The curtain rises on an astronomical scene playing out far, far away. A hot, young, molten planet spins wildly as it orbits its parent star. In the young solar system, there are plenty of objects being flung about. As time progresses, these will crash into planets and join them, burn up as they get too close to the star, smash into each other and become space dust, or eventually settle into a stable orbit.
During this early time, the planet is repeatedly pelted by this debris, slowly gaining mass. As many of the bodies joining the planet come from different areas of the solar system and were formed at different times, they have different compositions. Slowly, the young planet builds a unique set of elemental characteristics.
Then, one day, catastrophe happens. A giant body about 10 percent the mass of the planet smashes into it, causing molten rock from both objects to be flung in the air and enter orbit. As time again goes by, this molten rock coalesces and cools, becoming a moon.
The far, far away place I’m talking about actually is only far away in time. This is the widely accepted version of how the Earth’s moon was formed billions of years ago.
But new research is pointing to the story being a work of fiction.
Forty years after the Apollo missions, the rocks brought back are still providing valuable information about the moon. Junjun Zhang, graduate student in geophysical sciences at the University of Chicago, and four co-authors recently took a look at the levels of different titanium isotopes in the rocks and discovered that they’re shockingly similar to Earth’s.
As you may already know, the number of protons in the nucleus determines what element it is. Similarly, the number of neutrons in the nucleus determines which isotope of that element the nucleus is. Though chemically the same, the different isotopes of an element have slightly different masses.
So why is this important?
Because the Earth is so old and has been formed by numerous collisions joining its mass, it should have a rather unique ratio of elements and isotopes. If a giant body the size of Mars were to come along, chances are quite high that it would have a different composition. Thus, if said giant body were to collide with Earth to form the moon, the moon should have a composition that reflects contributions from both bodies to its formation.
In the study, Zhang and his colleagues looked at the ratio of titanium-50 to titanium-47. They chose titanium because it is extremely difficult to vaporize, so any amounts of the isotopes in early Earth or the giant body that crashed into it should have remained intact and joined the moon.
But that’s not what the team found. Instead, they discovered that the moon’s ratio of the two isotopes was the same as the Earth’s but different from other places in the solar system, casting doubt on the idea that a giant collision formed our only satellite.
This doesn’t completely disprove the giant impact theory of its formation, though. There is a tiny chance that the object that smacked Earth actually did have the same composition of titanium isotopes. Also, even though it is extremely difficult to do, it is possible that the titanium was still vaporized in the collision and then became incorporated in both bodies, erasing the colliding body’s titanium signature.
The doubt caused by the study also has sprung up some other old ideas, such that the Earth was rotating so quickly that the centripetal force cracked off a piece of mass that became the moon, or that the collision was caused by a giant ball of ice containing no titanium. But both those theories are unlikely, due to what we know about physics.
In the end, it is most likely that the original scenario described at the beginning of the post is correct, it’s just that scientists need to do some more digging and analyzing to fill in the correct details.