Water Flowed Over the Surface
Curiosity had barely roved anywhere on Mars when it stumbled right into an ancient riverbed. This area, nicknamed Yellowknife Bay, sits at the bottom an an alluvial fan, where a watershed could have flowed billions of years ago, distributing rocky debris. Though there had been evidence of this finding from pictures taken by orbiters, Curiosity provided the ground-truth results confirming that Gale crater had a wet past.
Scientists working with Curiosity think the area could have once been a fast-moving river that may have been ankle- or even hip-deep on a person standing there. Tiny rocks called clasts had accumulated in Yellowknife, sticking together to form a larger composite rock. The clasts were rounded, suggesting that they had been weathered and transported a long distance, most likely by water because they were too large for wind to have picked them up.
Further exploration at Yellowknife Bay showed evidence that water had percolated through the rocks there, indicating that the environment was dynamic. At certain points in its past the place may have been wet only to dry out later on, and then be filled with water again afterwards. The formations in the image above are fossilized underwater dunes that could have formed at the bottom of an ancient riverbed.
Image: NASA/JPL-Caltech/MSSS