Martian Find: Evidence of an Ancient, Rushing Stream

Polished pebbles in an old streambed, as found by the Curiosity rover, tell a tale of a planet's rich and watery past

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This pair of NASA handout images compares the Link outcrop of rocks on Mars, left, with similar rocks seen on Earth

Mars can break your heart, and it did the job exquisitely today. For all the splendor of the first pictures from the surface of the moon, it was clear that the place was a big, dead rock — and that it always had been. But every image every orbiter and lander sends back from Mars makes it clearer and clearer that our close neighbor was a near-miss planet, a place that coulda been a contender. It had abundant water and a protective atmosphere and — who knows? — even sentient creatures that might have kept us company in what otherwise has been a very silent universe.

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The images the Curiosity rover just beamed home are more poignant reminders of the planetary beauty Mars surely was. For a long time, the goal of Mars probes has been to confirm that what appear to be dry ocean basins, seabeds and river trails were once filled with water. On-the-ground surveys by the Spirit and Opportunity rovers sealed that deal when they found hematites, salt and other minerals that form only or largely in watery environments. So it was no surprise that Curiosity would find similar proof. Indeed, the ship’s landing site, Gale Crater, was chosen explicitly because it was likely once submerged. A delta-shaped ground formation known as an alluvial fan even reveals how the water flowed.

But the newest pictures are striking — and strangely dislocating — all the same. They reveal nothing more or less than smooth, water-polished pebbles, some as big as a golf ball and others as small as a grain of sand. You’ve seen them in every stream you ever played in as a child; you’ve collected them in paper cups. And now they’ve shown up on Mars.

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The size and shape of the pebbles suggest that the water was moving at about 3 ft. (1 m) per second — a brisk, vigorous flow — and that it was ankle- to knee-deep, according to Curiosity scientist 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,” he said in a NASA release. “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.”

Some of the pebbles are loose and separate; others are mixed into a dry conglomerate that is probably the remains of the ancient streambed. Project scientist John Grotzinger compares that mix to the composite chunks that get dislodged when a jackhammer meets a city sidewalk. The stream was probably fed by a channel in the alluvial fan that the investigators have named Peace Vallis. Still to come are closer investigations of the precise composition of the pebbles and soil, not to mention two years of tooling around Gale Crater and the base of Mount Sharp that rises in its center, which is carved with its own ancient waterways.

The scientists do not expect to find signs of ancient biology among the pebbles they’ve discovered. It’s not impossible for life to take hold in water moving so fast, but “it is not our top choice,” says Grotzinger. But if the same kinds of waterways on Earth indicate anything, it’s that just downstream from the churning currents, the water can can feed into calm, nearly amniotic pools. And there’s no telling what kind of Martian life might once have called such places home.

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