Origin of the Moon
You can watch the summary of this post as video here.
Overview
Our planet Earth‘s natural satellite, the Moon, is our closest astronomical object. There are several theories to explain the origin of the Moon.
While the two terrestrial planets Mercury and Venus do not have any moon orbiting them, Mars has two miniscule asteroid which Mars captured them. It raises the question: how did it come about that our Earth has such a massive Moon? Astronomers believe that the Moon formed as a result of an unusual event when the Earth was young.
What was the event that created the Moon? What are the theories that astronomers have suggested for Moon’s origin? Which theory is the most plausible one? What are the facts that we know about the Moon and the Earth? Let us explore these questions.
Facts / Constraints
There are several facts that we learnt either from observations or from the laboratory investigations of the Moon rocks. Any theory of origin must explain these facts.
- Earth is the only terrestrial planet to have such a massive moon. While Mercury and Venus have no moons, Mars has two miniscule moons which are captured asteroids. This suggests that our Moon formation must be from an unusual event, not a natural process.
- Our Moon has a peculiar orbit around the Earth. While the Earth is tilted by 23.5 degrees to the ecliptic plane, the Moon’s orbital plane is inclined to the ecliptic plane by about 5 degrees as shown in the diagram below. Ecliptic plane is the plane on which the Earth orbits the Sun.
- Another crucial fact is that Moon’s mean mass density is just 3344 Kg/m3 which is much lower than the Earth’s mean mass density of 5513 Kg/m3. In addition, the Moon’s overall mass density is much closer to the Earth’s terrestrial mantle’s mean mass density.
- The Moon is still moving away from the Earth. That indicates that it must have been once close to the Earth.
- Moon’s rocks are also 4.5 billion years old implying that the Moon is as old as our Earth.
- The ratio of oxygen isotopes in the Moon’s rocks are similar to that of Earth’s, indicating a common ancestry and nearby formation.
- Moonrocks are depleted of volatile elements. This suggests that the formation of the Moon occurred in high temperature environment which must have made the volatile elements to boil out into space.
- Moonrocks are enriched with non-volatile substances, with high melting points, indicating the remains of substances after high temperature event.
Any hypothesis on the origin of the Moon should explain all the above facts in a simple way without any assumption.
Early Hypotheses
First, let us investigate the three classical hypotheses for the origin of the Moon. Then we will look into the modern model that is acceptable to most astronomers as it satisfies all the constraints stated above. The three classical hypotheses are:
- Fission Hypothesis.
- Capture Hypothesis.
- Accretion model.
Fission hypothesis
It assumes that the Earth had no satellite of its own in its earliest youth. During that time, Earth was spinning so fast that a large portion of its mass tore away from it to form the Moon.
Probably this event happened after the Earth’s iron content had settled to the center. That naturally explains why the Moon is depleted in metals and has low mean mass density that matches that of the outer layers of Earth.
However, the fission hypothesis fails to account for the compositional differences between the Moon and the Earth, such as the depletion of volatiles on the Moon. Also, there are two more difficulties. First, the Earth would have to spin really fast – just 2.5 hours to complete one rotation. Secondly, it does not explain the inclination of Moon’s orbit to the ecliptic plane by 5 degrees. Had it spun off from the Earth, its orbit would have coincided with Earth’s equatorial plane. But that is not the case.
Capture Hypothesis
According to this theory, our Moon formed somewhere else in the solar system, and it strayed close to the Earth and our Earth’s strong gravity captured it into its orbit.
The advantage of this hypothesis is that it explains the compositional differences between the Earth and the Moon as the Moon assumed to have formed somewhere else. The main problem with this theory is how the capture could have taken place. In order to get captured, the straying object should have slowed down substantially. Otherwise, it would have resulted in gravitational boost that would hurl the object away from the Earth in the slingshot fashion. In addition, there is no explanation for the object to slow down before getting captured by the Earth. So, this theory too is not a satisfactory one.
Accretion Model
The third classical hypothesis is accreditation model. According to this model, the Moon and the Earth formed concurrently. The Moon formed from the disk of raw materials orbiting the young Earth just like how planets formed around the Sun from similar disk around the Sun.
This theory does not explain the compositional differences between the Earth and the Moon. Nor does it provide any reason for the absence of volatile elements in the Moon.
In addition, it does not answer why other terrestrial planets (Mercury, Venus and Mars) have no such moons. If this could occur naturally around the Earth, it could have occurred around other terrestrial planets too. But that did not happen.
Giant Impact Hypothesis
According to this hypothesis, about 4.6 billion years ago when the Earth was young, a massive object (about the size of Mars) sideswiped the Earth and dislodged the material from the Earth.
In hundred years after the giant impact, all the dislodged materials of the Earth, along with the pulverized pieces of the projectile, coalesced together to become the Moon. A recent simulation analysis of the giant impact shows that the Moon’s formation could have happened in a few hours. (Please see this video from NASA).
Now, let us analyze how this theory explains all the facts that we listed above under the section ‘The facts/Constraints.’
How It Explains Facts
Earth tilt: The giant collision could be the reason the Earth is tilted by 23.5 degrees from its upright position.
Moon’s Inclined orbit: The dislodged material from the Earth, along with the pulverized pieces of the projectile, must have been thrown into an orbit (around the Earth) with an inclination of 5 degrees to the ecliptic plane. So, even after it became a single piece, it continued the inclination. That explains why the Moon’s orbit has an inclination of 5 degrees.
Compositional differences: Because the Moon formed from the materials of the Earth and that of the projectile, the chemical composition of the Moon is naturally expected to be different from the Earth.
Mass density difference: When the collision occurred, iron had already sunk to the core of the Earth and the crust started to solidify on the partially molten rock. Because it was an off-center collision, the Earth material that went into the Moon was mostly from the outer layers of the Earth. That explains why the mean mass density of the Moon is much less than that of the Earth and why it matches that of the Earth’s mantle.
Absence of volatile elements: The impact of the collision must be so fiery that the intense heat must have boiled away most of the volatile elements from the debris which would later become the Moon. That explains why the Moon has no appreciable amount of volatile elements.
Conclusion
As the giant impact theory seems to successfully give reasons for all the known facts, astronomers believe that this is a plausible theory of the Moon’s Origin.
Such collisions were common in the formative period of the solar system. One such collision knocked off the outer mantle of Mercury. Now the planet Mercury is mostly metallic – a huge iron ball covered with thin layers of mantle and crust.
Another example of such a collision is Uranus’ extremely tilted axis of spin. It is spinning on an axis with a tilt of 97 degrees. Astronomers believe that the extreme tilt must be after a giant impact with a planet sized object.
The planet Venus’ retrograde spin must also be due to an impact Venus had with a massive object.
References
- The Cambridge Guide to the Solar System by Kenneth R. Lang, Cambridge University Press.