Image caption The meteorite dates to 4.4 billion years ago, a time when Mars was in its infancy

A rock discovered in the Sahara Desert is the oldest Martian meteorite ever found, scientists believe.

Earlier research had suggested it was about two billion years old, but new tests indicate the rock actually dates to 4.4 billion years ago.

The dark and glossy meteorite, nicknamed Black Beauty, would have formed when the Red Planet was in its infancy.

The research is published in the journal Nature.

Lead author Prof Munir Humayan, from Florida State University, US, said: "This [rock] tells us about one of the most important epochs in the history of Mars."

Mars rocks

There are about 100 Martian meteorites, but almost all of them are younger, dating to between 150 million and 600 million years old.

It is almost certainly coming from the southern highlands Professor Munir Humayan, Florida State University

They would have fallen to the Earth after asteroid or comet impacts had dislodged them, setting the rocks free to travel through space before eventually crash landing here.

This particular Martian meteorite, which is formed of five fragments, is much older.

An earlier analysis of one piece, called NWA 7034, put the age at 2 billion years.

But this latest research has found that another piece, NWA 7533, dates to 4.4 billion years ago, which suggests that NWA 7034 also must be older.

The team said it would have formed when the Red Planet was just 100 million years old.

"It is almost certainly coming from the southern highlands - the cratered terrain that makes up the southern hemisphere of Mars," said Prof Humayan.

This would have been a turbulent period of Martian history, when volcanoes were erupting all over the surface.

Mix of ages

Prof Humayan explained: "The crust of Mars must have differentiated really quickly, rather than gradually over time. There was a big volcanic episode all over the surface, which then crusted up, and after that the volcanism dropped dramatically.

This meteor continues to reveal its secrets - we're very excited about this Prof Carl Agee, University of New Mexico

"When it did this it also must have out-gassed water, carbon dioxide, nitrogen and other gases to produce a primordial atmosphere... and also a primordial ocean."

He added: "This is a very exciting period of time - if there were to be life on Mars, it would have originated at this particular time."

He said that team now plans to study the rock to see if there were any signs of past life. But he added that while the rock was lying in the Sahara Desert, living organisms probably would have occupied it, masking potential evidence.

Commenting on the research, Prof Carl Agee, from the University of New Mexico, who carried out the earlier analysis on NWA 7034, described the latest study as exciting work.

He said that the discrepancies in the ages might have come about because the meteorite contains a mixture of components - and his team was now also finding that parts of the rock were about 4.4 billion years old.

"There is definitely an ancient component in the rock, but we think there might be a mixture of ages," he said.

He explained that a comet or asteroid impact, a volcanic eruption or some other event that occurred about 1.5 billion years ago could have added younger material to the original crust.

He said: "It consists of at least half a dozen different rock types. We see different igneous rock types, sedimentary rock types... this is a very complex meteorite.

"This meteorite continues to reveal its secrets - we're very excited about this."

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