"The older you go in geologic time, the wetter it was on the surface, and the more evidence for flowing water and ground water, under conditions that are more earth-like," said Arvidson, who also is a professor at Washington University at St. Louis.
Later in Mars's history, the ground water was more acidic and salty, making it difficult for even the most extreme known bacteria to live, Arvidson said in a telephone interview.
The discovery was made on Endeavor, an ancient impact crater from the first period of geologic time, Arvidson said. It has since been heavily eroded by water. The rover found a clay mineral that forms in water under mildly acidic conditions. Though Endeavor is old, the Matijevic formation is even older, and is made of rocks that were sitting on Mars when the asteroid or comet that created the crater hit.
The researchers also found a kind of clay that commonly forms on earth when water flows through fractures in the rock. That suggests a lot of mild groundwater leeched material from the rocks, Arvidson said.
It isn't clear that the conditions were right to preserve any carbon, which can be a signature of life. If Curiosity, the rover that landed on Mars in 2012, were to drive to the Matijevic formation and sample it, that may aid in determining whether any carbon was there.
The younger rover is now headed toward Mt. Sharp, which is composed of about 5,000 meters (16,404 feet) of layered sedimentary rock, which Arvidson described as "a geological strip chart recorder" of Mars's history.
"This is way beyond saying we have water on Mars," he said. "This is trying to get to past chemical conditions of the water, and say something about habitability, and directing future missions to the juiciest havens."