The Past and the Future of the Earth-Moon System

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley Ph.D., John Hopkins University

The Earth-Moon system has a past full of impacts and collisions, big forces, and oxygen isotopes, i.e., signatures. The Moon keeps the Earth from flipping over but is getting further away from it all the time. What will happen if the Moon is someday too far?

Astronaut on lunar (moon) landing mission.
Evidence from the Moon shows significant similarities between the Earth and the Moon, and maybe a separation in the future. (Image: Castleski/Shutterstock)

Through billions of years, our Earth and its moon have formed a team that makes life possible on Earth. The Moon slowed down Earth’s rotation and prolonged the days, and it creates the tides along with the Sun. What else does the Moon do for the Earth?

Stabilizing Earth’s Rotation Axis

If the Moon was not around, the Earth’s spin axis could be heavily influenced by the gravitational forces from other planets, and the climate could change. The Moon stabilizes the Earth’s rotation axis through the angular momentum.

To change a moving object’s angular momentum, a significant force called ‘torque’ should be applied to it. The force needs to be in the new intended direction, and big enough to compete with the amount of angular momentum. Where does the angular momentum in the Earth-Moon system come from?

Learn more about Humans on the Moon: A Never-Ending Story.

Earth’s Angular Momentum

Three factors lead to the angular momentum of our Earth and its moon:

  1. The Earth’s spin about its rotation axis
  2. The Moon’s spin about its rotation axis
  3. The Moon’s orbital motion

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on Wondrium.

The last factor is the most effective one, being four times bigger than the Earth’s rotational angular momentum. This means changing Earth’s rotational axis requires a torque big enough to overcome the three factors mentioned, combined. The absence of a large moon could have led to considerable climate changes.

View of Moon limb with Earth rising on the horizon.
The Moon’s surface contains so much anorthosite rocks that probably floated over the hot molten crust. (Image: Elena11/Shutterstock)

How Was the Moon Formed?

The Moon is not the same age as the Earth because it would be a large body too close to the Earth, and the primary solar system formation could not allow it. They would simply combine and form a bigger Earth, without any moon. The oldest rocks from the Moon date back to about 50 million years after Earth formation.

It is not easy to find out how the Moon was actually formed since all the analyses are based on observations and pieces of evidence collected billions of years after formation. Thus, there are some theories, the best of which is called the Giant Impact Hypothesis.

Learn more about how planets migrated in our Early solar system.

The Giant Impact Hypothesis

The base of this theory is that a giant impact occurred between a Mars-sized protoplanet, called Theia, and the proto-Earth. Theia broke into pieces after the impact, and the Earth absorbed most of it. However, a small amount of Theia’s material remained, which was a bit bigger than the lunar mass. This mass was caught in a disk orbiting the Earth and eventually coalesced into the Moon.

Observations, calculations, and computer simulations so far approve the theory. It also explains the Moon’s composition and its tiny iron core: Theia’s solid iron core merged with the Earth’s, and the lighter material from the mantle of both planets, with less iron, flew away into the orbit. Another fact is that the Moon’s isotope signature is the same as the Earth.

The Moon’s Isotope Signature

Every planet in the solar system has a unique ratio of oxygen isotopes, and the Moon’s is extremely similar to the Earth’s. The oxygen isotopes are normally different enough to help us identify where the meteorites come from. There are two possibilities to explain this similarity.

The first theory is that Theia was formed so close to Earth that they were made of the same material. However, this makes explaining the impact and the rest of what happened difficultly. The second possibility is the exchange of material between the Earth and the Moon-forming disk after the Theia impact, which is more logical. Whichever the process, the Moon was formed incomparably fast!

View from the moon orbit and rotating around the planet earth.
It took the Earth millions of years to be formed, but our giant Moon took only a month to a year to be born. (Image: janez volmajer/Shutterstock)

It took one month to one year for the Moon to be formed, while the Earth took millions of years. The reason is perhaps that after the impact, the Moon was hot enough to have a 200- to 300-kilometer thick molten crust. This is evident in the igneous rock, called anorthosite, mostly forming the moon. What about the future?

Learn more about orbiting Earth: Up through the atmosphere.

The Future of the Earth-Moon System

The Moon is moving away from the Earth slowly. One day, maybe in 600 million years, it will be too small to create a total solar eclipse. What if it gets too far to be out of Earth’s gravitational reach?

As the Earth’s spin is slowing down, once a day on Earth equals the Moon’s orbital period, the effect of the tidal forces would stop. Then, the Earth would become permanently aligned with the Moon’s tidal bulge, showing the Moon only one side. This strange occurrence takes at least 50 billion years from now when the Sun has already become a red giant.

Thus, we do not need to worry about the future of the Earth-Moon system. Things will work out just fine until life goes on.

Common Questions about the Earth-Moon System

Q: What is the Earth-moon system?

The Moon was formed a little while after the Earth, and the two began affecting each other immediately. These effects stabilized conditions and made significant changes in both bodies. This team of two is called the Earth-Moon System.

Q: How big is the Earth-Moon system?

The Moon is located almost 60 Earth radii away from the Earth. This makes the entire Earth-Moon system as big as one-third of the Sun.

Q: Why is the moon important to Earth?

The moon stabilizes Earth’s orbit and keeps its conditions stable. Without the Earth-Moon system, the Earth would even face radical climate changes every few hundred years. Thus, life would have probably been significantly changed, if not destroyed.

Q: How does the moon affect the Earth?

Other than tides and keeping water on Earth the way it is, the Earth-Moon system has kept our planet’s rotation axis stable and right for life to continue.

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