4.6 billion years ago the Earth formed out of the material that also created the sun.
Because of gravity, dust and gas came together to form asteroids and eventually these asteroids
and other bodies of material collided with one another to form an even larger object,
which came to be what you’re standing on now.
So we all know how and when the Earth formed, but how did the moon form?
This is a great question because the answer is not obvious, actually you’ll come to
find we still don’t know the answer for sure.
It’s not an asteroid like so many of the others moons in the solar system, such as
Deimos and Phobos, the two Martian moons, that most likely were asteroids that were
captured by the Martian gravity.
There are four main theories to explain how the moon got to where it is today, three of
them have reasons why they couldn’t have happened but one of them is the most widely
accepted today.
Each theory has to naturally explain the following three facts.
One, the Moon’s low density shows us that it doesn’t have an iron core, like the Earth
does.
Two, a volatile is a substance that can evaporate at normal temperatures, so water for example.
And we noticed that Moon rocks contain very few volatile substances, which means that
the Moon would have needed to have been baked to explain how it removed the volatiles that
it once had, because to remove volatiles you need to heat it up.
And it would’ve only been the Moon that was baked because if the Earth was baked as
well then it would have removed the volatiles so Earth wouldn’t have water and no life
would have formed.
Three, the abundance of oxygen isotopes on Earth are identical on the Moon, which implies
that the Earth and Moon formed at the same distance from the Sun.
The first theory is called the Fission theory.
Charles Darwin’s son, George Darwin, suggested it in the 19th century.
This theory proposes that in the early stages of the solar system the Moon was actually
part of the Earth and somehow separated from the Earth.
That maybe in the far past the Earth was somehow spinning extremely fast and flung off a part
of its surface that then became the Moon.
And people look at the Pacific Ocean and say that could’ve been where the Moon came from.
This theory seems plausible since the Moon’s composition is very similar to the Earth’s
mantle.
However, the main problem against the Fission theory is that it doesn’t explain how the
moon’s material received that extra baking, in other words it doesn’t explain how the
Moon got rid of the volatiles, we talked about this earlier.
Next one up is the Capture theory.
This one is much simpler.
This theory proposes that the Moon was created somewhere else, whether it be somewhere else
in the solar system or somewhere else in the Milky Way, or some other place.
And, as the name suggests, this theory proposes that the Moon was captured by Earth’s gravitational
pull.
If this theory is correct then it would be practically impossible for us to track where
the Moon came from.
But this theory could explain the Moon’s different chemical composition, if it did
form somewhere else in the solar system.
However, this theory is highly improbable, because the Moon would have to be moving really
slowly in order for Earth’s gravity to be able to capture it.
For the Moon to move that slowly there would have needed to be a third celestial object
also pulling on the moon, slowing it down just enough, and well... we don’t see this
third object.
Also, like the Fission theory, this theory doesn’t naturally explain how the Moon received
that extra baking.
So, the next theory is the Condensation theory.
This says that the Earth and the Moon were created very near each other and out of the
same material.
And so because they were near each other the Moon will obviously orbit the Earth.
The problem with this theory is that if they were both created out of the same material
then they should both have the same composition.
In other words, the Moon should also have an iron core, like the Earth does, but well…
it doesn’t.
Also, like the Fission theory and the Capture theory, the Condensation theory doesn’t
naturally explain how the Moon received that extra baking.
So these three theories I’ve just talked about didn’t satisfy the astronomical community
and none of them provide an explanation that compliments or explains all the facts that
I talked about earlier.
But then a much more promising theory was thought of, named the Giant Impact theory.
This theory proposes that a celestial object, about the size of Mars, had an orbit very
similar to the Earth’s and eventually it caught up and crashed right into the Earth
4.5 billion years ago when Earth was only 50 million years old.
The colliding object is sometimes called Theia, I’m not sure I’m pronouncing this correct
by the way.
Theia is named after the mythical Greek Titan who was the mother of Selene, the goddess
of the Moon.
This sent heated debris from the outer layers of the two objects.
This would’ve caused a disk of material to orbit Earth, and this material eventually
stuck together to form the Moon in orbit around the Earth.
This theory is brilliant because it explains everything we see and notice today.
It explains why the Moon consists mostly of rock.
The energy of the giant impact explains how the Moon received that extra baking.
Unlike the other theories, this theory actually seems likely because we see evidence in several
places in the solar system where collisions were common in the early stages of the solar
system.
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Thank you so much for watching, I’ll see you later.