A fist-sized meteorite nicknamed "Black Beauty" could unlock vital clues to the evolution of Mars from the warm and wet place it once was to its current cold and dry state, the US space agency Nasa said.
Discovered in Morocco's Sahara Desert in 2011, the 320-gram space rock contains 10 times more water than other Martian meteorites and could be the first to have originated on the planet's surface or crust.
After more than a year of study, a team of US scientists determined the meteorite was formed 2.1 billion years ago at the start of the most recent geologic period on Mars, known as the Amazonian, Nasa said.
The abundance of water molecules in the meteorite - about 6,000 parts per million, 10 times more than other known rocks - suggests water activity persisted on the Martian surface when it was formed.
It is generally accepted that Mars had abundant water early in its existence, causing scientists to ponder if life might once have existed there. But the nature of its evolution to a cold and dry place remains a mystery.
"Many scientists think Mars was warm and wet in its early history, but the planet's climate changed over time," said lead scientist Carl Agee.
Known technically as NWA (Northwest Africa) 7034, "Black Beauty" is made of cemented fragments of basalt, a rock that forms from rapidly cooled lava.
"Perhaps most exciting is that the high water content could mean there was an interaction of the rocks with surface water either from volcanic magma, or from fluids from impacting comets during that time," co-author Andrew Steele said.
"It is the richest Martian meteorite geochemically and further analyses are bound to unleash more surprises."
Unlike most Martian meteorites, it is thought to be from the planet's surface.
"Researchers theorise the large amount of water contained in NWA 7034 may have originated from interaction of the rocks with water present in Mars' crust," Nasa said. More than 100 Martian meteorites have been found on earth, but most are fragments of three larger meteorites.
NWA 7034 has unique characteristics and it took scientists several months to ascertain it did indeed come from Mars and not another planet or asteroid belt.
"The age of NWA 7034 is important because it is significantly older than most other Martian meteorites," said Mitch Schulte, of the Mars Exploration Programme at Nasa headquarters in Washington. "We now have insight into a piece of Mars' history at a critical time in its evolution."
Agee said: "This Martian meteorite has everything in its composition that you'd want in order to further our understanding of the red planet.
"[It] tells us what volcanism was like on Mars two billion years ago. It also gives us a glimpse of ancient surface and environmental conditions … that no other meteorite has ever offered."
"The contents of this meteorite may challenge many long held notions about Martian geology," said John Grunsfeld, associate administrator for Nasa's science mission directorate.