New data from NASA's Opportunity rover suggests that mild, pH neutral water — the kind that may have supported microbial life — flowed on Mars before the giant impact crater known as Endeavor formed between 4.1 and 3.7 billion years ago.
Until now, Opportunity ("Oppy," as the rover has affectionately come to be called) had only found evidence of super-salty, acidic water on the Martian surface. But a new analysis of some ancient minerals near Endeavor's rim — the oldest rocks that the rover has encountered — reveals that water with strikingly different pH values washed over the region before and after the crater formed.
Raymond Arvidson from the University of Washington and colleagues from around the world report these latest results  from the Opportunity rover in the 24 January issue of the journal Science. Their findings also add to a growing body of research intended to assess the habitability of Mars.
"We found these rocks under the impact ejecta from Endeavor, so we know they're very old," explained Arvidson. "They represent a period of time in which the water was pretty habitable. That is, it had a mild pH-not particularly acidic and not particularly basic."
"If I were a microbe, I would love to live there," he quipped. "My fossil might even have been preserved there." (Arvidson discusses more of Opportunity's research in a related Science Podcast ).
These new results coincide with the 10th anniversary of Opportunity's 25 January landing on the red planet, and they highlight the career of this intrepid rover, which arrived on Mars with its twin, Spirit. They were followed more than eight years later by the Curiosity rover and its advanced geochemical laboratory, which made its dramatic touchdown on the planet in 2012. NASA had only expected Oppy to operate for a couple months and to travel for a thousand feet or so, but 10 years and 23.6 miles (38 kilometers) later, it's still going strong. (Spirit, however, became stuck back in 2009 and eventually stopped transmitting data to Earth.)
Data from the Mars Reconnaissance Orbiter, which has been studying Mars from orbit since 2006, first alerted Arvidson and his team to the presence of iron- and aluminum-rich clay minerals near Endeavor's rim. Once the researchers navigated Oppy to the corresponding location, they dug in and struck pay dirt.
It was there that the rover encountered fine-grained, layered rocks with the signature of the earliest episode of water activity documented on Mars. Unlike all of the younger rocks that Oppy had come across before, which told tales of highly acidic water that would have made life tough for even the hardiest extremophiles, these old rocks appear to have seen milder conditions.
Taken together, the team's findings indicate that the water that washed over the region before the Endeavor Crater formed would have been much more hospitable to microbial life than the water that flowed there after the impact.
Related results  from the Curiosity rover, reported in December 2013, suggested that an ancient lake theoretically capable of hosting life once covered the region of the red planet known as the Gale Crater. This ancient Martian lake would have had a relatively neutral pH, low salinity and a number of key biological elements, such as carbon, hydrogen, oxygen, sulfur, nitrogen and phosphorous, according to the researchers.
But all of these results only demonstrate that early Mars was habitable; they don't mean that the red planet was actually inhabited. Only more research can answer that question-and luckily, Opportunity and Curiosity are set to continue their exploration of the Martian landscape.
"Opportunity is solar-powered, so we're parked for the winter," Arvidson said. "But we've identified a small outcrop of rocks to the south that appear to be more aerially and vertically exposed, and we'll direct Opportunity toward them in the spring."