ALH 84001

A potato-sized rock from Mars that sparked a worldwide debate about life beyond Earth.

Key facts

Type

Martian meteorite (orthopyroxenite)

Fell or found

Found

Year

1984 (27 December)

Country

Antarctica (Allan Hills)

Mass

about 1.93 kg

Official name

Allan Hills 84001

On 27 December 1984, a team from the United States Antarctic Search for Meteorites programme was riding snowmobiles across the blue ice of the Allan Hills in Antarctica. Roberta Score, a member of the team, spotted a greenish-grey rock about the size of a large potato. Because it looked unusual, it was the first specimen from that season to be processed, and so it was given the label ALH 84001. Nobody guessed that, twelve years later, it would make headlines around the world.

Why Antarctica?

Antarctica is the best place on Earth to find meteorites. Meteorites that fall onto the ice sheet become buried and are carried slowly along as the ice flows. Where the ice runs up against mountains and is worn away by fierce, dry winds, the meteorites are brought back to the surface and concentrated on patches of bare blue ice. Dark rocks stand out clearly, and there are few Earth rocks around to cause confusion. Tens of thousands of meteorites have been collected there.

A rock from Mars

ALH 84001 was first classified as a diogenite, a type of meteorite believed to come from the asteroid Vesta. In 1993, a closer look at its chemistry showed that it actually belonged with a small group of meteorites known to come from Mars. Scientists can recognise Martian meteorites because their minerals have a distinctive balance of oxygen isotopes, different forms of oxygen atoms, and because some contain trapped gases that match the Martian atmosphere as measured by spacecraft.

ALH 84001 is made almost entirely of the mineral orthopyroxene, so it is classified as an orthopyroxenite. It crystallised from molten rock roughly 4.1 billion years ago, making it one of the oldest pieces of Mars known. Later, water seeping through cracks in the rock deposited tiny orange globules of carbonate minerals, about 3.9 to 4 billion years ago, at a time when Mars was probably warmer and wetter than today. About 16 to 17 million years ago an asteroid impact blasted the rock off Mars, and after drifting through space it landed in Antarctica roughly 13,000 years ago.

Signs of life?

In August 1996 a team led by David McKay at NASA's Johnson Space Center published a paper suggesting that ALH 84001 might hold evidence of ancient Martian microbes. They pointed to several features: the carbonate globules; complex organic molecules called polycyclic aromatic hydrocarbons; tiny crystals of magnetite, a magnetic iron oxide, similar to those made by some bacteria on Earth; and minute worm-like shapes that looked a little like fossilised bacteria. The announcement was so dramatic that the President of the United States, Bill Clinton, spoke about it publicly.

The debate

Other scientists quickly tested each claim. The worm-like shapes were far smaller than most known bacteria, and some may have been produced during sample preparation. Organic molecules like those found can form without life and could partly be contamination from Antarctic ice. Magnetite crystals with similar shapes can be produced by heating carbonate minerals, for example during a shock from an impact. Today most scientists think that every feature can be explained without life, although the debate shaped research for decades.

Why it matters

Even though it did not prove that life existed on Mars, ALH 84001 had an enormous impact. It helped launch the modern science of astrobiology, the study of life's possible origins and distribution in the universe, and encouraged new missions to search for signs of past life on Mars. It also showed how science should work: an exciting claim, careful testing by others, and conclusions drawn from the weight of evidence.

The meteorite itself remains a precious record of early Mars, preserving the chemistry of water that flowed through its rocks four billion years ago. Most of it is held under careful curation by NASA, with small samples lent to researchers around the world.

Did you know?

See more

Allan Hills 84001 in the catalogue