What did ALH 84001 tell us about life on Mars?
In 1996 a NASA-led team suggested ALH 84001 held possible signs of ancient Martian microbes. Most scientists now think non-biological processes can explain all the features, but the meteorite did reveal that early Mars had liquid water.
ALH 84001 is a meteorite of about 1.9 kilograms found in the Allan Hills of Antarctica in December 1984. It was first classified as a diogenite, a type of meteorite linked to the asteroid Vesta, before researchers realised in the early 1990s that it came from Mars. It is very old, having crystallised roughly 4.1 billion years ago, which makes it a sample of the ancient Martian crust.
In August 1996 a team led by NASA scientists published a paper in the journal Science suggesting possible evidence of past life. They pointed to four lines of evidence: tiny orange globules of carbonate minerals that formed in water; organic molecules called polycyclic aromatic hydrocarbons; very small, uniform crystals of the magnetic mineral magnetite, similar to those made by some bacteria on Earth; and worm-like structures tens of nanometres long that they suggested might be fossil microbes. The announcement drew worldwide attention, and the US President made a statement about it.
Then came years of testing. Other researchers showed that the carbonates and magnetite could form by heating and chemical reactions without life, that some organic material could be contamination from Antarctic ice, and that the "nanofossils" were far smaller than known bacteria and might partly be artefacts of sample preparation. The scientific consensus today is that the meteorite does not provide convincing evidence of Martian life, although the original team did not accept every criticism.
The episode was still hugely valuable. ALH 84001 confirmed that liquid water existed in the Martian crust billions of years ago, and its carbonates have been used to study the early Martian atmosphere. The debate sharpened the tests scientists use for biosignatures and helped inspire the wave of Mars missions that followed, including rovers designed to search for evidence of ancient habitable environments.