One study shows that vast regions of the ancient highlands of Mars, 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 for the study derives from images taken by the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, and other instruments on the orbiter.
"The big surprise from these new results is how pervasive and long-lasting Mars' water was, and how diverse the wet environments were," said Scott Murchie, CRISM principal investigator at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.
The clay-like minerals, called phyllosilicates, preserve a record of the interaction of water with rocks dating back to what is called the Noachian period of Mars' history, approximately 4.6 billion to 3.8 billion years ago.
This period corresponds to the earliest years of the solar system, when Earth, the moon and Mars sustained a cosmic bombardment by comets and asteroids.
The phyllosilicate-containing rocks on Mars preserve a unique record of liquid water environments possibly suitable for life in the early solar system.
"The minerals present in Mars' ancient crust show a variety of wet environments," said John Mustard, a member of the CRISM team from Brown University.
"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. 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," he added.
Another study finds that the wet conditions on Mars persisted for a long time.
Thousands to millions of years after the clays formed, a system of river channels eroded them out of the highlands and concentrated them in a delta where the river emptied into a crater lake.
"The distribution of clays inside the ancient lakebed shows that standing water must have persisted for thousands of years," said Bethany Ehlmann, another member of the CRISM team from Brown.
"Clays are wonderful at trapping and preserving organic matter, so if life ever existed in this region, there's a chance of its chemistry being preserved in the delta," she added.