NASA’s steady reconnaissance of Mars with the Curiosity rover has produced another major discovery: evidence of an ancient lake with water so low in salinity and so neutral in acidity that it could plausibly be described as drinkable.

Scientists have known that the young Mars was warmer and wetter, but this is the best evidence yet that the planet had what people would call swimming holes (though scientists say it would have been chilly – you’d have wanted to wear a wetsuit).

Additional Photos

This NASA image from February shows a self-portrait taken by the Mars rover Curiosity. The rover has uncovered signs of an ancient freshwater lake on Mars that may have teemed with microbes for tens of thousands of years. The Associated Press

The “freshwater” lake could have harbored life, in theory. 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.

The findings are being published Monday online by the journal Science and will be discussed in San Francisco at the annual meeting of the American Geophysical Union.

Scientists had announced this year that they’d found signs of an ancient lake within Gale Crater, but the new reports provide a much more detailed analysis. The lake existed 3.6 billion years ago and stuck around for thousands of years, at least, the new reports conclude. Previous discoveries suggested that Mars once had surface and ground water 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.

“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. During its initial traverse of about one-third of a mile, the rover studied rocks that showed signs of being 4 billion years old. The rover paused at a rocky sandstone outcropping with fine sand and dust accumulating in its shadow.

The scientists named this site Rocknest. They 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 that resembled organic carbon, but the investigation viewed that as an ambiguous finding.

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.

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.

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