The Moon harbors a secret that has puzzled planetary scientists for decades: its isotopic composition is almost indistinguishable from Earth's. According to the canonical giant impact model, the Moon should have inherited most of its material from the impactor Theia, a Mars-sized planet that slammed into the young Earth. But analyses of lunar samples returned by the Apollo missions showed the opposite: oxygen, titanium, and tungsten isotopes match Earth's to within a few parts per million. This paradox, dubbed the lunar isotopic crisis, cast doubt on the entire story of our satellite's birth. New work offers an unexpected way out: instead of tweaking the impact dynamics, revise Theia's composition and let the lunar magma ocean stratify like a cocktail.
Picture a layered cocktail: heavy syrup sinks to the bottom, while a lighter layer stays on top. In the first hours after the catastrophe, the Moon was covered by an ocean of molten rock hundreds of kilometers deep. The model's author assumed that Theia's mantle contained more iron than the proto-Earth's mantle, which is consistent with data from Mars and Vesta. Calculations using the alphaMELTS program showed that at 0 GPa and 2000 °C, the density of Theia melt was 2.78 g/cm³, while that of proto-Earth melt was 2.67 g/cm³. The difference is only 4%, but under lunar gravity that was enough for Theia's material to start sinking. Blobs of melt with a radius of 1 km descended at about 20 m/s, reaching the bottom in 23 hours. Diffusive mixing would have taken 3000 years, so the layers remained separate.
Then crystallization came into play. As the ocean cooled, minerals began to grow at the boundary between layers: orthopyroxene, clinopyroxene, and olivine. Calculations with the pMELTS code showed that at pressures from 1.5 to 3 GPa (depths up to 420 km), these crystals floated up into the overlying melt, forming an insulating barrier. Over the temperature range from 1549 °C to 1240 °C, about 27% of the upper layer's mass near the boundary consisted of floated crystals, which blocked mixing. As a result, the Moon's upper solid shell formed from proto-Earth material, while the lower mantle came from Theia. This explains why the lunar crust has an Earth-like isotopic composition, even though most of the satellite's mass may have come from the impactor.
This work does more than resolve an old paradox; it opens a new way to think about planet formation. Previously, scientists tried to explain the isotopic similarity by changing the impact energy or assuming that Theia and Earth were nearly identical in composition. Now it suffices to allow that Theia's mantle was slightly richer in iron, and the magma ocean stratification does all the work. This shifts the focus from impact dynamics to melt thermodynamics, and thus a whole class of new models lies ahead. In the future, such models can be tested with seismic data: if the Moon's lower mantle truly consists of Theia material, it will show up in seismic wave velocities. Perhaps we will finally understand what the Moon is really made of and glimpse the first hours of Earth's life.
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.