Magma may have formed the moon

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Earth is remarkable in our solar system because it is the only rocky planet with such a large moon, and is also the only planet besides Pluto to have a moon so fundamentally similar to itself in composition. Most bodies in the solar system have unique chemical compositions, but lunar rocks brought home during the Apollo missions show that their isotopic fingerprint is almost identical to that of minerals from Earth.

Magma may have formed the moonThese results have been seen as proof that the formation of the moon was the result of the so-called “Giant Impact Hypothesis”, which states that the moon was created from the collision of two protoplanets, or embryonic worlds. One of those was the new-born Earth, and the other was a Mars-size rock called Theia. The moon then coalesced from the debris.

Computer models of the giant-impact scenario often say that more than 60% of the moon should be made of material from Theia, but this is not the case. One recent lunar formation model therefore suggested the moon might have formed from an impact so violent, it vaporised a large portion of the early Earth, with the moon emerging from the resulting doughnut-shaped mass called a synestia. Another suggests the collision involved a fast-spinning proto-Earth.

However, these models require unlikely impact conditions. Now a team of researchers have come up with a more likely theory: that our planet’s only natural satellite formed following the impact with Theia, but that instead of debris coalescing, liquid magma from the surface of the early Earth was ejected into space. This magma solidified, along with a small proportion of Theia’s material, to form the moon as we know it today.

According to study lead author Natsuki Hosono, a planetary scientist at the Japan Agency for Marine-Earth Science and Technology in Yokohama, this new model confirms previous theories about how the moon formed without the need to propose unconventional collision conditions. “In our model, about 80% of the moon is made of proto-Earth materials. In most of the previous models, about 80% of the moon is made of the impactor. This is a big difference,” adds co-author, Shun-ichiro Karato, a geophysicist at Yale University.

The researchers claim that shortly after the Earth formed, it was covered by a sea of hot magma, while the impacting object was likely made of solid material. The team performed a computer simulation showing that the impact would have heated the magma much more than solids from the impacting object. According to this simulation, the magma expands in volume and shoots into orbit to form the moon, explaining why there is much more Earth material in the lunar makeup. Even a glancing blow from the impactor would have been enough to knock more than 70% of the moon-forming debris from the magma ocean.

“Prior work on lunar formation basically ignored the effect of the magma ocean,” Hosono told “Our research concluded that the magma ocean is one of the most important things for the moon-forming giant impact.”

The new model suggests that the amount of debris from an impact off a molten Earth was comparable to the current mass of the moon. Prior theories, on the other hand, suggested that in order to build the moon, the giant impact first needed to generate an amount of debris equal to about three or four times the moon's mass.

However, Hosono and his team are not leaving it there. In the future, the researchers will tweak other variables such as the mass of the impactor and its rotation to see if the amount of generated debris will form a moon of the right size.


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