ALH 84001 and other Martian rocks
The ancient Martian rock once claimed to hold fossils, and 'Black Beauty'.
- Classification
- Stony meteorites › Achondrites › Planetary achondrites › Martian meteorites › ALH 84001 and other Martian rocks
- In our catalogue
- 130 meteorites
- Heaviest
- Northwest Africa 7319, 6.41 kg
Allan Hills 84001, found in Antarctica in December 1984, is an orthopyroxenite about 4.1 billion years old, one of the oldest Martian rocks known. In 1996 NASA scientists announced that tiny structures and minerals inside it might be traces of ancient bacteria. Most researchers now think these formed without life, but the claim launched modern astrobiology.
Northwest Africa 7034, nicknamed 'Black Beauty', is a Martian regolith breccia rich in water, containing zircon grains about 4.4 billion years old. It is a sample of the ancient Martian crust.
Allan Hills 84001
Allan Hills 84001 is an orthopyroxenite, made of about 97% orthopyroxene with chromite, plagioclase glass and small orange globules of carbonate. It was found on 27 December 1984 in the Allan Hills of Antarctica by Roberta Score, and at first was classed as a diogenite. Only in 1993 was it recognised as Martian. It crystallised about 4.1 billion years ago, so it comes from Mars's ancient crust and records a time when the planet still had a magnetic field and, probably, liquid water at the surface.
Its carbonate globules formed about 3.9 to 4.0 billion years ago from water moving through the rock. In 1996 a team led by David McKay announced that tiny chain-like magnetite crystals, organic molecules and worm-like shapes in the carbonates might be traces of ancient life. Later work showed that most of these features can form without biology, and most scientists do not accept them as evidence of life, but the debate drove the creation of modern astrobiology and new missions to Mars.
Northwest Africa 7034 'Black Beauty'
Northwest Africa 7034, found in the Sahara in 2011, and its many paired stones are regolith breccias of the Martian crust. They contain fragments of basalt, impact melt and soil, and much more water than most Martian meteorites, about 0.6% by weight. Zircon grains in them are about 4.4 to 4.5 billion years old, among the oldest pieces of Mars known. Its chemistry matches measurements of the Martian surface by rovers and orbiters better than any other meteorite. A 2022 study proposed that it was launched from a crater in the southern highlands.
Why they matter
These two meteorites sample the ancient southern highlands of Mars, which make up most of the planet's surface but are otherwise represented only by measurements from spacecraft. They show that Mars formed a crust within about 100 million years of its birth, and that water was active in its crust early on.
How to recognise them
Allan Hills 84001 is a pale greenish-grey, coarse-grained rock crossed by dark shock-crushed zones. Black Beauty is dark, almost black, with many small light and dark fragments. Both need laboratory classification.
Famous examples
- Allan Hills 84001 Martian (OPX), Antarctica, 1984 · 1.93 kg
- Northwest Africa 7034 Martian (basaltic breccia), 2011 · 320 g
Related groups
- Martian meteorites Rocks from Mars, recognised by gas from the Martian atmosphere trapped inside them.
- Shergottites The most common Martian meteorites: young basaltic and olivine-rich rocks.
- Diogenites Pyroxene-rich rocks from deep inside Vesta.
Photos shared under Creative Commons licences (Wikimedia Commons).