Planetary differentiation in the context of "Lunar surface"

The Moon is the only natural satellite of Earth. It orbits around Earth at an average distance of 384,399 kilometres (238,854 mi), a distance roughly 30 times the width of Earth. It completes an orbit (lunar month) in relation to Earth and the Sun (synodically) every 29.5 days. The Moon and Earth are bound by gravitational attraction, which is stronger on their facing sides. The resulting tidal forces are the main driver of Earth's tides, and have pulled the Moon to always face Earth with the same near side. This tidal locking effectively synchronizes the Moon's rotation period (lunar day) to its orbital period (lunar month).

In geophysical terms, the Moon is a planetary-mass object or satellite planet. Its mass is 1.2% that of the Earth, and its diameter is 3,474 km (2,159 mi), roughly one-quarter of Earth's (about as wide as the contiguous United States). Within the Solar System, it is larger and more massive than any known dwarf planet, and the fifth-largest and fifth-most massive moon, as well as the largest and most massive in relation to its parent planet. Its surface gravity is about one-sixth of Earth's, about half that of Mars, and the second-highest among all moons in the Solar System after Jupiter's moon Io. The body of the Moon is differentiated and terrestrial, with only a minuscule hydrosphere, atmosphere, and magnetic field. The lunar surface is covered in regolith dust, which mainly consists of the fine material ejected from the lunar crust by impact events. The lunar crust is marked by impact craters, with some younger ones featuring bright ray-like streaks. The Moon was volcanically active until 1.2 billion years ago, surfacing lava mostly on the thinner near side of the Moon, filling ancient craters, which through cooling formed the today prominently visible dark plains of basalt called maria ('seas'). The Moon formed out of material from Earth, ejected by a giant impact into Earth of a hypothesized Mars-sized body named Theia 4.51 billion years ago, not long after Earth's formation.

A planetary surface is where the solid or liquid material of certain types of astronomical objects contacts the atmosphere or outer space. Planetary surfaces are found on solid objects of planetary mass, including terrestrial planets (including Earth), dwarf planets, natural satellites, planetesimals and many other small Solar System bodies (SSSBs). The study of planetary surfaces is a field of planetary geology known as surface geology, but also a focus on a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences, and astronomy. Land (or ground) is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.

In differentiated bodies, the surface is where the crust meets the planetary boundary layer. Anything below this is regarded as being sub-surface or sub-marine. Most bodies more massive than super-Earths, including stars and giant planets, as well as smaller gas dwarfs, transition contiguously between phases, including gas, liquid, and solid. As such, they are generally regarded as lacking surfaces.

The age of Earth is estimated to be 4.54 ± 0.05 billion years. This age represents the final stages of Earth's accretion and planetary differentiation. Age estimates are based on evidence from radiometric age-dating of meteoritic material—consistent with the radiometric ages of the oldest-known terrestrial material and lunar samples—and astrophysical accretion models consistent with observations of planet formation in protoplanetary disks.

Following the development of radiometric dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. The oldest such minerals analyzed to date—small crystals of zircon from the Jack Hills of Western Australia—are at least 4.404 billion years old. Calcium–aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.5673 ± 0.00016 billion years old giving a lower limit for the age of the Solar System.

A protoplanet or planetary embryo is an astronomical body originated within a protoplanetary disk that has undergone internal melting to produce a differentiated interior.

Protoplanets are thought to form out of kilometer-sized planetesimals that gravitationally perturb each other's orbits and collide, gradually coalescing into larger bodies through a process known as "runaway growth". Once accumulated enough mass, protoplanets will begin to assume a spherical shape due to hydrostatic equilibrium and become dwarf planets, those of which that subsequently succeed in dominating their own orbit will become planets proper.

A mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are made of rock or ices, and are generally the largest and most massive layer of the planetary body. Mantles are characteristic of planetary bodies that have undergone differentiation by density. All terrestrial planets (including Earth), half of the giant planets, specifically ice giants, a number of asteroids, and some planetary moons have mantles.

A planetary-mass moon is a planetary-mass object that is a natural satellite of another non-stellar celestial object. Because of their mass, these moons are large and ellipsoidal (sometimes spherical) in shape due to hydrostatic equilibrium caused by internal partial melting and differentiation and/or from tidal or radiogenic heating, in some cases forming a subsurface ocean.

Planetary-mass moons are sometimes called satellite planets by some planetary scientists such as Alan Stern, who are more concerned with whether a celestial body has planetary geology (that is, whether it is a planetary body) than its solar or non-solar orbit (planetary dynamics). Thus they consider planetary-mass moons to be a subset of the planets. This conceptualization of planets as three classes of objects (classical planets, dwarf planets and satellite planets) has not been accepted by the International Astronomical Union (the IAU).

Having a mean density of 3,346.4 kg/m, the Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and planetary core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust.

Geochemical mapping from orbit implies that the crust of the Moon is largely anorthositic in composition, consistent with the magma ocean hypothesis. In terms of elements, the lunar crust is composed primarily of oxygen, silicon, magnesium, iron, calcium, and aluminium, but important minor and trace elements such as titanium, uranium, thorium, potassium, sulphur, manganese, chromium, and hydrogen are present as well. Based on geophysical techniques, the crust is estimated to be on average about 50 km thick.