— NASA’s steady reconnaissance of Mars with the Curiosity rover has produced another major discovery: evidence of an ancient lake — with water that could plausibly be described as drinkable — that was part of a long-standing, wet environment that could have supported simple forms of life.

Scientists have known that the young Mars was more Earthlike than the desert planet we see today, but this is the best evidence yet that Mars had swimming holes that stuck around for thousands or perhaps millions of years. (It would have been very chilly — bring a wet suit.)

The findings were being published Monday online by the journal Science and were discussed in San Francisco at the fall meeting of the American Geophysical Union.

Scientists had announced this year that they’d found signs of an ancient, fresh-water lake within Gale Crater, but the new reports provide a much more detailed analysis, including the first scientific measurements of the age of rocks on another planet. The research suggests that Martian winds are sand-blasting rock outcroppings and creating inviting places to dig into rocks that may retain the kind of organic molecules associated with ancient microbes.

Gale Crater is in an area with rocks about 4.2 billion years old. The lake, which scientists think existed a little more than 3.5 billion years ago, was roughly the size and shape of one of New York’s Finger Lakes. The freshwater lake may have come and gone, and sometimes been iced over, but the new research shows that the lake was not some momentary feature, but rather was part of a long-lasting habitable environment that included rivers and groundwater.

An animated video from NASA shows the area on Mars where the Curiosity rover recently discovered evidence of an ancient lake. Narration by John Grotzinger, a Caltech planetary geologist who is the chief scientist of the Curiosity rover mission. (Video courtesy of NASA)

Previous discoveries by Mars rovers had suggested that the Red Planet once had surface and groundwater with the quality of battery acid, but the water in this lake looks much more benign.

“If we put microbes from Earth and put them in this lake on Mars, would they survive? Would they survive and thrive? And the answer is yes,” said John Grotzinger, a Caltech planetary geologist who is the chief scientist of the Curiosity rover mission. He is the lead author of a paper titled “A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars.”

“In March, we did know that we had a lake, but what we weren’t sure of was how big it was and how long it lasted, and also we were not sure about the broader geological context that supports the presence of lakes coming and going for a very long time,” Grotzinger said in an interview.

“This is really similar to an Earth environment,” he said at the AGU news conference.

The duration of this environment matters when it comes to habitability, said Jennifer Eigenbrode, a geochemist at NASA Goddard Space Flight Center and a co-author of three of the new papers.

“If you have it sustained for a while, life can be there and do something and persist,” she said.

The chemistry of the lake would have been congenial to organisms known as chemolithoautotrophs — mineral-eaters. Whether such organisms, which thrive on Earth in exotic environments such as caves and deep-sea hydrothermal vents, actually existed on the young Mars is a question Curiosity lacks the tools to answer.

“I’m most excited about the nature of the water,” said Jim Bell, an Arizona State University scientist who has worked with the cameras on Curiosity as well as two precursor rovers, Spirit and Opportunity, and is a co-author of four of the new papers. “Previous results from Spirit and Opportunity pointed to very acidic water, but what we’re seeing in Gale Crater is evidence of fresh water. Very neutral. Drinkable.”

A fleet of NASA spaceships has flown to Mars in the past decade. The exploration program has used orbiters to study the landscape from on high to search for the most intriguing places to send landers and rovers. Three rovers — Spirit, Opportunity and Curiosity — have rolled their way across that parched landscape in that decade, their movements painstakingly choreographed by technicians at the Jet Propulsion Laboratory in Pasadena, Calif.

Curiosity landed in the summer of 2012 in the 95-mile-wide Gale Crater. The rover carries a suite of instruments for scraping samples and drilling into rock. Scientists have conducted what amounted to a full set of tabletop laboratory experiments on the soils, with Curiosity following commands sent across millions of miles of interplanetary space.

The soils were scooped, photographed, baked and sniffed, and when heated produced a number of gases, including water, sulfur dioxide, carbon dioxide and oxygen. Curiosity also detected molecules of organic carbon, but that’s an ambiguous finding. The organics could have been produced by Curiosity’s instruments, or they could have come to Mars via meteorites. They could also be indigenous, and figuring that out will be part of Curiosity’s mission in months and years to come. But even if the organics are Martian in nature, that doesn’t mean they’re produced biologically.

Mars has as much land surface as Earth, and only a tiny fraction of the planet has now been explored by the rovers. The fact that Curiosity found signs of an ancient lake with benign chemistry suggests that Mars was broadly “habitable” — potentially an abode of life — billions of years ago.

Mars has lost much of its atmosphere since, and dried out, and become a cold, hostile environment with no obvious signs of extant life, though there could be “cryptic” life below the surface.

“Curiosity’s measurements demonstrate clearly that Mars was a habitable world. We still don’t know whether it was or is inhabited, but we know life could have flourished early in Martian history in the aqueous environment we found,” said Laurie Leshin, dean of the School of Science at Rensselaer Polytechnic Institute in New York and a scientist who has worked on the Curiosity mission.

Scientists do not know how life originated on Earth. They don’t know if it fizzes into existence wherever there is the right combination of elements, or if it’s a rare, or even unique phenomenon. The general consensus is that whatever happened on Earth could happen elsewhere, but the discussion is hampered by the fact that the data set of planets with life contains only the one example — the datum that is Earth-life.

There could be 4-billion-year-old microfossils in the Mars rocks, but finding them would probably require a sample-return mission, something NASA would like to pull off with international partners in the next decade.

NASA has approved a plan for a new rover in 2020, but a sample-return project would be an elaborate production involving three separate spacecraft, launching from Earth at two-year intervals. That more ambitious program hasn’t been approved and remains aspirational in a time when NASA’s planetary exploration budget has been squeezed, in part due to cost overruns.

The Curiosity rover was delayed two years, and its $2.5 billion cost is about $900 million over budget, according to G. Scott Hubbard, former head of NASA’s Mars exploration program and now a professor at Stanford. But he applauded the new science results and said it validated the agency’s Mars strategy.

“This is wonderful. It’s a capstone of the decade’s worth of systematic exploration,” Hubbard said.