Water covers more than 70 percent of the surface of Earth. It’s the only world in the solar system with liquid water on its surface. But that hasn’t always been the case. Water once filled rivers and lakes on Mars. And it might have filled an ocean that covered a third of the planet.

If the ocean existed, it filled a large basin in the northern hemisphere. Today, the elevation of that region is thousands of feet lower than average – a good “bowl” for holding water.

Parts of the bottom of the bowl are covered with sediments that are hundreds of feet thick. They could have been deposited by winds or some other mechanism, but water is considered the most likely culprit. Some of the landforms there resemble underwater canyons seen on Earth. And there are features that look like a shoreline.

A Chinese rover recently added to the discussion. Scientists reported that it found evidence of a shoreline, plus other contours that could have been part of an ocean. Their analysis showed that the ocean could have formed almost 3.7 billion years ago. It continued filling up for about 200 million years. After that, the water froze, etching a shoreline. The ice then evaporated or seeped into the ground. Some of it might still be there, below a thick layer of dust.

Despite the new evidence, the idea of an ocean isn’t settled. Scientists may need an even closer look to confirm that an ocean once covered much of the surface of Mars.

Script by Damond Benningfield