Mercury, also known as 水星 (Shuǐxīng), is the smallest and closest planet to the Sun among the eight planets in the Solar System, with an orbital period of 88 Earth days.
From Earth, the planet appears to have a synodic period of approximately 116 days, moving significantly faster than other planets. This swift motion led the Romans to name the planet after Mercurius, the god of communication and swift messengers.
Due to the planet’s almost nonexistent atmosphere to retain heat, Mercury experiences the greatest temperature fluctuations among the planets, ranging from 100 K (−173°C; −280°F) at night to 700 K (427°C; 800°F) during the day. Mercury’s axial tilt is the smallest in the Solar System (about 1/30 degrees), yet it has the largest orbital eccentricity.
Color image of Mercury taken by MESSENGER.
Structure of Mercury
Mercury is one of the four terrestrial planets in the Solar System and is a rocky planet similar to Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km.
Mercury’s composition is approximately 70% metal and 30% silicate. Its average density of 5.427 g/cm3 is the second highest in the Solar System, slightly less than Earth’s density of 5.515 g/cm3. Ignoring the effects of gravitational compression, materials on Mercury could be denser with a density of about 5.3 g/cm3, compared to Earth’s 4.4 g/cm3 in this scenario.
The average density of Mercury can be used to infer its internal structure. While Earth’s high density is significantly contributed by the gravitational compression, especially at its core, Mercury’s smaller volume and lesser compressed core must mean that its high density indicates a relatively larger core containing more iron.
Geologists estimate that Mercury’s core occupies about 42% of its volume, compared to 17% for Earth. Recent studies suggest that Mercury has a molten core.
Mercury’s core contains more iron than any other planet in the Solar System, and several hypotheses have been proposed to explain this. The most widely accepted hypothesis suggests that Mercury originally had a silicate-to-metal ratio similar to that of common chondrite meteorites, thought to be characteristic building blocks of the Solar System, and that it had a mass approximately 2.25 times its current mass.
Orbit of Mercury
Orbit of Mercury (yellow, brown) in 2006.
Mercury rotates on its axis three times in two orbital periods around the Sun, when using the fixed stars as a reference frame. When viewed from the Sun, in the orbital reference frame, Mercury completes only one rotation in two of its orbital periods. This precise ratio is due to the influence of tidal locking. If a person were standing on Mercury, they would see the Sun move across the sky only once, resulting in one “day” for every two “years” on Mercury.
Surface Features of Mercury
The surface of Mercury is heavily cratered and rugged, resembling the Moon’s surface, with plains and large impact craters indicating it has undergone a long period of geological quiet for billions of years.
Mercury was bombarded heavily by comets and meteoroids shortly after it formed about 4.6 billion years ago, and again during a second bombardment about 3.8 billion years ago. During this second bombardment phase, numerous impact craters appeared across the planet’s surface.
“Strange terrain” formed at the antipodal point of the Caloris impact basin.
Impact Craters and Basins
The impact craters on Mercury range from small pits to large craters with diameters of hundreds of kilometers. They are all in a state of gradual “weathering”, from relatively new ray-crater features to ancient craters that remain as faint traces. The impact craters on Mercury are distinctly different from those on the Moon due to the smaller range of materials ejected after meteoroid impacts, which is a consequence of Mercury’s stronger gravitational field compared to the Moon.
Plains on Mercury
There are two geologically distinct regions of plains on Mercury.
- 1. The intercrater plains are the oldest features on the surface, predating the bombardment of the planet by meteoroids. These volcanic plains appear to have been obscured by earlier craters and typically contain craters with diameters under 30 km.
- 2. The smooth plains are vast low-elevation areas resembling “seas” on the Moon.
Another unusual feature on Mercury’s surface is the presence of numerous cliffs that cut across the plains. As the planet’s core cooled, its crust contracted, leading to the deformation of the surface terrain and the formation of these cliffs.
Surface Environment and Atmosphere of Mercury
Despite its small size and slow rotation, Mercury possesses a notable magnetic field that seems to be distributed across the entire planet.
Mercury’s magnetic field is strong enough to deflect solar wind around the planet, creating a magnetosphere.
Mercury Exploration Missions
Two spacecraft from Earth have visited Mercury: Mariner 10 in 1974 and 1975; and MESSENGER, launched in 2004, which orbited Mercury over 4,000 times in four years. The spacecraft ran out of fuel and crashed onto the planet’s surface on April 30, 2015.
