The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission is a scientific investigation of the planet Mercury, the smallest and least explored of the terrestrial planets. It has the oldest surface and thinnest atmosphere. Temperatures vary from nearly the highest in the solar system to among the coldest. It's the only planet besides Earth to possess a global magnetic field. It has an enormous iron core that takes up at least 60% of its total mass – twice as high a fraction as for Earth. Understanding Mercury and the forces that have shaped it is fundamental to understanding the origin and evolution of the four rocky inner planets in our solar system.