In Mars' Nili Fossae, clays are concentrated on slopes and along canyon walls
Water was once widespread on Mars, data from a Nasa spacecraft shows, raising the prospect that the Red Planet could have supported life.
Researchers found evidence of vast lakes, flowing rivers and deltas on early Mars, all of which were potential habitats for microbes.
They also discovered that wet conditions probably persisted for a long time on the Red Planet.
Details appear in the journals Nature and Nature Geoscience.
One study shows that vast regions of Mars' ancient highlands, which cover about half the planet, contain clay minerals - which can form only in the presence of water.
Volcanic lavas buried the clay-rich regions during subsequent, drier periods of the planet's history, but impact craters later exposed them at thousands of locations across Mars.
The data comes from the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument on the US space agency's Mars Reconnaissance Orbiter spacecraft.
CRISM works by "reading" over 500 colours in reflected sunlight to detect particular minerals on the Martian surface - including those that formed in the presence of water.
"The big surprise from these new results is how pervasive and long-lasting Mars' water was, and how diverse the wet environments were," said CRISM's chief scientist Scott Murchie, from the Johns Hopkins University Applied Physics Laboratory in Maryland.
The clay minerals, known as phyllosilicates, preserve a record of the interaction of water with rocks dating back to the Noachian period of Martian history, which lasted from about 4.6 billion years ago to 3.8 billion years ago.
This was a time in which the Earth, the Moon and Mars were being pummelled by comets and asteroids.
Rocks of this age have largely been destroyed on Earth by plate tectonics. They are preserved on the Moon, but were never exposed to liquid water.
Rivers once flowed into Mars' Jezero Crater, carrying clay minerals
So rocks containing phyllosilicates on Mars preserve a unique record of watery environments in the early Solar System, some of which could have been stable long enough for life to get started.
Importantly, CRISM detected hydrated silicates - so called because they contain water in their crystalline structure - within sediments that had been clearly deposited by water.
The clay minerals were found in fans and deltas within the Holden, Eberswalde and Jezero craters on Mars.
"In most locations the rocks are lightly altered by liquid water, but in a few locations they have been so altered that a great deal of water must have flushed though the rocks and soil," said Jack Mustard, professor of planetary geology at Brown University in Rhode Island.
"This is really exciting because we're finding dozens of sites where future missions can land to understand if Mars was ever habitable and if so, to look for signs of past life."
Deep and cool
The European and US space agencies have targeted several phyllosilicate-rich regions as potential landing sites for their next rover missions to Mars.
The team also found phyllosilicate deposits in thousands of places in and around craters, including the pointed peaks located at the centres of some impact depressions.
MSL LANDING SITE SHORTLIST
Nili Fossae Trough - 25km-wide linear trough with exposed clays and volcanic deposits
Holden Crater - 150km-wide crater once filled by a lake which experienced a massive flood
Mawrth Vallis - Valley with many exposed, clays deposited over a long time
Eberswalde Crater - 65km-wide depression once filled with water and featuring an ancient delta
Northern Meridiani - Region at equator rich in minerals that typically form in presence of water
Miyamoto Crater - Impact depression in Meridiani region with exposed clay minerals
This example suggests that water was present 4-5km below the ancient Martian surface, the researchers said.
Crater-causing collisions are thought to have excavated underground minerals that were then exposed on the crater peaks.
"Water must have been creating minerals at depth to get the signatures we see," Professor Mustard explained.
The clay minerals must have been formed at relatively low temperatures.
"What does this mean for habitability? It's very strong," explained Professor Mustard.
"It wasn't this hot, boiling cauldron. It was a benign, water-rich environment for a long period of time."
In a separate study published in the journal Nature Geoscience, a team led by Bethany Ehlmann, from Brown University, analysed sedimentary deposits in two deltas within Mars' Jezero crater - which once hosted a body of water measuring some 40km (25 miles) across.
The deltas suggest a river transported clay minerals into the basin from a watershed.
"Not only was water active in this region to weather the rocks, but there was enough of it to run through the beds, transport the clays and run into the lake and form the delta," said Ms Ehlmann.
She added that the deltas appeared to be excellent candidates for finding stored organic matter, because the clays brought in from the watershed and deposited in the lake would have trapped any organisms, leaving a "cemetery of microbes".
Nasa will send a robotic rover, Mars Science Laboratory (MSL), to the planet in September 2009 to look for signs of past or present life.
The European Space Agency (Esa) also plans to send a rover to investigate Mars' habitability. This mission, called ExoMars, is scheduled to launch in 2013.