Unpeeling the history of water on Mars

Years ago, I visited the Grand Canyon with my family. The beauty of it was overwhelming, and everything they say about it is true. It's magnificent. That grandeur is only amplified by the obvious scientific significance of it. The layers of sedimentary rock, exposed by the eons-long patient erosion of the Colorado river, are a dramatic open textbook of the geological history of our planet, as if the Earth itself is saying "Look here, and learn of the past!" Learn we have. And the Earth, as we have also learned, is not entirely unique. From millions of kilometers away, another canyon beckons us to uncover a planet's past.