A 'fresh' Martian meteorite that fell to Earth last year contains unique evidence of water weathering on the planet's surface.
The rock, blasted off Mars some 700,000 years ago by an asteroid or comet impact, also bears traces of the planet's atmosphere, say scientists.
Analysis of samples of the meteorite indicates it originated on or near the Martian surface.
There are also signs of elements being carried into cracks in the rock by water or fluid - something never seen before in a Martian meteorite.
Martian meteorites allow scientists to study the planet's geology on Earth without having to mount expensive space missions.
However most are not found until long after their arrival, allowing them to be spoiled by Earthly contamination and chemicals.
The new meteorite, known as Tissint, is one of only five to be collected after being spotted falling to Earth.
Having broken up in the atmosphere, fragments of the rock landed on a desert region of southern Morocco near the town of Tissint on July 18 last year.
The pieces were analysed by an international team of scientists that included experts at London's Natural History Museum.
A one kilogram lump of the meteorite measuring about 12 centimetres across is now on display in the museum's The Vault gallery.
Dr Caroline Smith, meteorites curator at the Natural History Museum, said: 'Any meteorite that is seen to fall - they're called 'fall meteorites' - are particularly interesting because they suffer from very little contamination.
'One of the main things we found was that some of the chemical signatures in this meteorite indicate it must be from quite close to the surface of Mars, or even on the surface.'
The meteorite contains large amounts of black glass, created by heat from a shock impact melting rock.
Scientists know that elements found inside the glass cannot have resulted from contamination from Earth.
One of them turned out to be cerium, an element from the Martian surface. The unusual levels of cerium suggested they had been deposited by a leeching process involving water.
'This enrichment has happened because of the weathering process you get at the near surface of Mars,' said Dr Smith. 'Water, or fluids, have picked up the cerium and gone into gaps in the rock.'
No-one knows when this occurred, but it would have been some time before the rock was ejected from Mars.
Nitrogen originating from Martian atmosphere was also identified in the black glass.
Findings from the research, conducted by 20 scientists in five countries, appear today in the journal Science.
Lead researcher Professor Hasnaa Chennaoui Aoudjehane, from the Hassan II University in Casablanca, Morocco, said: 'The Tissint Martian meteorite found in Morocco is very important because it is so fresh. We have conclusively shown, for the first time, the chemical signatures of weathering processes on Mars.'
Dr Smith said scientists on Earth could analyse the rock with a precision not possible using robots such as the rover Curiosity, which recently landed in a Martian crater.
'Meteorites are known as the poor man's space probe,' she said.
She said there was absolutely no suggestion of any sign of Martian life in the meteorite.
'I can say hand on heart that people have not found anything indicative of little green men or little green bugs,' she said.
In 1996 scientists at the American space agency Nasa claimed to have discovered evidence of fossil microbes in a Martian meteorite from Antarctica known as ALH84001.
Since then other experts have argued that the 'bugs' probably have a non-biological origin.