A meteorite found in the Sahara last year by Bedouin tribesmen is a rock from Mars, revealing that the Red Planet's crust formed 4.4 billion years ago, scientists reported on Wednesday.

Sold to the elite club of meteorite collectors, the extraordinary rock has been analysed by US and French geologists.

They found tough crystals within the meteorite called zircons, which held traces of uranium, whose rate of decay can be used as a calculator of age.

The dating puts the meteorite at a whopping 4.4 billion years old.

"This date is about 100 million years after the first dust condensed in the solar system," said Munir Humayun, a professor of Earth, ocean and atmospheric science at Florida State University.

"We now know that Mars had a crust within the first 100 million years of the start of planet-building, and that Mars' crust formed concurrently with the oldest crusts on Earth and the moon."

The rock, measuring 40 millimetres (1.5 inches) across and weighing 84 grammes (2.9 ounces), carries the official designation of NWA 7533, with the letters NWA meaning North West Africa, where it was found.

It was found near Bir Anzarane, in southern Morocco.

It has a sister, an even more famous stone called NWA 7034, that has been called "Black Beauty" -- its dark surface indicating a volcanic basalt origin.

At least three other recovered meteorites in this area are believed to be part of a rock that came from Mars, presumably dislodged by an asteroid that whacked the planet's surface.

It then drifted in orbit before eventually landing on Earth, disintegrating in the final stages of a fiery descent.

Using mass spectrometers at the US National High Magnetic Field Laboratory, the scientists also found high concentrations in NWA 7533 of trace metals such as iridium.

These are typically found when a planetary surface is pulverised by asteroid bombardment, and soils in the impact crater suddenly fuse in the heat.

This geochemical mash-up probably explains why "Black Beauty" has a far younger estimated age, of 2.1 billion years, as determined by decay of the radioactive element rubidium.

That earlier date is likely to come from a mixture of the soils, all of differing ages, that fused together.

The parent rock is likely to have come from Mars' southern highlands, according to the paper, published in the journal Nature.

"This cratered terrain has been long thought to hold the keys to Mars' birth and early childhood," Humayun said.