The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of 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 ...

. The LMO was likely present on the Moon from the time of the Moon's formation (about 4.5 or 4.4 billion years ago) to tens or hundreds of millions of years after that time. The LMO was a

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...

consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-

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 another planetary body. As the Moon accreted from the debris from the giant impact,

gravitational potential energy Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum Work (physics), mechanical work t ...

was converted to

thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...

. Due to the rapid accretion of the Moon (in about a month to a year), thermal energy was trapped since it did not have sufficient time to thermally radiate away energy through the lunar surface. The subsequent thermochemical evolution of the LMO explains the Moon's largely anorthositic crust, europium anomaly, and KREEP material. The LMO was initially proposed by two groups in 1970 after they analyzed anorthositic rock fragments found in the

Apollo 11 Apollo 11 was a spaceflight conducted from July 16 to 24, 1969, by the United States and launched by NASA. It marked the first time that humans Moon landing, landed on the Moon. Commander Neil Armstrong and Lunar Module pilot Buzz Aldrin l ...

sample collection. Wood et al. used fragments of bulk sample 10085 for their analyses. Ferroan anorthosite (FAN) rocks found during the

Apollo program The Apollo program, also known as Project Apollo, was the United States human spaceflight program led by NASA, which Moon landing, landed the first humans on the Moon in 1969. Apollo followed Project Mercury that put the first Americans in sp ...

are composed primarily (over 90%) of the mineral

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

. More specifically, FAN rocks found on the Moon consist of the

calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...

(Ca) end-member of plagioclase (i.e.,

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

calcium

Initial chemical composition and depth

Initial chemical composition of the LMO is estimated based on the chemistry of lunar samples, along with the chemical composition and thickness of the current lunar crust. For computer modeling purposes, the initial chemical composition is typically defined by weight percent based on a system of basic molecules such as SiO₂, MgO, FeO, Al₂O₃, and CaO. Seven example initial chemical compositions of the LMO from the literature are shown in the figure to the right. These compositions are generally similar to the composition of the Earth's mantle with the main difference being some (e.g., Taylor Whole Moon) or no enhancement (e.g., Lunar Primitive Upper Mantle) of

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

elements. The estimated initial depth of the LMO varies from 100 km to the radius of the Moon.

Crystallization sequence

The exact sequence of

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): Mi ...

s that crystallize out of the LMO depends on its initial state (viz. chemical composition, depth, and temperature). Following the idealized Bowen's Reaction Series,

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

is generally expected to crystallize first, followed by orthopyroxene. These minerals are denser than the surrounding magma and thus sink towards the bottom of the LMO. As such, the LMO is initially expected to solidify from the bottom up. After about 80% of the LMO has crystallized, the mineral plagioclase crystallizes along with other minerals. Lunar rocks that are primarily made of plagioclase (i.e., anorthosite) form and float towards the surface of the Moon, making its primordial crust.

Duration

The LMO may have lasted tens to hundreds of millions of years after the Moon's formation. The Moon is estimated to have formed between 52 and 152 million years after calcium-aluminum-rich inclusions (CAIs), the oldest known solids in the

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 ...

that serve as a proxy for its age of 4.567Ga. The exact formation time of the LMO is somewhat uncertain. End points may be indicated by the age of FAN sample 60025 (4.360±0.003 Ga) and the estimated age of ur-KREEP (4.368±0.029 Ga). If the Moon formed early (i.e., 52 million years after Solar System formation) and both samples indicate full LMO crystallization, then the LMO would have lasted for about 155 million years. In this case, computer models show that one or more heat sources (such as tidal heating) are required to prolong crystallization of the LMO. If the Moon formed late (i.e., 152 million years after Solar System formation) then again using the FAN sample 60025's age and the estimated age of ur-KREEP, the LMO lasted for about 55 million years. Meaning the LMO was not prolonged by one or more additional heat sources. Early Lunar Timeline

In the past, the age difference between the oldest and youngest FAN samples were used to determine the duration of the LMO. This was problematic due to the large errors of sample ages and due to some sample ages being reset by impacts. For instance, the oldest FAN sample is 67016 with a Sm-Nd age of 4.56±0.07 Ga and the youngest is 62236 with a Sm-Nd age of 4.29±0.06 Ga. The difference between these ages is 270 million years. This would again mean that the LMO had an additional heat source, such as tidal heating.

Zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...

analysis of

Apollo 14 Apollo 14 (January 31February 9, 1971) was the eighth crewed mission in the United States Apollo program, the third to Moon landing, land on the Moon, and the first to land in the Geology of the Moon#Highlands, lunar highlands. It was the las ...

samples suggests the lunar crust differentiated 4.51±0.01 billion years ago, indicating lunar formation 50 million years after the beginning of the Solar System.

Alternative models

One of the alternative models to the LMO model is the Serial Magmatism model.

References

{{The Moon Geology of the Moon Pre-Nectarian