SPECTACULAR images from a Martian "grand canyon" have revealed signs of abundant ground water that may have flowed through cracks in the rock many millions of years ago.
The pictures are more evidence that Mars might have once been a habitable planet with breathable air, streams, lakes, rain - and possibly life. They were taken by a powerful camera on the United States space agency NASA's Mars Reconnaissance Orbiter
spacecraft.
Last autumn, scientists turned the HiRISE (High Resolution Imaging Science Experiment) camera on to the Candor Chasma, one of several canyons that make up the giant rift valley called Valles Marineris, just south of the Martian equator.
Transplanted on to Earth, it would stretch across the whole of the United States: in places, it is six to seven times deeper than the Grand Canyon.
The first detailed images of this dramatic landscape surprised and excited the NASA experts.
They show exposed layers of dark and light rock, crossed by sand dunes. Within the layers are a series of linear fractures, or joints, surrounded by "halos" of light-coloured bedrock. Scientists say the "halos" are caused by chemical bleaching and are clear evidence that fluid, most likely water, once flowed through the giant fractures.
The findings were presented at the start of the world's biggest science conference, the annual meeting of the American Association for the Advancement of Science (AAAS), being held in San Francisco.
Professor Alfred McEwen and Dr Chris Okubo, who analysed the images at the University of Arizona in Tucson, wrote: "On Earth, bleaching of rock surrounding a fracture is a clear indication of chemical interactions between fluids circulating within the fracture and the host rock."
They said the fluid was very likely to have been water. This could be confirmed by direct analysis of the rock composition using instruments carried on future Mars rovers.
HiRISE, which takes pictures with different coloured filters, has also captured stunning images of other regions, including Becquerel Crater in Arabia Terra, a densely cratered area in the northern hemisphere, where rock layers can also be seen.
Fluid processes similar to those that shaped Candor Chasma are thought to be evident in Victoria Crater, now being explored by the NASA Mars rover Opportunity. It has been roaming the Martian surface for three years.
HiRISE images of the crater, which sits on a vast plain just south of the Martian equator, revealed structures along the eastern slopes that may have been formed by water flowing through fractures.
Layered deposits have intrigued scientists since they were first discovered by the Mariner 9 and Viking orbiters in the 1970s.
"The origin, history and nature of the light-toned layered deposits are of great interest," said Prof McEwen. "There has been lots of debate about the significance of these materials."
Recent Mars missions have produced a wealth of evidence that long ago, perhaps when life was just emerging on Earth, the planet had an active water cycle.
And the Mars Global Surveyor, another NASA orbiter, has found evidence that water might have flowed briefly in two Martian gullies within the past few years.
FEARS FOR ANTARCTICA
FRAGILE Antarctic ecosystems are already feeling the effects of global warming, scientists warned yesterday.
Penguin populations are being forced south, threatening the abundance of shrimp-like krill that are essential to the local food chain. Larger fish, mammals and birds, including whales and seals, depend on krill for their survival.
"We're already seeing the marine ecosystems respond dramatically to increases in temperatures along the Antarctic peninsula," Berry Lyons, of Ohio State University's polar research centre, told the American Association for the Advancement of Science.
