The giant impact that created the Moon left behind a disk of molten debris, in which, according to numerical models, more than 40% of the mass comes from the impactor Theia. Isotopic analyses of lunar samples, however, show nearly complete overlap with terrestrial rocks, casting doubt on the canonical model. Alongside this work are studies explaining the lunar similarity through high-energy scenarios with rapid proto-Earth rotation or through collisions of comparable-mass bodies, but all require an additional mechanism to shed angular momentum. This paper takes a different path: it does not change the impact dynamics but changes Theia's mantle composition, making it denser and causing it to sink in the lunar magma ocean.
The modeling is based on the assumption that Theia's mantle contained more iron than the proto-Earth's mantle, supported by data from Mars and Vesta. Using the alphaMELTS program, melt densities were calculated at 0 GPa and 2000 °C: for Theia material, the density was 2.78 g/cm³, and for proto-Earth material, 2.67 g/cm³. Then the dynamics of sinking of Theia melt droplets with a radius of 1 km in the proto-Earth magma ocean was estimated: drag force, gravity, terminal velocity about 20 m/s, sinking time about 23 hours. Mixing time due to diffusion was estimated at 3000 years, many orders of magnitude longer, so the layers do not mix. Subsequently, using the pMELTS code, fractional crystallization of two layers near the interface was modeled at pressures from 1.5 to 3 GPa to check whether crystals from the lower layer would float into the upper layer.
Calculations showed that after the Moon formed, the molten ocean stratifies: Theia material, being denser, sinks, while proto-Earth material remains on top. During ocean solidification, a crystal barrier forms at the layer boundary: orthopyroxene and clinopyroxene from the upper layer float onto the lower melt, and olivine from the upper layer floats onto the lower melt in the temperature range from 1549 °C to 1240 °C. In total, about 27% of the upper layer's mass at the boundary ends up as floating crystals, isolating the layers. At 2 GPa (depth about 420 km, radius about 1300 km), isolation is pronounced, and at 2.5 and 3 GPa even stronger. As a result, the Moon's upper solid layer consists of proto-Earth material, and the lower layer of Theia material, explaining the terrestrial isotopic composition of the lunar crust.
The work offers a natural explanation for the lunar isotopic crisis without invoking high-energy scenarios with angular momentum problems. Unlike neighboring works that alter impact dynamics or require isotopic identity between Theia and Earth, here it suffices to assume a higher iron content in Theia's mantle, consistent with data from other small bodies. This shifts the focus from impact dynamics to melt composition and thermodynamics, opening a new class of Moon formation models.
The topic could develop toward more detailed modeling of fractional crystallization accounting for full mantle compositions and realistic pressure distributions. Predictions could be tested via seismic data on the Moon's internal structure: if the lower layer indeed consists of Theia material, it might show up in seismic wave velocities. Also promising is applying the approach to other moons and planets where magma ocean stratification is expected.
The results will influence Moon formation models and the interpretation of geochemical data from lunar samples. They may also be useful for understanding the evolution of other bodies with magma oceans, such as Mars or large asteroids.
The next step could be modeling crystallization with more realistic compositions and testing the sensitivity of results to iron content. It is also important to compare model predictions with seismic data on the Moon's internal structure.
This work directly addresses the unresolved problem of the Moon's origin and the lunar isotopic crisis. It also touches on the broader problem of magma ocean evolution on terrestrial planets and its influence on the observed composition of crust and mantle.
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What the work does
This paper proposes a solution to the lunar isotopic crisis through the stratification of the magma ocean after a giant impact. The author suggests that Theia's mantle was richer in iron, making its melt denser, so it sinks, leaving an upper layer of proto-Earth material. Using the programs alphaMELTS and pMELTS, they calculated densities, sinking dynamics, and crystallization at the layer boundary, showing the formation of a barrier of buoyant crystals that isolates the layers.
Where its strength is
The strength of the work is that it offers a natural explanation for the isotopic similarity without altering impact dynamics or running into the angular momentum problem, relying on the compositional difference between Theia's mantle and the proto-Earth's.
Where it could go next
Worth trying
Why this may matter
If the stratification mechanism works, it could be key to explaining not only the Moon's isotopic composition but also the geochemical evolution of other bodies with magma oceans, expanding our understanding of planet formation.
Nearby works in our archive
The closest work in the archive sits at 0.64 by meaning — the usual level for our corpus.
The sinking time for a Theia melt droplet with a radius of 1 km in the lunar magma ocean was just 23 hours, less than an Earth day. In contrast, mixing the layers by diffusion would take about 3000 years.
The idea of a molten celestial body stratifying echoes Arthur C. Clarke's novel 'Rendezvous with Rama', where the internal structure of a giant cylinder has distinct layers with different properties. In the Moon's case, magma ocean stratification determined the isotopic composition of its surface.