Episode 17: The Missing Moon

[upbeat music] There have been several theories over time about how the Moon was formed, but we didn't settle on our current accepted theory until the Apollo missions in the 1960s and '70s.

Before we explored the Moon, there were three main ideas.

One was Capture Theory, which proposed that the Moon was captured by Earth's gravity, having started as an asteroid formed somewhere else in the solar system and passing by the Earth.

However, the fact that the Moon and the Earth have similar ratios of certain chemical isotopes doesn't suggest that the Moon came from elsewhere in the solar system.

Also, the orbital pattern of the Moon doesn't match with the typical patterns for moons that are captured.

In proportion to the Earth, our Moon is unusually large, and in order for it to start orbiting the Earth coming from its own independent trajectory, it would've had to lose a lot of energy.

Even if this did occur, it would be far more likely for it to go into an eccentric orbit, which does not match the almost circular orbit that we observe the Moon to follow.

Eccentricity is a measure of how far the shape of an object's orbit differs from a perfect circle.

The second theory is the Accretion Hypothesis.

This suggests that the Moon formed from the same disk of dust, plasma, and gas that formed the Earth.

This material condensed and collapsed together to form the Earth, and the theory suggests the Moon, also created from this material, began to orbit the Earth as the Earth had amassed a greater amount of material and developed the stronger gravitational pull out of the two.

One of the drawbacks of this theory is that there are some discrepancies in the proportion of particular elements on Earth compared to the Moon.

For example, the Moon has a much smaller iron core for its size compared to Earth, which doesn't add up if they were formed from the same original material.

Another issue is that the angular momentum of the system is too high.

The accretion theory would point to more of a slow synchronization of the orbiting pattern, which differs from the high angular momentum of the Earth-Moon orbit that actually occurs and suggests that there was a large transfer of energy to the system, not a gradual balancing of the orbit.

The third theory is called the Fission Theory.

This is the idea that the Earth was spinning so fast that some material basically flew off and started to orbit it.

Now, whilst the Earth was initially spinning much faster than it is now, modern calculations can disprove this hypothesis.

The calculations show that in order for a chunk of material of that size to spontaneously separate from the Earth, the Earth would have to be spinning so fast that it itself would probably disintegrate instead of somehow ejecting one stable object.

It is also difficult to see how something that was not just made of the same material that Earth formed from, but literally used to be a part of the Earth, could have, on some counts, very different chemical characteristics.

[upbeat music] Those were the main theories of the past, but what do we think today? With information gained from analyzing samples of material from the Moon, we have settled on a different idea.