In the dusty, rust-colored expanse of Gale Crater on Mars, NASA’s Curiosity rover has uncovered a ghostly echo of a long-lost world. Etched into the rock are tiny undulations — ripples frozen in time, preserved for billions of years. These ripples, scientists say, are the telltale marks of waves that once lapped the shores of an ancient Martian lake.

The discovery offers some of the strongest evidence yet that Mars was once a planet of liquid water, not just ice. The ripples, formed 3.7 billion years ago, suggest that the Red Planet’s climate during that time was warm and dense enough to support liquid water exposed to the atmosphere. This hints that Mars may have been habitable for longer than previously thought.

Small but Meaningful Ripples

The notion that Mars once harbored liquid water on its surface is almost unanimous among scientists. Evidence includes ancient river valleys, dried-up lakebeds, and mineral deposits like clays and sulfates that form in the presence of water. Additionally, the presence of hematite, a mineral that often forms in water, and the detection of hydrated minerals further support the idea that Mars had a watery past. 

However, whether any of this water was ever in liquid form while exposed to the atmosphere is less clear.

The ripples were found in two distinct rock formations within Gale Crater, a 96-mile-wide basin that Curiosity has been exploring since 2012. One set of ripples, located in an area called the Prow outcrop, lies within what was once a field of wind-blown sand dunes. The other, found in the sulfate-rich Amapari Marker Band, points to the presence of a shallow lake. Both sets of ripples are small — just 6 millimeters high and spaced 4 to 5 centimeters apart—but their implications are enormous.

“The shape of the ripples could only have been formed under water that was open to the atmosphere and acted upon by wind,” says Claire Mondro, a postdoctoral scholar at Caltech and lead author of the study. “This means the water wasn’t frozen. It was liquid, and it was exposed to the air.”

A Change in the Martian Timeline

The ripples’ size and spacing suggest the lake was shallow, less than 2 meters deep. But what’s most striking is their age: 3.7 billion years old. This places them in a period when Mars was thought to be drying out, losing its water to space. The discovery suggests that pockets of liquid water persisted longer than scientists had assumed, potentially extending the window of habitability for microbial life.

Many models have previously suggested that water on the planet’s surface would have been locked under thick ice, given the faintness of the young Sun and Mars’s greater distance from it. However, these ripples would beg to differ. Mars seemed to have had open water on its surface and may have supported these conditions for quite some time. The Prow ripples formed earlier than those in the Amapari Marker Band, suggesting that conditions suitable for liquid water occurred multiple times — or lasted for an extended period. This raises the possibility that Mars’ climate oscillated between wet and dry phases, rather than undergoing a single, irreversible drying event.

“At the time of ripple formation, climate conditions must have supported ice-free liquid water on the surface of Mars,” the study states.

What This Means for Life on Mars

John Grotzinger, a geologist at Caltech and former project scientist for the Curiosity mission, calls the discovery a “game-changer” for Mars paleoclimate science. “We’ve been searching for these features since the Opportunity and Spirit missions began in 2004,” he says. “Earlier missions found evidence of flowing water, but it was unclear if that water ever pooled into lakes or seas. Now, we have proof of ice-free lakes.”

Liquid water is a key ingredient for life as we know it, and the presence of open water increases the chances that microbial life could have thrived. “Extending the length of time that liquid water was present extends the possibilities for microbial habitability later into Mars’ history,” Mondro says.

While scientists have yet to present any solid proof of ancient life on Mars, the new findings have painted a picture of a planet that was once far more Earth-like. Gale Crater, for instance, is thought to have been a large lake system that persisted for millions of years. 

The findings appeared in the journal Science Advances.