BBC correspondent Leo Enright reports on the new rock from Mars

They are analysing a small sample of a meteorite which landed in the Sahara desert.

A UK team is also trying to raise enough money to send their own robot explorer to study the surface of the red planet.

The meteorite was discovered last May

The meteorite left Mars a million years ago and is the size of a melon.

The discovery comes two years after American scientists said they had found microscopic fossils in another Mars meteorite.

Very expensive rock

Nasa scientist Everett Gibson, who led that investigation, said: "We found the remains of biological activity.

"I'm optimistic and looking forward to hopefully getting a few grammes of this material and we can put the powerful analytical tools to begin to decode the messages carried by this rock."

Scientists want a larger slice of the rock

But the new Mars rock is in the hands of a private collector who could sell it for millions.

British scientists have been given a tiny sample and the leading expert has made a passionate appeal to the collector for a bigger slice.

Dr Monica Grady of the Natural History Museum said: "Please give the scientific community about 20 to 50 grams of meteorite to work with."

She added: "You won't regret it."

UK calling Mars

Analysing meteorites is the cheapest way of gathering information on Mars. But there are now plans to send a UK probe to the planet.

Top UK space scientist Professor Colin Pillinger has unveiled his plans for Project Beagle, a tiny robot that would land on the surface of Mars.

It would travel to the planet on board Europe's Mars Express and parachute onto the surface of Mars to probe and analyse the soil for signs of life.