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The hinh xes vncar-sized Curiosity rover has spotted evidence of once quite hospitable environs on Mars.

As shown in the imagery below, the NASA robot investigated dried-up lake beds and captured views of ripple formations on their ancient shorelines. Like on Earth, these ripples were almost certainly formed by small waves on open-air (not ice-covered) lakes, planetary scientists say. It provides evidence that Mars was warm, wet, and habitable at a time some research suggests the planet started cooling and transforming into an extremely dry and frigid desert.

These ripples formed some 3.7 billion years ago. (For reference, the earliest known fossils on Earth formed some 3.5 billion years ago.)

"Extending the length of time that liquid water was present extends the possibilities for microbial habitability later into Mars's history," Claire Mondro, a Caltech postdoc who researches the planet's past and led the new study, said in a statement. The research was recently published in the peer-reviewed journal Science Advances.

Mars today is 1,000times drier than the driest desert on Earth. Though no evidence of primitive Martian life has yet been found, it's grown clear that the planet hosted watery environs conducive for such microbes to potentially form. In this case, the evidence of shoreline rippling underscores that these shallow lakes — at least some 200 to 500 meters (650 to 1640 feet) across — were open-air bodies of water, meaning they weren't blanketed in ice cover, as we see on winter lakes or ponds on Earth. This points to hospitable environs.

"The shape of the ripples could only have been formed under water that was open to the atmosphere and acted upon by wind," Mondro explained.

The views below show these ancient ripples, formed in ancient soil and now preserved as Martian rock. They're small, each at some six millimeters (about a quarter inch) high.

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Though Mars once harbored bounties of water, the Red Planet gradually lost its insulating atmosphere, in part to effects of solar radiation and a weakened magnetic field. Ultimately Mars' once thick atmosphere diminished, and bounties of water escaped. Without this insulating blanket, the planet dried out.

Yet for millions of years, Mars at least had the opportunity for life to flourish in lakes, or the moist clays of river deltas. NASA hopes to robotically return pristine Mars rock samples home in the 2030s; the space agency thinks they could potentially show evidence of past surface life.

But even if Martian life never dwelled on the surface, it's possible that life thrived, or even thrives, deep beneath the ground, shielded from the extremes of the callous desert and pummeling radiation.