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A new analysis of pebble-containing slabs investigated by NASA's Curiosity rover confirms a stream once ran through Gale Crater on Mars.
During a pit stop last year, Curiosity came upon hundreds of smooth, round pebbles that look strikingly similar to deposits in river banks on Earth.
Scientists believe the rover rolled onto an ancient streambed, but needed to study the stones in more detail. So Curiosity snapped high-resolution pictures and fired its laser at several pebbles to analyze the chemical makeup.
Researchers say the roundness of the stones was shaped by a fast-flowing stream that probably was ankle to waist-deep. Curiosity landed in the crater near the equator last summer.
Rebecca Williams of the Planetary Science Institute, the lead author of the new report, said that researchers were able to determine the depth and speed of the water that once flowed at the site.
"These conglomerates look amazingly like streambed deposits on Earth," Williams said. "Most people are familiar with rounded river pebbles. Maybe you've picked up a smoothed, round rock to skip across the water. Seeing something so familiar on another world is exciting and also gratifying."
Sanjeev Gupta, a co-author of the report, said that analysis of the amount of rounding on the pebbles indicates that the stream was flowing at a sustained, vigorous speed.
"The rounding indicates sustained flow. It occurs as pebbles hit each other multiple times. This wasn't a one-off flow. It was sustained, certainly more than weeks or months, though we can't say exactly how long," Gupta said.
The stream carried the gravel at least a few miles, the researchers estimated.
The analysis appears in Friday's issue of the journal Science.
The Associated Press contributed to this report.