Titanium Paternity Test Fingers Earth as Moon's Sole Parent

Shocker! The moon probably didn't form from "a giant collision between Earth and a Mars-sized object gave birth to the moon 4.5 billion years ago."

"When we look at different bodies, different asteroids, there are different isotopic signatures. It's like their different DNAs," Dauphas said. Meteorites, which are pieces of asteroids that have fallen to Earth, contain large variations in titanium isotopes. Measurements of terrestrial and lunar samples show that "the moon has a strictly identical isotopic composition to the Earth," he said.

"We thought that the moon had two parents, but when we look at the composition of the moon, it looks like it has only one parent," Zhang said.

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So am I correct to assume that this cast a large doubt on a collision ever happening in the first place?

That's what I take from the article. It's highly unlikely that a "Theia" would have exactly the same isotopic signature as Earth if every asteroid differs. So we have a lot of explaining to do.

When I first read about the collision theory, the default assumption was that the proto-Earth was smaller than it is now and the Mars-sized collider was added to the Earth, increasing it to its current size and leaving the lost ejecta to form the Moon (with a few molten droplets to accidentally hit hyperbolic trajectories and slingshot into other parts of the solar system, no doubt). If that original assumption were correct, then the isotopic composition of the current Earth is actually already a combination of original Earth and Mars-sized intruder, masking the parentage issue (essentially, Earth isn't Luna's parent, so much as its sibling, both formed from proto-Earth and the Collider).

The earth-as-single-parent theory isn't in danger yet.

If the earth at the time of the collision was large enough to swallow the colliding body the moon might have formed from earth material thrown off by the collision,

1. the isotope signatures of earth and moon would remain the same, and

2. the earth material thrown off would more likely keep the same face to earth, as the moon does.

If any of the colliding body had became part of the moon, the collision would have been a glancing blow and the moon would more likely rotate and not keep the same face to the earth.

What's in danger is the theory of the colliding body's size.

Less likely, probably, as gravity attracted enough material to form the early earth, the trajectories of the approaching bodies might have resulted in an elongated, dumbbell-shaped body that separated. Since the moon is slowly becoming more distant, the math can be reversed and the time of separation can be estimated.

Good information!  Thanks for sharing!

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