Today, we know of Mars as a cold, dry desert, with patches of subterranean ice and ice caps at its poles. Billions of years ago, however, liquid water flowed freely across the planet. And, while NASA's various Mars rovers have uncovered signs that such water once existed on Mars, there's perhaps no better evidence of an ice-free, shallow lake than these two sets of ripples in Martian rock.
In November 2022, NASA's Curiosity rover imaged the Amapari Marker Band in the foothills of Mount Sharp, located in Gale Crater. Within the marker band itself — a thin, dark layer of rock — scientists recognized wave ripples in what would've once been a sandy shoreline. A few weeks later, Curiosity imaged another set of wave ripples in the nearby Prow outcrop, which would have been on the lake bed.
The ripples are thought to have formed some 3.7 billion years ago, during a period when Mars was thought to be drying up. But their presence indicates that Mars' climate would have been fairly warm — and its surface wet — at the time. "The shape of the ripples could only have been formed under water that was open to the atmosphere and acted upon by wind," California Institute of Technology (Caltech) postdoctoral researcher Caire Mondro said in a statement. And if the water was blown by wind, that means it was not covered in ice, as some scientists have hypothesized.
From these ripples, researchers not only determined the existence of a shallow lake here, but also its depth. Using a computer model, Caltech professor of geology Michael Lamb found that the size and spacing of the ripples indicate the lake was less than six feet (two meters) deep.
"Earlier missions, beginning with Opportunity in 2004, discovered ripples formed by water flowing across the surface of ancient Mars, but it was uncertain if that water ever pooled to form lakes or shallow seas," John Grotzinger, the former project scientist for Curiosity's mission, said in the statement. "The Curiosity rover discovered evidence for long-lived ancient lakes in 2014, and now 10 years later Curiosity has discovered ancient lakes that were free of ice, offering an important insight into the planet's early climate."
A paper on the discovery was published in the journal Science Advances on Jan. 15.
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