The Origin and Importance of the Moon

We've been following a series at Evolution News and Views on the uniqueness of the earth, a uniqueness which suggests that life may be very rare in the universe. Properties of the universe, and particularly those of our immediate neighborhood, seem incredibly fortuitous. Myriad aspects of our planet, our solar system and our galaxy have to be just right for life to exist on earth.

One of those parts of the solar system upon which life on earth depends is our moon. It turns out that had we no moon life couldn't be sustained.

One of the many processes on earth that make life possible is the cycling of carbon from the atmosphere preventing the buildup of greenhouse gases. The process is dependent upon plate tectonics and plate tectonics depend upon the crust of the earth being relatively thin so that crustal plates can slide easily over the underlying mantle. Plate tectonics exist on no other planet in our solar system and may be very unusual elsewhere as well. To have plate tectonics a planet has to be about the size of the earth, have extensive surface water, and have a thin crust.

Why is the earth's crust relatively thin? The answer has to do with the fact that we have an unusual moon:

How did the Earth get such a large moon? It is far too large for the Earth to have captured it, and because of its chemistry and orbital parameters, the only plausible model is an impact scenario. The Moon's creation event is also the most plausible explanation for the resultant thin crust of the Earth that enables its efficient plate tectonics. The most recent models suggest the Earth collided with a body with a mass comparable to itself. These two bodies merged, and became the Earth-Moon system.

The Earth ended up with more mass, and received the cores of both bodies, giving it a large iron rich core, and more radioactive elements. This may explain some of the differences between Venus and Earth. With more iron and heat-generating radioactive materials, the Earth is able to generate its large magnetic field, which Venus lacks. The Moon ended up with more crustal lighter density materials, leaving the crust of the Earth thinner than it would have been.

Earth's plate tectonics can be so efficient because the crust is very thin, cracked, and can slide over the mantle much more easily. Plate tectonics are also responsible for the continent-versus-land distribution. Without it, Earth would be a waterworld, and there would be no carbonate-silicate cycle to maintain the climate over billions of years. We wouldn't be here, and likely no technological civilization of any kind.

Current simulations suggest that only about 2 percent of Earth-sized planets should form an Earth-Moon type system. And by an amazing coincidence, the Moon's size is also quite important to the stability of the Earth's tilt. If it had been any larger, it would destabilize our tilt, for example. On the other hand, much smaller, and Earth's climate would have suffered more frequent and severe ice ages. The other planets in our solar system contribute to this as well, as the interactions between the Moon's influence on the Earth, and the other planets' gravitational effects could have been comparable, [if] the large planets [were] not spaced widely, [there'd be] chaotic swings in the tilt of the Earth's axis. So again the large planets, and their orbits, play a role in the habitability of the Earth.

Here's an animation of what scientists think happened:

Something the video tacitly illustrates is that one advantage of having a large moon is that its gravity sucks up a lot of debris that would otherwise bombard the earth making life, especially early life, a lot dicier than it already was.