The findings were announced in an edition of the journal Science released Friday to coincide with the 10th anniversary of the Mars Opportunity Rover and its twin, Spirit, landing on the Red Planet.
A major focus on NASA's decade of research on Mars surface has been whether the planet may ever have been habitable, scientists said.
"While Mars is too cold now to have the liquid water needed for life, we've had evidence for past water activity on the planet from satellite images of valleys and analysis of rocks by the rovers," said Paulo de Souza of Australia's Commonwealth Scientific and Industrial Research Organization, a member of the rover science team led by Cornell University Professor Steven Squyres.
"But the water that once shaped those landscapes and minerals was as acidic as vinegar," he said.
However, some of the oldest minerals ever analyzed by Opportunity suggest that wasn't always the case, Souza said.
"Our latest research has found not only the earliest episode of water activity documented yet by the Opportunity Rover, but that the geochemistry of the 4-billion-year-old rocks indicates extensive deposits of past water that's among the freshest, most life-sustaining found so far anywhere on Mars," he said. "If there was ever life on Mars, then this would have been the mud for it to live in."