The discovery of them on Mars by the Curiosity rover made history Thursday. These rocks signified that water had once flowed on Mars, and had flowed for a long time, and that the stream had been fast-flowing and deep.
Above, you can see an image of the rocks that once contained ancient streambeds on now bone-dry Mars.
How the scientists concluded these were evidence of water
The sizes and shapes of the rocks gave clues as to how fast the water flowed and how far:
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," Curiosity science co-investigator William Dietrich of the University of California, Berkeley, said in the NASA press release.
Scientists have long speculated that Mars once held water, but the water-transported gravel is the first evidence that those hypotheses were correct.
The rounded shape of the stones, which range in size from that of a grain of sand to a golf ball, suggests that the water flowed from far above the rim of the Gale Crater where Curiosity landed and is exploring. A number of channels in that area indicates that water flowed continuously or repeatedly over a long time.
"The shapes tell you they were transported and the sizes tell you they couldn't be transported by wind. They were transported by water flow," Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Tucson, Ariz. said.
NASA said that long-flowing streams can be habitable environments, though they are not ideal for preserving evidence of once-living organisms. For that reason, Curiosity will go, as originally intended, to Mount Sharp (which can be seen in this photograph), "but this is insurance that we have already found our first potentially habitable environment," said Mars Science Laboratory Project Scientist John Grotzinger of the California Institute of Technology in Pasadena.
The scientists estimate that water existed on Mars for at least thousands of years, if not millions.
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