The landing site of the Mars rover Curiosity was once covered with fast-moving and possibly waist-high water that could have possibly supported life, NASA scientists announced Thursday.

Although planetary scientists have often speculated that the now-desiccated surface of Mars was once wet, Curiosity’s cameras provided the first proof that flowing water was present on a least one part of Mars for “thousands or millions of years.”

The early finding led John Grotzinger, the top mission scientist at the Mars Science Laboratory, to conclude that Curiosity had found a potentially “habitable” site — a central goal of the mission — well before heading to its primary destination.

The area may not have other attributes needed for life, he said, but the team now has a “hall pass” on the question of flowing water, and the Gale Crater landing site seemed even more appealing.

“A long-flowing stream can be a habitable environment,” he said. “We’re still going to Mount Sharp [a three-mile-high mound at the center of the crater], but this is insurance that we have already found our first potentially habitable environment.”

Curiosity team scientists determined that flowing water was once present near the Gale Crater landing site based on the telltale size, shape and scattering of pebbles and gravel nearby, especially those found in conglomerate rocks at three sites.

The roundedness of the pebbles is especially significant, they said, and strongly suggests that the rocks were carried down a roughly 20- to 25-mile stream or river and were smoothed along the way.

William Dietrich, professor of geomorphology and member of the Curiosity imaging science team, presented some rounded earthly pebbles, which he said are similar to those found in the images.

“Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them,” Dietrich said. “This is the first time we’re actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it.”

Curiosity made its dramatic landing in early August and has spent much of its time since testing systems and instruments and preparing for its two-year drive.

But the rover’s cameras began sending back images of the conglomerate rock with small pebbles soon after landing, and they provided sufficiently detailed pictures to convince scientists that the pebbles and gravel had a watery past.

Gale Crater was selected as a landing site in part because satellite imaging had earlier found what appeared to be a sizable cut in the crater wall that looked like a dried river or streambed. The bed continued into the crater and then spread out in the shape of a delta. Similar features have been found in many other Martian locations.

The Curiosity team thought the rover had not landed exactly on that dried delta — or “alluvial fan,” as geologists describe it — but the finding of the water-borne rocks is forcing them to rethink the size of the fan.

The confirmation of water flows came in the early days of a mission that had very consciously discarded the long-standing NASA directive to “follow the water” in Mars exploration. Although finding and studying the signatures of past water flows are important for Curiosity’s goal of identifying habitats that could have supported life, the mission motto is now “follow the carbon.” That element is present in all organic compounds, which are the building blocks of life on Earth and are expected to have been similarly essential to any possible Martian life.

Curiosity has two miniature chemistry labs that will test for those organic compounds and other telltale elements.

The rover’s ultimate destination is the three-mile-high mound in the center of the crater, but it will first detour to a nearby and unusual geological meeting of three rock types. Scientists think one of the rock types may have been formed from fine clays, the lightest material carried by the water and so the last to drop out.

Announcement of the long-ago presence of Martian surface water is an early coup for the mission but is consistent with the rover’s unusually good fortunes.

Since the rover made its near-perfect landing, its major systems and instruments have checked out successfully. There have been a few glitches — a set of wind sensors for the weather station was damaged at landing, and unwanted Florida air was discovered in some instruments — but NASA officials say they foresee no lasting obstacles.

“Our biggest anomaly has been that we have no real anomalies,” said Michael Meyer, lead scientist for NASA’s Mars Exploration Program, at NASA Headquarters in Washington. “Definitely surprising, given the complexity of what we’re doing.”