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Long ago, a large lava flow passed through the Athabasca Valles region of Mars. We can tell which direction it was flowing by examining the surface of the flow and the remaining "lava wakes." Although you can't sail a boat on a sea of lava, hills and craters that stick up higher than the lava flow act like barriers. When a boat is driven through the water, there is a bow wave at the front of the boat, and a wake that trails off behind that indicates which way the boat is moving. In a lava flow, when a hill sticks up, the lava piles up on the upstream side (just like a bow wave) and can leave …
