Mars likely had an ocean ...

... which probably covered up to 33% of its surface.

The younger, inner shoreline is evidence that an ocean about 10 times the size of the Mediterranean Sea, or about the size of North America, existed on the northern plains of Mars a few billion years ago. The larger, more ancient shoreline that covered a third of Mars held an ocean about 20 times the size of the Mediterranean, the researchers estimate.

Where there is (was) water, there is (was) also the strong possibility of life.