A little rock can pack a mighty punch. The object that smashed into Earth 4.5 billion years ago to create the moon was relatively small聽鈥� roughly one-tenth the mass of Earth, according to the latest modelling.
Since the 1970s, astronomers have suspected that the moon was created when a giant protoplanet called Theia struck the newly formed Earth. The collision created a cloud of debris, which quickly coalesced into our planet鈥檚 partner. There鈥檚 just one nagging problem: this idea, known as the giant-impact hypothesis, can鈥檛 explain why the moon and Earth are chemically identical.
Later, two different hypotheses arose that could explain why the moon is Earth鈥檚 chemical clone, but they predict radically different masses for Theia. In one scenario, two half-Earths merged to form the Earth-moon system. But the second hypothesis suggests Theia was a small, high-velocity projectile that smacked into a large and fast-spinning young Earth.
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at the Paris Institute of Earth Physics and his colleagues set out to determine how each idea matches up with our planet鈥檚 chemistry.
Matching the mantle
鈥淲e know that the giant impact occurred within the first 100 million years and that the impactor, regardless of whether it鈥檚 a small impactor or a big impactor, must have left a chemical imprint in the Earth鈥檚 mantle,鈥� Badro says. That chemical imprint differs depending on Theia鈥檚 mass.
The researchers ran more than 2 billion simulations of the crash, tweaking Theia鈥檚 mass and other parameters of the young Earth, such as whether it was entirely molten or not. No matter the scenario, they found that an impactor larger than 15 per cent of the mass of Earth 鈥� slightly heavier than Mars聽鈥� cannot produce the chemistry we see in Earth鈥檚 mantle. A heavier object assembles a mantle far too rich in nickel and cobalt.
鈥淚t鈥檚 a really creative approach to use chemistry to try to constrain the moon formation,鈥� says at Harvard University.
The team cannot pin down the exact dynamics of the impact, like how fast Theia might have been spinning when it hit the young Earth. Regardless of its spin, though, Theia was probably not the behemoth we thought it could be.
Geophysical Research Letters
Read more: Lunar volcanoes and lava lakes gave the early moon an atmosphere
