Did Earth once have two Moons?

The two sides of the Moon look nothing like each other : the close side is flat and low , while the far side is incredibly craggy . We may now be able to work out this topographical mystery … and it take a second moon .

The current skilful hypothesis for the parentage of the Moon is the “ elephantine impact ” model . This holds that a Mars - sized object , sometimes called Theia , once shared Earth ’s domain and collide with it over four billion class ago , and the remains of this cataclysm ultimately organize into the Moon . Now it seem that that cosmic play repeated itself in illumination with the Moon itself . scientist are rather awesomely calling this “ The Big Splat . ”

It ’s not just that the far side of the Moon is mountainous whereas the near side is flat – it in reality has a substantially thickheaded crust than its lowland opposite number . That ’s hard to excuse if the Moon formed all at once , as the formation summons tend to even out such satisfying irregularities . But planetary scientist at UC Santa Cruz suggest a fresh result : what if the elephantine wallop between Earth and Theia created not one , but two moons ?

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This 2d satellite would have been considerably small than the Moon we know today . It probably would have only been about 1/30 the mass of the Moon , which is still decent sized by the satellite banner ( that ’s a lot bigger than Mars ’s lunar month , for instance ) . Both satellite would likely have share the same orbit around Earth , with the second moon situate at one of the two Trojan distributor point of stability relative to our Moon . We discussed Trojan show in some particular latterly with the find of Earth ’s first Trojan asteroid – for a more detailed explanation , gohere .

terra firma ’s first known Trojan asteroid follows our orbit like a 2nd Earth

So how did the two collide ? The 2nd moon was able-bodied to remain static in the early years of the Earth - Moon organisation , because the still forming larger Moon was ab initio much closer to Earth . As the initial topsy-turvyness passed , the primary Moon moved out further into its current arena . The second moonshine was no longer able to stay on in a stable orbit , and it began a tedious , gentle with its big companion – well , slow and soft by the standard of two moonlight smash up into each other , that is .

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Erik Asphaug , one of the leader of the Modern study , explains why this 2nd moon surmisal is so compelling :

“ Our model works well with fashion model of the moon - forge giant shock , which predict there should be monolithic debris left in scope about the Earth , besides the moon itself . It agree with what is known about the dynamical constancy of such a system , the timing of the cooling system of the moonshine , and the age of lunar rocks . Of course , impact modeler essay to explain everything with collision . In this typesetter’s case , it requires an odd hit : being sluggish , it does not organize a volcanic crater , but splats material onto one side . It is something young to think about . ”

As Asphaug indicates , this was no ordinary collision . Because the colliding object was move at a relatively low velocity , crater formation and thawing of the lunar surface was minimal . alternatively , the entire 2nd moon simpleton flattened and accumulated on one side of its former orbital partner , piling on 10 of kilometers worth of newfangled crust . This pile would not have been completely even , however , and that ’s where you get the lunar upland .

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And it ’s not just the meridian differences that this hypothesis accounts for . It also explain why the two sides of the Moon have such radically different compositions . The near side is rich in potassium , P , thorium , uranium , and various rare - world element , all of which were deposited on that side by the Moon ’s primeval magma ocean . Now we love why they all ended up on side – the hit flattened the far side and pushed the resource - rich magma around to the other side of the satellite .

Indeed , the collision explains why there ’s a far and nigh side at all . It made the Moon lopsided , make the intact artificial satellite to reorientate itself so that the lighter side face the Earth while the heavier side point away . All that said , there are still other possibilities to describe for why the Moon is influence the way it is . It ’s possible that tidal violence could account for the Moon ’s composition without call forth a second satellite .

But even one of the primary proponents of the tidal force modelling , fellow UC Santa Cruz researcher Francis Nimmo , says there ’s a good chance that a 2nd moon really did have a part to play in the Moon ’s current look :

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“ The fact that the good side of the moon looks so unlike to the far side has been a puzzle since the dawn of the space age , perhaps second only to the origin of the moon itself . One of the graceful panorama of Erik ’s article is that it relate these two puzzles together : perhaps the jumbo collision that work the moon also spalled off some smaller bodies , one of which later fell back to the Moon to cause the duality that we see today . ”

So then , while we ca n’t quite think it confirm , there ’s a very reasonable chance that , at some point in Earth ’s far remote past , you could have looked up at the night sky and seen two moons up there . ( Well , assuming you had a prison term machine handy to get back to before the aurora of life , of course . ) There ’s no two ways about it – that ’s pretty damn awe-inspiring .

ViaNature .

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