Reading the Rocks

On the Cover: We study rocks to discover clues about a planet's geologic and climatic history. On Earth, gray hematite (center) is commonly formed in liquid water. When scientists detected gray hematite on Mars, they pondered whether it, too, formed in water—could it be evidence of a warmer, wetter past? <br \=""> <br \=""> Gray hematite has been identified in three locations on Mars—Valles Marineris (background), Meridiani Planum (left), and Aram crater (bottom).