Mars had ancient lakes which lasted long enough that they could have been places for life to begin on the Red Planet billions of years ago, scientists say.

NASA's Curiosity rover has discovered evidence of the long-ago existence of a series of freshwater lakes inside the Gale Crater, they say.

While they were seemingly transient, drying up and then refilling many times, the entire system of lakes and streams within the 96-mile-wide crater could have lasted for hundreds of thousands of years or even more, the researchers report in the journal Science.

Curiosity has been exploring Gale Crater since landing there in 2012, and since September of 2014 has been studying the base of a three-mile-high mountain dubbed "Mount Sharp" inside the crater.

"Observations from the rover suggest that a series of long-lived streams and lakes existed at some point between 3.8 billion to 3.3 billion years ago, delivering sediment that slowly built up the lower layers of Mount Sharp," says mission scientist Ashwin Vasavada.

The researchers say Curiosity has seen evidence of ancient water environments within Gale Crater, including rivers, deltas and lakes.

Scientists believe the crater was formed by a massive impact on the Red Planet's surface around 3.8 billion years ago.

Even with alternating between drying out and refilling, the lakes within the crater could have provided water even when they disappeared, because a groundwater table beneath them would have persisted, the researchers say.

"If life had evolved on Mars, you now have a habitat which is perpetually wet that would allow microbes to be sustained," says Curiosity team member and study lead author John Grotzinger from the California Institute of Technology. "Those environments would have existed probably for millions, if not tens of millions of years throughout the rocks that we see."

The Curiosity observations strongly suggest liquid water pooled in the crater in both surface and subsurface reservoirs for a period of time that is "geologically and perhaps biologically relevant," the study authors reported.

Evidence of liquid water that lasted for long periods of time also suggests ancient Mars had a much thicker atmosphere billions of years ago, which would have more closely resembled our own planet.

"The more the geology looks like Earth, the more likely it seems that some life-form(s) could have developed in the Martian waters," Marjorie Chan of the University of Utah wrote in a commentary accompanying the published study.

"The geology of Mars still holds the tantalizing possibility that extraterrestrial life might exist or have been preserved, because the evidence of water is so plentiful," she wrote.