NASA’s unmanned Curiosity rover has found the most direct evidence to date that ancient Mars once had running water. The robot explorer discovered rock outcroppings thrusting from the Martian surface that are the remains of an ancient stream bed consisting of water-worn gravel that was washed down from the rim of Gale Crater where the nuclear-powered rover landed. This means that Mars was once a much wetter place and increases the chances that it once harbored life ... or still does.

One of the great disappointments of the 20th century was the discovery that Mars, for all practical purposes, is a dead planet. In the 19th century, the observations of the Italian astronomer Giovanni Schiaparelli and his American counterpart Percival Lowell had given the public the popular image of Mars as an ancient world that was dying, yet still maintained water and life.

This Mars, from the time of H.G. Wells to Edgar Rice Burroughs to Robert Heinlein, was the home to an ancient civilization that struggled to hold back the inevitable by building gigantic networks of canals that carried water from the Martian poles to the equator and irrigated regions large enough to be seen from Earth.

Unfortunately, the canals proved to be illusions and NASA’s Mariner probes of the 1960s put paid to a watery planet and replaced it with a Mars scarred by meteors and having hardly any water or air. In recent decades, this gloomy picture has softened a bit as later missions found evidence that water might once have flowed on Mars millions or perhaps billions of years ago, and may even still be there in the form of permafrost deep beneath the surface.

Now, according to NASA, the 4x4-sized Curiosity probe has found an ancient stream bed where water once flowed regularly from the rim of Gale Crater. The first hints that such a geological formation existed was at Bradbury Landing where Curiosity touched down. There, the sky crane that lowered the rover to the surface of Mars blasted away dust from an area dubbed “Goulburn” by mission control at the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Mosaic view of the Hottah outcropping (Photo: NASA/JPL-Caltech/MSSS)

When Curiosity finished post-landing checks and began its explorations in earnest, it found outcroppings of gravel at the areas Link and Hottah, named after rock formations at Link Lake and Hottah Lake in the Northwest Territories of Canada.

Examined with the Curiosity’s high-definition mast camera, the outcroppings resemble shattered pavements – almost like badly mixed concrete. This is not far off. They are actually a conglomerate of gravel ranging in size from sand to small pebbles that have cemented together to form a layer. What is exciting about this is that it’s a classic sedimentary formation. Ever more important, many of the pebbles are rounded in such a way that indicates they were carried along by running water. In other words, this is an ancient Martian stream bed.

Topographical image of Curiosity's exploration area showing the alluvial fan that carried gravel from Peace Vallis (Image: NASA/JPL-Caltech/UofA)

"From the size of gravels it carried, we can interpret the water was moving about three feet (1 m) 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 stream bed material to direct observation of it."

Based on earlier images of the region taken from orbit, the stream bed is part of an alluvial fan of material that washed down over a long period of time from the rim of Gale Crater by way of a channel called Peace Vallis. The distance traveled from there would account for the rounded appearance of some of the stones.

"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," said Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Tucson, Arizona.

Map showing Curiosity's path on Mars and the three stream bed outcroppings (Photo: NASA/JPL-Caltech/Univ. of Arizona)

These stones are important not only because they are evidence of water flowing on ancient Mars on a regular basis, but also because it gives scientists samples from the rim of Gale Crater. These are of particular interest because the rim is thought to contain clay and sulfate minerals, which may contain preserved organic chemicals that might indicate the existence of past or present life.

"A long-flowing stream can be a habitable environment," said Mars Science Laboratory Project Scientist John Grotzinger of the California Institute of Technology in Pasadena. "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."

Because of this and the desire to learn more about the wet environment formed by these deposits, JPL may have Curiosity study the chemical composition of the gravel.

The study of the stream bed is part of Curiosity's two-year mission to explore Mars in search of sites where life might have or still does exist. The rover arrived on Mars on August 6. Since then, JPL has put the nuclear-powered explorer through a rigorous three-week shakedown followed by a series of test drives. During this time, Curiosity fired its rock-vaporizing laser, streamed the first human voice from another planet, wrote messages in the Martian soil, gave itself a thorough self-examination and studied its first rock using its robotic arm.

The JPL video below discusses the recent stream bed discovery.

Source: NASA

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