Washington — NASA's Jet Propulsion Laboratory (JPL) has unveiled photographs revealing the remnants of a streambed on the Martian surface, a sure sign that water once flowed rapidly over the surface of the Red Planet.

The photos were transmitted from the rover Curiosity, which landed on Mars almost eight weeks ago. The Mars Science Laboratory —- that is the car-sized rover together with an array of scientific instruments — is designed to find evidence of environments where life might have existed.

"Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them," said William Dietrich, a science co-investigator and a professor at the University of California, Berkeley. "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."

Dietrich and other scientists involved in the mission explained the latest findings at a JPL briefing September 27.

The photos taken by the rover's cameras and transmitted to JPL reveal a conglomerate rock containing stones and gravel with rounded shapes. The material looks like rocks commonly found on Earth, and geologists know just how they are formed.

"The shapes tell you they were transported, and the sizes tell you they couldn't be transported by wind," said Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Arizona. "They were transported by water flow."

Curiosity's onboard instruments could allow the mission's science team to gain a better understanding of the materials that bind the conglomerate rock, which would reveal more about the characteristics of the watery environment that produced it. Further analysis might also yield more information about the regional geology of Gale Crater, the rover's landing site.

Mission planners chose this landing site based on detailed study of many ground images returned from the orbiters that have circled Mars since the 1990s. Aerial views showed a number of geologic features where scientists thought the rover might find clues about the history of Mars' evolution from a hot, wet environment to a cold, dry one.

Curiosity's destination is Mount Sharp, a mountain rising from the center of the crater's plain. Scientists are sure that the clay and sulfate minerals detected there by the orbiters can be good preservers of carbon-based organic chemicals that are potential ingredients for life.

"A long-flowing stream can be a habitable environment," said MSL project scientist John Grotzinger of the California Institute of Technology. "It is not our top choice as an environment for preservation of organics, though. We're still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment."

Given the size of the gravel in the ancient streambed, the science team calculates that the water flow that carried it along moved at about one meter per second and was "somewhere between ankle and hip deep," Dietrich said.

MSL landed on Mars with a two-year prime mission, carrying 10 instruments to collect samples and analyze surface materials to seek out conditions favorable for microbial life.