Mercury: Structures & Vocabulary

Mercury is the smallest major planet and the closest to the Sun, with an unusually large metallic core, a heavily cratered surface, almost no substantial atmosphere, and a strongly eccentric orbit. Despite intense daytime heating, permanently shadowed polar craters can preserve water ice.

Mercury's 3:2 spin-orbit resonance means it rotates three times for every two trips around the Sun. Its eccentric orbit causes large variations in solar heating and apparent solar motion. Interior cooling and core contraction shrank the planet slightly, compressing the crust and producing scarps. Polar craters receive almost no direct sunlight, allowing cold traps to retain ice.

Spacecraft such as Mariner 10 and MESSENGER mapped the surface, gravity, magnetic field, composition, and polar deposits. Radar observations and neutron measurements support the presence of water ice in permanently shadowed regions. Mercury's high bulk density requires a much larger metal fraction than the Moon or Mars.

Mercury contains a large iron-rich core beneath a comparatively thin silicate mantle and crust. Surface units include ancient cratered terrain, smooth volcanic plains, giant impact basins such as Caloris, tectonic scarps, and permanently shadowed polar craters containing water ice.