The moon and Earth are remarkably similarNASA/Bill Anders By Leah CraneThe moon may be made of magma, according to a new idea about how Earth’s cosmic companion formed. The leading hypothesis for the birth of the moon is that a Mars-sized object called Theia hit Earth, blasting up a cloud of debris which coalesced to become the moon. In many simulations of this process, most of the cloud comes from Theia, which would make the moon unlike Earth. In reality, the compositions of Earth and the moon are extraordinarily similar, so planetary scientists think the cloud ought to have contained lots of material from our planet. Advertisement Natsuki Hosono at the Japan Agency for Marine-Earth Science and Technology and his colleagues performed a new set of simulations that may solve that conundrum. In these simulations, instead of hitting a solid planet, Theia hits an Earth covered in a global magma ocean. Read more: Earth may have a pair of ‘ghost moons’ made of dust trapped in orbit The magma would splash into space much more easily than a rocky mantle, making the moon-forming cloud 70 to 80 per cent Earth material, enough to make the moon and Earth match. About half of the magma ocean would be ejected into space, and Theia’s core would eventually sink into the young Earth. The magma would eventually crystallise to form rocky crusts like the two worlds have today. “It’s not impossible that there should be a magma ocean, but the timing is critical if this was the mechanism for the moon’s formation,” says Jay Melosh at Purdue University in Indiana. If this hypothesis turns out to be true, it could help us figure out exactly when the moon formed. The simulations also matched two other important properties of the Earth-moon system: the moon’s relatively high speed as it orbits the planet, and the fact that the moon has more iron oxide than Earth – iron oxide would have been more abundant in liquid rock than solid. If this is truly how our moon formed, it could make us rethink the formation of other moons in our solar system, Melosh says. We might have to consider whether Mars had a magma ocean when its moons were formed by an impact, or whether Pluto’s subsurface liquid ocean was closer to the crust and helped to form its huge moon Charon. Journal reference: Nature Geoscience, DOI: 10.1038/s41561-019-0354-2 More on these topics: planets moons
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