Mercury, the smallest and densest planet in the solar system, was first observed at least five thousand years ago, but its proximity to the sun makes it difficult for scientists to study. This appealing resource illuminates Mercury for young readers with satellite images, helpful diagrams, and vocabulary boxes. Topics covered include Mercury's formation, surface, atmosphere, orbit, and rotation, as well as Mariner 10 and Messenger, the unmanned spacecraft that have flown by Mercury and provided more insight into the planet's features.