Parts of Mars were once "drenched" with so much water that life could easily have existed there, NASA said on Tuesday.
The robot explorer Opportunity has seen clear evidence of the main goal of Mars exploration -- that water once flowed or pooled on the Red Planet's surface.
"Opportunity has landed in an area of Mars where liquid water once drenched the surface," NASA associate administrator Ed Weiler told a news conference. "Moreover, this area would have been good habitable environment."
That does not mean that evidence of life has been found -- but it suggests that life could have evolved on Mars just as it did on Earth, NASA said.
It does mean NASA can go ahead with a plan to eventually send people to Mars. Finding strong evidence of water has been a prerequisite for more ambitious missions.
Evidence of frozen water has been seen in several places on Mars and photographs taken from orbiters have shown structures that could have been formed by flowing or gushing water, but the Opportunity's instruments provide the strongest evidence yet of something resembling the way water flows and collects on or just under the surface of the Earth.
Opportunity landed on Jan. 24 in a small crater on the vast flat Meridiani Planum near the planet's equator. It has been studying finely layered bedrock in the crater's wall.
Scientists have been puzzling over whether the layers were formed by wind, volcanic lava flows or water and if little round balls nicknamed "blueberries" may have been formed by water.
They have also been intrigued by the discovery of a gray shiny mineral called hematite, which on Earth is formed in water.
The scientists said the hematite, the blueberries and the heavy salt content of the area all add up to one conclusion -- salt water.
"We have concluded the rocks here were once soaked with liquid water," said Steve Squyres of Cornell University in Ithaca, New York, who leads the scientific investigation.
"It changed their texture and it changed their chemistry," he added. "We cannot yet tell you with certainty that these rocks were laid down in a lake, in a pool, in a sea."
They may have been formed by water percolating through layers of volcanic ash, he said.
"[This area] would have been suitable for life," Squyres said. "That doesn't mean life was there. But this was a habitable place on Mars at one period of time."
More will be known when a mission can be sent to bring back Mars rocks, Squyres said. "The best way to get at the age is going to be to bring some of this stuff back," he said.
"It is clear that we are going to have to do a sample return," agreed Weiler. He said work will start right away on preparing for an eventual human mission to Mars.
In the meantime, another robotic mission will be set up, probably to pick up some rocks and soil and bring them back to Earth for close analysis.
Pictures from the rover's panoramic camera and microscopic imager show a rock it has been looking at called "El Capitan" is pocked with indentations about a centimeter long.
"This distinctive texture is familiar to geologists as the sites where crystals of salt minerals form within rocks that sit in briny water," NASA said in a statement.
Benton Clark, chief scientist of space exploration at Lockheed Martin Space Systems Astronautics Operations in Denver, said the salty area resembled a dried-up seabed -- and the composition was comparable to the saltiness in the Dead Sea, between Israel and Jordan.
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