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.