This Bathtub Ring of Minerals is More Evidence for an Ancient Warm, Wet Mars

NASA's MSL Curiosity rover found a bathtub ring-like deposit of zinc, manganese, and iron in Gale Crater. These metals precipitate out of water in the right conditions, and there's not really any other way they could've become concentrated here. Adding to the excitement, these deposits also form in lakes on Earth, where the concentrated metals are food for some types of bacteria.