Lunar magma ocean

The LMO was a thermodynamic consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-Earth and another planetary body.

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 sample collection.

[7] Ferroan anorthosite (FAN) rocks found during the Apollo program are composed primarily (over 90%) of the mineral plagioclase.

[9] This suggests that at least upper layers of the Moon were molten in the past due to the purity of lunar anorthosites and the fact that anorthite generally has a high crystallization temperature.

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.

Lunar rocks that are primarily made of plagioclase (i.e., anorthosite) form and float towards the surface of the Moon, making its primordial crust.

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 that serve as a proxy for its age of 4.567Ga.

A simplified animation of the Lunar Magma Ocean crystallization sequence
Animation showing the cross-section of the LMO as it crystallizes over time. The first solids to form (e.g., olivine ) are denser than the surrounding magma, thus sink towards the interior. After about 80% of the LMO has crystallized, less dense solids (i.e., plagioclase ) begin to form and float towards the surface, forming the primordial crust of the Moon.
Lunar ferroan anorthosite rock from Apollo 16
Lunar FAN rock from Apollo 16 ( Sample 60025 ).
Bar chart showing seven published estimates of the initial Lunar Magma Ocean chemical composition by weight percent
Seven published estimates (A-G) of the initial LMO chemical composition shown by weight percent. Minor components such as TiO 2 & Cr 2 O 3 are not shown. [A] Taylor Whole Moon from Taylor (1982) [ 11 ] as modified in Elardo et al. (2011). [ 12 ] [B] O’Neill (1991) [ 13 ] as modified in Schwinger and Breuer (2018). [ 14 ] [C] Lunar Primitive Upper Mantle from Longhi (2006) [ 15 ] as modified in Elardo et al. (2011). [ 12 ] [D] Elkins-Tanton et al. (2011). [ 16 ] [E] Morgan et al. (1978). [ 17 ] [F] Ringwood and Kesson (1976). [ 18 ] [G] Warren (1986). [ 19 ]
Timeline of early lunar history showing estimated Moon formation times with respect to the age of the Solar System and available lunar crust sample ages
The most reliable FAN sample age is shown with a red square (error bars are smaller than the marker) and the best estimate for formation of the original KREEP layer at depth (i.e., ur-KREEP) is shown with a dark cyan triangle. [ 23 ] Oldest [ 26 ] and youngest [ 27 ] FAN samples are shown by gray circles.