Large quantities of a green mineral gemstone have been found on Mars.
Rocky outcrops of the mineral olivine were spotted by a space craft orbiting the planet.
On Earth, the mineral is known as peridot, an inexpensive gemstone used in jewellery.
Its presence gives clues to the ancient history of Mars, suggesting the planet has been cold and dry for billions of years.
The exposed mineral is weathered away in warm, wet conditions.
If it has been there for billions of years, as geological evidence suggests, then Mars must have been cold and dry for much of that time.
Scientists now need to pin down the precise age of the mineral, found over a 30,000-square-kilometre area in a long, narrow, shallow depression known as Nili Fossae.
The region probably formed at least 3.6 billion years ago when an asteroid or comet crashed nearby, carving a crater known as the Isidis basin.
It is probable at that time that the mineral was exposed. An alternative, but less likely scenario, is that it was pushed to the surface much more recently by another geological event.
Transparent green-coloured mineral
Alters to other chemicals in the presence of water
Called peridot on Earth and used in rings, bracelets and necklaces
"If the olivine was exposed shortly after the impact event, the Martian surface may have been dry and cold for more than three billion years", a team of researchers from the US Geological Survey in Denver, Colorado, write in the journal Science, "but if the olivine was recently uncovered at the surface, then it could have been cold and dry for as little as a few thousand years."
The mineral was detected by an instrument on the US space agency's (Nasa) Mars Global Surveyor.
The unmanned probe arrived at the planet in September 1997 and has been making observations ever since.
It has shown good evidence that the cold, inhospitable planet was once warm and wet like the Earth. These conditions could have harboured primitive life.