River Rocks
When NASA’s Huygens lander reached the surface of Titan in 2005, it captured the first-ever evidence of liquid on the surface of another planet. Landing near the edge of a dried lakeshore, it also photographed the rocks seen in the left-hand image.
With their flat shape and rounded edges, the Titan pebbles bear a very strong similarity to rocks from a terrestrial shoreline, suggesting that similar processes generated them.
Image: NASA/JPL/ESA/University of Arizona and S.M. Matheson