matter, possibly formed by a living organism.
On the other hand, most terrestrial bacteria are 100 times larger than this material.
Meteorites can contain very small fossils (less than one hundred millionth of a millimeter).
Nanobacteria come in this size.
In 1961, another meteorite showed signs of life.
But it was soon discovered that they were pollen grains and furnace ash grains.
Signs of life were found to come from Earth itself.
This may also be the case with the Antarctic meteorite.
What makes scientists more optimistic is that some of these mentioned elements were found in cracks, and the cracks could only have formed before the meteorite stopped in Antarctica.
So maybe, just maybe, the signs of microbial life we're seeing are from when the rock was on Mars.
In 1976, the Viking space probe found no such bacteria on Mars.
But perhaps they landed on the lifeless part of Mars.
Or maybe bacteria were present on Mars millions of years ago but are no longer there.
Scientists are examining ALH84001 very carefully.
And even US President Bill Clinton promised to support NASA's new spacecraft to Mars.
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