Mercury: Observation, Measurement & Evidence

Mercury has a large iron-rich core occupying much of its radius, a comparatively thin silicate mantle and crust, widespread impact terrain, giant basins such as Caloris, long lobate scarps produced by planetary contraction, and a weak global magnetic field. Its exosphere is extremely tenuous rather than a dense atmosphere.

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'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.

Mariner 10 and MESSENGER imaging, spectroscopy, laser altimetry, gravity, and magnetic measurements established the modern model. Radar first indicated polar ice, while spacecraft data confirmed hydrogen-rich cold traps and mapped surface composition. Mercury's magnetic field demonstrates that at least part of its metallic core remains fluid.