The Earth May Have Once Had Two Moons

Lunar Far-Side Highlands

A mysterious feature on the dark side of the moon, known as the lunar far side highlands, provide evidence that the Earth may once have had two moons. The far side highlands are a series of vast mountain ranges that are abnormally large in relation to the rest of the Moon's topography. There is a new theory that they were formed in a slow collision with a large rocky body that we now think was once in orbit around the Earth. We believe that our current Moon was created by a giant impact with a Mars sized body when the Earth was still molten around 4.4 billion years ago.

This impact would have blasted out a vast swathe of rock which would have been caught in the Earth gravitational field and come together to form the Moon in just a few tens of thousands of years. The new theory would suggest that some of the material would have formed a smaller companion body which would have stayed in orbit in a stable 'pocket' of gravity, known as a Trojan point, for a few million years. While on the subject, it is worth mentioning that we have recently discovered an asteroid that is trailing the Earth on its own orbit inside a Trojan point. It presents no threat. It simply trails us like a dog.

A computer model of the impact event

Moving back to the subject in hand, the theory states that the Trojan point destabilized and the smaller moon began a slow collision with our Moon. As the collision would have been very slow, there would have been no magma, hell-fire or brimstone; instead just a slow disintegration and deposition of the smaller body's material over the larger moon. Dr Eric Asphaug and his colleague, Martin Jutzi, created a computer model that worked with a body about a thirtieth of the mass of the Moon today.

This was based upon calculations about the impact that formed our moon. The pair believed that there would have been enough material left over for a second moon to form. Jutzi stated that companion moons are common bi-products of giant impact events. Using the model, they tracked the paths of the debris and found that the creation of a second moon would have been very possible. This is currently the best explanation to the lunar dichotomy and they hope that future samples of lunar material alongside spacecraft data will confirm the theory or at least back it up.

Several lunar mysteries to do with topography and geology can be explained by this not too outlandish theory. It is well known that the near side of the Moon is rather flat and low while the far side is is very mountainous. A large, fast impact would have destroyed part of the moon and knocked it off its orbit. However a slow impact such as the one hypothesized here would have had an effect similar to a vast tectonic plate, pushing up mountain ranges while slowly subducting or in this case disintegrating over the surface of the body.

A diagram of the KREEP crustal difference

We also know that the near side has a thin crust just a few kilometres thick while the far side has a crust that is many tens of kilometres thick. This thick crust is cooled KREEP magma. KREEP magma is a rare type of molten rock characterized by high levels of potassium (K), rare earth elements (REE) and phosphorus (P). The only known examples of this type of magma date from the Hadean eon, the time when the Earth was molten and still forming and also the formation date of the moon.

The Moon and its possible twin would have been covered in a layer of this rock type. A collision such as the one suggested here would have spread the KREEP material over the one side of the moon forming the extra thick crust. The collision would have had other effects as well. First and foremost, a slow collision means that there would have been no impact crater. Instead the debris would have piled up, creating lopsided Moon and causing one side to always face us as is the case today.

You might be wondering why I have put a geology based article into a palaeontology blog. It is here because the formation of the moon was vital to the development of life and this new theory could explain certain persistent mysteries surrounding this subject. Life, as far as we know cannot exist on a molten planet such as the early Earth. The Earth cooled, a stable crust formed and liquid water could therefore exist, allowing life to develop. However exactly how the Earth cooled is a mystery.

We know that the heavier and hotter elements such as iron, nickel and uranium would have sunk to the center of the Earth, drawing heat away from the surface. However this would have not been enough. Io, one of the moons of Jupiter has an elliptical orbit around its parent body. Jupiter's gravity causes it to stretch and compress, generating heat and keeping Io in a molten state. We would have had a similar effect but in reverse with a binary satellite system.

The orbits of the two moons would have stretched and compressed the Earth, preventing a stable crust from forming. A collision would have removed one source of gravity, keeping the stretching and compression constant, allowing the Earth to cool to the point that solid rock could begin to form. A final point is that some of the collision debris would have fallen to Earth.

Phosphorus is part of a group of chemicals that form a T shape within the periodic table; the others are carbon, nitrogen and oxygen with phosphorus forming the base of the T. All of these chemicals are vital to life. Phosphorus along with a type of sugar forms the backbone that supports the double helix structure of DNA. The deposited KREEP magma could have provided a sudden rush of phosphorus that might just have kick started the synthesis of DNA and ultimately the creation of life.