Mercury is classified as a terrestrial planet, with roughly the same surface gravity as Mars. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of 1,550 km (960 mi) and one-third the diameter of the planet (4,880 km or 3,030 mi). Similarly to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants.

Mercury's sidereal year (88 Earth Days) and sidereal day (58.65 Earth Days) are in a 3:2 ratio. This relationship is called spin-orbit resonance, and sidereal here means "relative to the stars".

Consequently, one solar day (sunrise to sunrise) on Mercury lasts around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days.

Combined with its high orbital eccentricity, the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from -170 degrees Celsius at night to 420 degrees Celsius during sunlight. Due to the minimal axial tilt, the planet's poles are permanently shadowed. This strongly suggests that water ice could be present in the craters. Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field strong enough to deflect solar winds. Mercury has no natural satellite.

As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in the solar system, Mercury was formed approximately 4.5 billion years ago. Its mantle is highly homogeneous, which suggests that Mercury had a magma ocean early in its history, like the Moon. According to current models, Mercury may have a solid silicate crust and mantle overlaying a solid outer core, a deeper liquid core layer, and a solid inner core. Many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimals and rock vaporization.

Nomenclature

The ancients knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one. They knew the planet as Στίλβων Stilbōn, meaning "twinkling", and Ἑρμής Hermēs, for its fleeting motion, a name that is retained in modern Greek.

The Romans named the planet after the swift-footed Roman messenger God, Mercury whom they equated with the Greek Hermes, because it moves across the sky faster than any other planet. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus; a Christian cross was added in the 16th century: ☿

Physical Characteristics

Mercury is one of four terrestrial planets in the Solar System, which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 kilometers. Mercury is also smaller albeit more massive - than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.

Mercury appears to have a solid silicate crust and mantle overlying a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core. The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon, and perhaps sulfur and carbon, plus trace amounts of other elements. The planet's density is the second highest in the Solar System at 5.427 g per cubic centimeter, slightly less than Earth's 5.515 g per cubic centimeter. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than Earth's, with an uncompressed density of 5.3 g per cubic centimeter versus Earth's 4.4 g per cubic centimeter. Mercury's density can be used to infer details of its inner structure. Although Earth's high-density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are less compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.

Surface Geology

Mercury's surface is similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more heterogeneous than the surface of Mars, or the Moon, both of which contain significant stretches of similar geology, such as maria and plateaus. Albedo features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and ray systems. Larger albedo features correspond to higher reflectivity plains. Mercury has "wrinkle-ridges" (dorsa), Moon-like highlands, mountains (montes), plains (planitiae), escarpments (rupes), and valleys (valles).

The planet's mantle is chemically heterogeneous, suggesting the planet went through a magma ocean phase early in its history. Crystallization of minerals and convective overturn resulted in a layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early chemically reducing conditions that are found on other terrestrial planets. The surface is dominated by iron-poor pyroxene and olivine, as represented by enstatite and forsterite, respectively, along with sodium-rich plagioclase and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite.