Yep, There Were Definitely Rivers on Mars

The Citizen's Guide to the Future
Sept. 27 2012 5:03 PM

Curiosity Rover Finds a Riverbed on Mars

In this image from NASA's Curiosity rover, a rock outcrop called Link pops out from a Martian surface that is elsewhere blanketed by reddish-brown dust.
In this image from NASA's Curiosity rover, a rock outcrop called Link pops out from a Martian surface that is elsewhere blanketed by reddish-brown dust. The smooth, rounded rocks are evidence of an ancient stream.

NASA/JPL-Caltech/MSSS

Scientists have known for years that water must have once flowed on Mars. Today NASA announced that the Curiosity rover has sent back the most compelling and detailed evidence yet: the first-ever photographs of rocks composed of gravel from a streambed. The photos come from dried-up channels near the base of Aeolis Mons, the towering, 18,000-foot peak in the center of Gale Crater. From NASA’s statement on the discovery:

"From the size of gravels it carried, we can interpret the water was moving about three feet per second, with a depth somewhere between ankle and hip deep," said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. 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."   
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The large size and rounded shape of the gravel indicates that it was carried long distances by the water, probably starting above the crater’s rim, in a channel called Peace Vallis, and flowing down into it. The find comes from a pair of angled rock outcroppings, called “Hottah” and “Link,” which apparently formed the foundation of ancient streambeds in an alluvial fan.

Hottah rock outcropping
The rover found the streambed gravel at a few sites, including the rock outcrop pictured here, which the science team has named "Hottah" after Hottah Lake in Canada's Northwest Territories.

NASA/JPL-Caltech/MSSS

It’s too early to tell whether those fast-flowing streams ever housed living organisms, but it’s at least theoretically possible. Curiosity’s ultimate destination remains Aeolis Mons, also known as Mount Sharp, which contains minerals that could provide further clues as to whether there ever was life on Mars.

Mars rocks and Earth rocks
This set of images compares the Link outcrop of rocks on Mars (left) with similar rocks seen on Earth (right).

NASA/JPL-Caltech/MSSS

Future Tense is a partnership of SlateNew America, and Arizona State University.

Will Oremus is Slate's senior technology writer.

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