Magma oceans are vast fields of

surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

that exist during periods of a

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

's or some

natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...

's accretion when the

celestial body An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...

is completely or partly molten. In the early

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, magma oceans were formed by the melting of

planetesimal Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

s and planetary impacts. Small planetesimals are melted by the heat provided by the

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

aluminium-26 Aluminium-26 (26Al, Al-26) is a radioactive isotope of the chemical element aluminium, decaying by either positron emission or electron capture to stable magnesium-26. The half-life of 26Al is 717,000 years. This is far too short for the isotope ...

. As planets grew larger, the energy was then supplied from giant impacts with other planetary bodies. Magma oceans are integral parts of planetary formation as they facilitate the formation of a core through metal segregation and an atmosphere and hydrosphere through degassing. Evidence exists to support the existence of magma oceans on both the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and the

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

. Magma oceans may survive for millions to tens of millions of years, interspersed by relatively mild conditions.

Magma ocean heat sources

The sources of the energy required for the formation of magma oceans in the early solar system were the radioactive decay of

, accretionary impacts, and core formation. The abundance and short half life of aluminium-26 allowed it to function as one of the sources of heat for the melting of

planetesimals Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

. With aluminium-26 as a heat source, planetesimals that had accreted within 2 Ma after the formation of the first solids in the solar system could melt. Melting in the planetesimals began in the interior and the interior magma ocean transported heat via

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

. Planetesimals larger than 20 km in radius that accreted within 2 Ma are expected to have melted, although not completely. The

kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

provided by accretionary impacts and the loss of

potential energy In physics, potential energy is the energy of an object or system due to the body's position relative to other objects, or the configuration of its particles. The energy is equal to the work done against any restoring forces, such as gravity ...

from a planet during core formation are also large heat sources for planet melting. Core formation, also referred to as metal-silicate differentiation, is the separation of metallic components from silicate in the magma that sink to form a planetary core. Accretionary impacts that produce heat for the melting of planet embryos and large terrestrial planets have an estimated timescale of tens to hundreds of millions of years. An example would be the Moon-forming impact on Earth, that is thought to have formed a magma ocean with a depth of up to 2000 km. The energy of accretionary impacts foremost melt the exterior of the planetary body, and the potential energy provided by core differentiation and the sinking of metals melts the interior.

Lunar magma ocean

The findings of the Apollo missions were the first articles of evidence to suggest the existence of a magma ocean on the Moon. The rocks in the samples acquired from the missions were found to be composed of a mineral called

anorthite Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

Anorthite consists mostly of a variety of plagioclase feldspars, which are lower in density than magma. This discovery gave rise to the hypothesis that the rocks formed through an ascension to the surface of a magma ocean during the early life stages of the Moon. Additional evidence for the existence of the Lunar Magma Ocean includes the sources of mare basalts and KREEP (K for potassium, REE for rare-earth elements, and P for phosphorus). The existence of these components within the mostly anorthositic crust of the Moon are synonymous with the solidification of the Lunar Magma Ocean. Furthermore, the abundance of the trace element europium within the Moon's crust suggests that it was absorbed from the magma ocean, leaving europium deficits in the mare basalt rock sources of the Moon's crust. The lunar magma ocean was initially 200-300 km thick and the magma achieved a temperature of about 2000 K. After the early stages of the Moon's accretion, the magma ocean was subjected to cooling caused by convection in the planet's interior.

Earth's magma ocean

During its formation, the Earth likely featured a series of magma oceans resulting from giant impacts, the final one being the Moon-forming impact. The best chemical evidence for the existence of magma oceans on Earth is the abundance of certain siderophile elements in the mantle that record magma ocean depths of approximately 1000 km during accretion. The scientific evidence to support the existence of magma oceans on early Earth is not as developed as the evidence for the Moon because of the recycling of the Earth's crust and mixing of the mantle. Unlike Earth, indications of a magma ocean on the Moon such as the flotation crust, elemental components in rocks, and KREEP have been preserved throughout its lifetime. Today

Earth's outer core Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately beneath Earth's surface at the core-mantle boundary and ...

is a liquid layer about thick, composed mostly of molten

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

and molten

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

that lies above Earth's solid

inner core Earth's inner core is the innermost internal structure of Earth, geologic layer of the planet Earth. It is primarily a solid ball (mathematics), ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. T ...

and below its mantle.{{Cite journal , last1=Zhang , first1=Youjun , last2=Sekine , first2=Toshimori , last3=He , first3=Hongliang , last4=Yu , first4=Yin , last5=Liu , first5=Fusheng , last6=Zhang , first6=Mingjian , date=2014-07-15 , title=Shock compression of Fe-Ni-Si system to 280 GPa: Implications for the composition of the Earth's outer core , journal=Geophysical Research Letters , volume=41 , issue=13 , pages=4554–4559 , doi=10.1002/2014gl060670 , bibcode=2014GeoRL..41.4554Z , s2cid=128528504 , issn=0094-8276, doi-access=free This layer may be considered as an ocean of molten iron and nickel inside Earth.

References

Planetary science Geology of the Moon Hadean volcanism Geochemistry Planetary geology

Magma ocean heat sources

Lunar magma ocean

Earth's magma ocean

See also

References