go to homepage



ALH84001, meteorite determined to have come from Mars and the subject of a contentious scientific claim that it contains the remains of ancient life indigenous to the planet. Recovered from the Allan Hills ice field of Antarctica in 1984, the 1.9-kg (4.2-pound) igneous rock is thought to have crystallized from magma on Mars 4.5 billion years ago and later to have been shocked and altered, perhaps by one or more nearby meteoroid or asteroid impacts. Still later, carbonate mineral grains were introduced into shock-induced fractures in the rock. Another large impact blasted the rock off the Martian surface and into solar orbit with a velocity greater than the planet’s escape velocity of 5 km per second (11,000 miles per hour). Over time the rock’s orbit was altered such that it approached Earth, eventually falling in Antarctica about 13,000 years ago.

  • An elongated structure resembling a fossil microorganism (centre of image), revealed in a …

In 1996 NASA scientists who carried out microscopic and chemical analyses on ALH84001 touched off a controversy by suggesting that the carbonates in the meteorite had been produced by Martian microorganisms. The carbonate grains are associated with organic compounds, contain minute crystals of iron minerals similar in size and shape to those produced by bacteria, and exhibit elongated objects resembling microscopic fossils. In subsequent investigations, other scientists contested the interpretation of these lines of evidence, demonstrating that each could be adequately explained by nonbiological processes or was not entirely consistent with what is known about microfossils and living microorganisms on Earth. The hypothesis that ALH84001 contains evidence for extraterrestrial life has not found wide acceptance, although there are strong indications that Mars may once have been hospitable to life.

Learn More in these related articles:

...degree, which is in line with other evidence that liquid water was present at least periodically on Mars at some time in the past. The most unique Martian meteorite is another Antarctic specimen, ALH84001. This rock, an orthopyroxenite, has a crystallization age of about 4.5 billion years, which is roughly the same age as asteroidal meteorites (see below The ages of meteorites...
McKay is best known for his work on ALH84001, a meteorite originally discovered in Antarctica in 1984. The meteorite, believed to be about 4.5 billion years old and weighing 1.9 kg (4.2 lb), had initially been classified as a diogenite, a common type of rock. It was not until 1994 that it was determined to be of Martian origin. One of only 12 such known meteorites, the specimen quickly...
  • MLA
  • APA
  • Harvard
  • Chicago
You have successfully emailed this.
Error when sending the email. Try again later.
Edit Mode
Tips For Editing




Leave Edit Mode

You are about to leave edit mode.

Your changes will be lost unless you select "Submit".

Thank You for Your Contribution!



Uh Oh


Keep Exploring Britannica

Excavation Earth: Fact or Fiction?
Alan M. Turing, 1951.
Alan Turing
Isaac Newton, portrait by Sir Godfrey Kneller, 1689.
Sir Isaac Newton
United Nations (UN)
Leonardo da Vinci
Planet Earth section illustration on white background.
Exploring Earth: Fact or Fiction?
Earth’s horizon and airglow viewed from the Space Shuttle Columbia.
Earth’s Features: Fact or Fiction
Thomas Alva Edison
solar system
A Model of the Cosmos
Edwin Hubble
Joseph Priestley
Albert Einstein.
Albert Einstein
Email this page