NASA's Curiosity Mars rover has made its first major science discovery, and it's one for the ages.

Scientists announced Thursday that water—fast-running and relatively deep—once coursed over the now bone-dry surface, a finding based on the presence of rounded pebbles and gravel near the rover's landing site in Gale Crater.

What's more, the team has concluded that the water was present for "thousands or millions of years," though the researchers said it would take far more research to get a clearer picture of the flow's longevity.

The discovery is the first proof that surface water once ran on Mars. Planetary scientists have hypothesized that the cut canyons and riverlike beds photographed by Mars satellites had been created by running water, but only now do researchers have on-the-ground confirmation—and the promise of learning much more about the nature and duration of the water flows.

"We've now identified pebbles and gravel at the landing site that clearly have been carried down by water, have been broken down and very much smoothed out," said William Dietrich, a geomorphologist working with the Curiosity imaging science team. "This is the beginning of our process of learning how much water was running and how long this area was wet."

Past Potential for Life?

The evidence in the newfound streambed led Curiosity lead scientist John Grotzinger—known as a cautious and careful scientist—to conclude that the rover had already found a site that was potentially habitable in the distant past. That doesn't mean life existed there or anywhere else on Mars, he said, but rather that some key physical conditions appear to have allowed for its possible emergence.

"Habitability requires water, a source of energy, and a source of organic carbon, and now we have a hall pass for the water observation," he said at a press conference Thursday at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, where the Curiosity mission control is headquartered. Chemical assessments will come later, when the two miniature labs on the rover begin doing their work, though not necessarily at the current site.

"We're still going to Mount Sharp," a three-mile-high (five-kilometer-high) mound at the center of the crater, said Grotzinger, "but this is insurance that we have already found our first potentially habitable environment."

While Curiosity is not a life-detection mission, it is considered an astrobiological exploration—a search for the building blocks of life as we know it and habitats where it might have emerged. The rover has been on Mars now for 51 Mars days, which translate into several additional Earth days.

Pebbles Like M&Ms

The full river and drainage system by the Curiosity landing site is about 200 square miles (520 square kilometers), Dietrich said. It includes the elevated area beyond the crater wall, with fossil streams that feed into a deep, 11-mile (18-kilometer) channel slowly falling down that cliff, and then a 20-square-mile (50-square-kilometer) alluvial fan—the roughly triangular deposit left behind by the long-gone flow. The canyon channel was about 2,000 feet (610 meters) wide.

The most interesting gravel and pebbles—the size of M&Ms or hard candies—were found in conglomerate rocks at three sites close to the landing site. Curiosity's thrusters had dug out the first, Goulburn, at landing. The two others, called Link and Hottah, showed the same pavement-like formations.

According to geologist Rebecca Williams of the Planetary Science Institute, the team did not expect to see the remains of the alluvial fan—now known as Peace Vallis—as far down as the landing site. That they were found, she said, appears to expand the size of the area once touched by water.

"The shapes tell you [the rocks] were transported, and the sizes tell you they couldn't be transported by wind," Williams said. "They were transported by water flow."

A Rich Crater Site

Grotzinger said the team had decided to release the dramatic new information so early because the data were so strong and because they showed the essential connection between satellite imaging (which had initially identified the canyon and fan) and on-the-ground geology.

That geology was conducted entirely by analyzing photos taken by Curiosity's large suite of cameras.

The rover will soon be headed to an area named Glenelg, which many team members believe is more clearly in the alluvial fan. The two chemistry labs are expected to be used for the first time at the Glenelg site—where three rock formations join—and could shed more light on the nature of the water that once flowed there.