Mars was once a water world, concludes a new study that dives into the controversy over whether an ocean covered much of the Red Planet’s northern hemisphere early in its history.

Ever since researchers found hints in the late 1980s that Mars’ northern lowlands are ringed by what appears to be a dried-up shoreline, planetary scientists have debated whether the region was covered by water about 3.5 billion years ago. Now, Gaetano Di Achille and Brian Hynek of the University of Colorado in Boulder have analyzed spacecraft data to find that 29 of 52 dry river deltas and thousands of river valleys within and surrounding the northern lowlands all lie at about the same elevation. That’s just what would be expected if a sea once blanketed the region, leaving behind a level coastline, the researchers report online June 13 in Nature Geoscience.

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