The Moon’s Origin and the Tungsten Puzzle
Formation of the Moon
According to the widely accepted giant impact hypothesis, the moon formed about 4.5 billion years ago when a Mars-sized body called Theia collided with Earth. Simulations and analysis of moon rocks suggest that the moon is primarily composed of material from Theia’s mantle, which is similar in composition to Earth’s mantle.
Chemical Composition of the Moon
However, planets typically have distinct chemical compositions. If Theia formed far from Earth, its composition should have been different, and the moon’s composition should not resemble Earth’s mantle.
The Tungsten Puzzle
One element that complicates the moon’s origin story is tungsten. Tungsten is an iron-loving element that tends to sink towards the cores of planets. The moon and Earth should therefore have very different amounts of tungsten, as Theia’s tungsten-rich mantle would have been incorporated into the moon during the impact.
Isotopic Similarities
Two independent studies examined the ratio of two tungsten isotopes in moon rocks and Earth samples. They found that moon rocks have slightly more tungsten-182 than Earth, an intriguing finding because tungsten-182 is produced by the radioactive decay of hafnium-182, which has a short half-life.
The Late Veneer Hypothesis
The simplest solution to the tungsten puzzle is the late veneer hypothesis. This hypothesis suggests that Earth and the proto-moon initially had similar tungsten isotope ratios. However, Earth, being larger and more massive, continued to attract planetesimals after the impact, adding new material to its mantle. This late veneer would have had more tungsten-184 relative to tungsten-182, while the moon would have retained the ratio from the impact.
Evidence for a Late Veneer
The late veneer hypothesis is supported by the fact that Earth has more siderophile elements (elements that love iron) in its mantle than expected. These elements should have sunk into the core but must have been brought to Earth after core formation by meteorite impacts.
Similarity of Tungsten Isotope Ratios
For the proto-moon to match Earth’s tungsten ratio, Theia and Earth must have started with very similar tungsten abundances. Solving this puzzle will require further planetary studies, but the lunar origin story is becoming clearer.
Role of Planetesimals in Lunar Formation
Simulations have shown that it is more likely for large impacts to occur between bodies that formed close together and therefore have similar compositions. This supports the idea that Theia formed relatively near Earth.
Planetesimals and the Late Veneer
Planetesimals continued to bombard the young solar system after the moon’s formation. Earth picked up more of this late veneer material than the moon, further contributing to the differences in their compositions.