Traci Watson, Special for USA TODAY

It's dry and dusty now, but Mars was once home to a vast, shallow lake or series of lakes that made the Red Planet a welcoming spot for microscopic life for millions of years, scientists said Monday.

Pictures and other data collected by NASA's Mars rover Curiosity show that rivers once flowed into a lake or lakes at the bottom of Gale Crate, an enormous dimple carved out by an incoming space rock. The persistence of the lakes means that the Mars of eons ago probably boasted an ocean or some other large water body, and perhaps rain, the researchers said.

Scientists have known since last year that Mars once featured a life-friendly freshwater lake, but they didn't know how long that lake, at a spot called Yellowknife Bay, had stuck around. Now new data gathered by Curiosity suggest that the water body nicknamed Gale Lake offered a haven to microbes for millions or even tens of millions of years.

The lake's lifespan provided "sufficient time for life to get started and thrive," said Michael Meyer, the lead scientist for NASA's Mars Exploration Program, at a briefing Monday. Scientists don't know whether Gale Lake itself was habitable, but the Yellowknife Bay waters were fresh and probably drinkable.

The latest portrait of a lush Mars comes courtesy of NASA's hard-working Curiosity rover, which touched down on Martian soil in 2012. Pictures taken by Curiosity's bevy of cameras show rock outcrops very much like those that form at Earth's own river deltas, where a river flows into a lake or other still body of water.

Nearby, Curiosity found rocks that bear telltale traits of having formed on the floor of the lake itself, Curiosity scientists said.

The thickness of the rock outcrops indicates that the lake – or lakes – must have sloshed around the bottom of Gale Crater over the course of millions of years, though the lake probably dried up and then reappeared a number of times, the researchers said.

In a remarkably Earth-like scenario, snow on the mountain ridges ringing the crater could have melted to feed the rivers, which might also have been fed by groundwater.

The rap on Mars has long been that it was warm and wet for only the first billion years of its history, but the new-found rocks indicate that the planet was overflowing with water after that, said Ashwin Vasavada, Curiosity deputy project scientist.

Longstanding lakes in the bottom of Gale Crater mean that the Martian humidity must have been high, the temperature relatively balmy, he said, and that "practically requires a standing body of water, like an ocean."

The rover's expedition has also solved the mystery of the origins of Mount Sharp, the Mount-Rainier sized peak rising out of the middle of Gale Crater. The rock outcrops show that Sharp's bottom layers at least were built by slowly accumulating lake sediments that the winds then sculpted into a towering crag.

In the coming months, Curiosity will scale Mount Sharp, looking for evidence of the fate of the lake, which could help explain the radical changes in Mars climate until it became the mostly cold, dry and barren world of today.