Following the water on Mars has long been a strategy to try to understand the planet's past potential for life.
The new results, however, add to astronomers' growing understanding of water on Mars, and could lead to clues of whether life exists, or ever existed, on the Red Planet.
MSN: New proof that water existed underground on ancient Mars
It seems as though NASA or other space agencies "rediscover" water on Mars every few years, so maybe this latest announcement will be regarded as just more of the same.
But water on Mars has a varied history.
MSN: New proof that water existed underground on ancient Mars
Scientists have previously established the existence of water on Mars in the form of ice at the poles and water vapor, and pointed to geological features that appear to have been carved by water ages ago.
"The role of liquid water on Mars is of great importance for its habitability and this study using Mars Express describes a very large zone where groundwater was present for a long time, " said Olivier Witasse, a project scientist with ESA's Mars Express.
MSN: New proof that water existed underground on ancient Mars
Last June, the scientific community was stunned by the announcement that there were signs of liquid water on modern Mars after all.
These grainy photographs may be the first evidence that liquid water can exist on Mars.
Yet in order to have liquid water on the surface, Mars had to have a much thicker atmosphere.
Michael Meyer, lead scientist for NASA's Mars Exploration Program, said the observations provided the strongest evidence to date that water still flowed occasionally on the surface of Mars.
That could give scientists clues as to how, where and when the water flowed on the ancient surface of Mars.
As Spirit began ascending some hills about two miles from its landing site, it found signs that water was once on this part of Mars, too.
Robotic explorers have found evidence of water, a key ingredient for life on Mars and on the moons of Jupiter.
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Yet, some of the most promising locations to search for ancient life on Mars are places where water may have been in contact with volcanic rocks, ancient hydrothermal systems where conditions may have been conducive to life.
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MOLA, which works by bouncing beams of light off the planet's surface, has found mountains twice as high as Everest, a crater ten kilometres deep, and huge valleys carved by more water than scientists believed had ever existed on Mars.
That said, over the past decade, there has been an enormous amount of evidence that water once flowed freely on the ancient surface of Mars.
Nasa's Curiosity rover has only been on the surface of Mars seven weeks but it has already turned up evidence of past flowing water on the planet.
Liquid water is not something scientists expect to be apparent on Mars because the planet is so cold and dry, Squyres said.
Mars rover Curiosity has found the remnants of an ancient streambed on Mars, providing still more evidence that water once flowed freely there.
In this way, things like rocket engines, satellites, Mars-exploring rovers, space suits, and water purifiers on the International Space Station become part of everyday life on Earth.
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They enable Mars Rover Curiosity to search for water and life and tell people on Earth what they find.
This find is just one more piece of the puzzle about what Mars looked like hundreds of millions of years ago, when there was water on the surface.
But what's novel about the CEAC design is that it incorporates a bio-regenerative life-support system -- the kind of technology that could one day not only feed an outpost on the moon or Mars, but also provide it with oxygen and recycle its water.
Micro-organisms on Earth are much more common in fresh water than acidic water, and so rather than Mars just having been a potential haven for some extreme forms of life, Curiosity's discovery means that Mars had -- and still preserves evidence of -- environments that were habitable for an enormous variety of micro-organisms that thrive on our own planet.
It's hard to know how long ago liquid water would have been there because there's no mechanism to date the rocks that rovers find on Mars, Squyres said.
Previous sites explored by the rover are thought to have been shaped by the activity of acidic water, which would pose a more challenging proposition for any simple microbes that might have evolved on the early Mars.
Much of it resulted from volcanoes blasting out gases like carbon dioxide and water vapour, which allowed ice (also a mineral) to form near the poles of Earth, and possibly on Mars too.
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