In August 1996, scientists from the National Aeronautics and Space Administration (NASA) announced they had detected possible signs of life on Mars; the announcement caused heated discussions within the scientific community. A series of more recent discoveries has caused scientists to once again debate the possibility of life on Mars.Where was NASA's 'evidence' found?
The early evidence didn't arrive in the form of a little green man or even a little green bug. It was a rock. Not much bigger than a potato and given the dry name of ALH84001, this rock was discovered in Antarctica in 1984. It stands out as a rock among rocks because it is a meteorite and it came from Mars (Box 1: Mars: Earth's cool cousin). We know where it comes from because it has a chemical 'fingerprint' that matches samples taken from Mars.
Life on Mars...PAH!
NASA scientists noticed several intriguing things about ALH84001 which they thought indicated that life may once have existed on Mars.
The first of these were tiny blobs, the size of fullstops. Called carbonate rosettes, they were similar to the rosettes produced by bacteria in ponds on Earth as they metabolise minerals.
In addition, chemical compounds known as polycyclic aromatic hydrocarbons (PAHs) were found in and around the carbonate rosettes. ALH84001 contains an unusual mixture of certain lightweight PAHs. The NASA scientists concluded that these might have been produced as once-living organisms decomposed.
Another piece of evidence centred on the discovery of tiny crystals of magnetite and iron sulphide embedded in places where the carbonate rosettes had dissolved. The scientists noted that some bacteria on Earth also manufacture similar crystals.
The fourth and final piece of evidence was perhaps the most controversial. Using an electron microscope, the NASA scientists found elongated and egg-shaped structures within the carbonate rosettes which they interpreted to be tiny fossils of Martian microbes.
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