Theia and the New Theory on the Moon’s Origin

New evidence suggest that the Moon and Earth are even more alike than we previously thought.

Artist’s illustration of a planetary collision between two planets: image credit NASA/JPL-Caltech



Look up to the skies on most clear nights and you will see the Moon shining down upon you. Since the dawn of mankind we have seen its familiar face brightening our dark nights, inspiring poets and romantics with its gentle glow. Although the moon looks nothing like Earth, a new study shows us that the Earth and the Moon may be more like twin brothers than second-cousins.

For most of mankind’s history, the Moon and its origin was explained using gods, mythology and fantastic beasts. Once astronomers moved from the fantastical to the practical, we began to develop our first scientific theories on the Moon’s origin. Up until the early 1980’s we had multiple competing theories for the moon’s origin.


The Old Theory of Earth-Moon Accretion
By the 1800’s it was widely accepted that planets and moons were created by a star’s left-over dust and gas. After a star is born, left-over matter that is in orbit around the star is pulled together slowly by the force of gravity. Over millions of years this matter collides and accumulates under immense pressure to eventually form planets and moons.

Accretion of the Solar System
Artist’s conception of accretion in a star system. (credit: NASA/Goddard Space Flight Center)

In the past astronomers believed that the Earth and Moon were both formed at the same time via this process. This is known as the Earth-Moon Accretion model. The creation of planets by accretion is still widely accepted, but we no longer believe that our Moon was formed in this manner.

Accretion is another word for the increase in the size of an object by adding more onto it. For example the accretion of ice under a rooftop will eventually form icicles.

If the Earth and Moon were formed around the same time from the same matter in orbit around the Sun, their densities should be close to one another. However the Earth’s average density is almost double that of the Moon. If you expanded the Moon until it was the size of the Earth, the Moon would weigh just over half as much as Earth.

Additionally for the Accretion model to fit, the Moon’s orbit would need to be at the Earth’s equator. But the moon has an orbit that is angled 20 to 30 degrees from the Equator, far outside what we would expect to see if both objects formed via accretion. This theory just doesn’t fit our modern observations and measurements.


The Old Theory of Earth-Moon Fission
Up until the 1980’s some astronomers believed that the Moon was flung off from the outer shell of Earth during the early formation of the Solar System. This was known as the Earth-Moon Fission model.

Fission is defined as the act of splitting whole things into parts. Nuclear fission splits atoms to release incredible amounts of energy. Fusion is the opposite of fission.

Pacific Ocean as Viewed from the GOES Weather Satellite
The Pacific Ocean as Viewed from the GOES Weather Satellite (credit: NASA-GSFC, data from NOAA GOES)

It was thought that our Earth was spinning much more rapidly billions of years ago than it is now. It was rotating so fast along its axis that a day on Earth would take only two and a half hours. The centrifugal force of the rapidly-spinning Earth would have caused the edge of the planet to bulge outward like a squished top.

Centrifugal force is an outward force imposed on any object that rotates along its axis. It is caused by the inertia of the object. A Yo Yo spinning around your head has centrifugal force and its inertia wants the Yo Yo to move outward from your hand. The string keeps it going in a circle instead of outward.

The centrifugal force at the Earth’s equator would be so great that it would rip the Earth’s outer layer off and eject it into space around the planet. Gravity would then bring the pieces of this outer layer together to form the Moon.

The Fission model matched our real-life observations of the density difference of the Earth and Moon. The numbers showed that the Moon could have been peeled off the Earth’s surface. It was even believed that the Pacific Ocean was a left-over scar from this cataclysmic event that occurred billions of years ago.

However over the last thirty years we have greatly increased our understanding of both the Moon and the oceans. We now know that the Pacific Ocean is caused by tectonic movement of plates on the ocean floor, not from a sudden removal of the Earth’s crust. We have also measured the age of the rocks in these plates and discovered that they are much younger than the Moon. Just like the the older Accretion model, the orbit of the moon around Earth was also in the wrong place. There are just too many inconsistencies to still believe that the Moon was flung off by the force of a rapidly-rotating Earth.


The Old Theory of Earth-Moon Capture
When astronomers began to compare the Earth and Moon to at all known moons and planets in our Solar system we noticed some oddities.

Moon and Earth from DSCOVR
A photograph from the Deep Space Climate Observatory (DSCOVR) of the Moon as it passes in front of the Earth. (credit: DSCOVR/NASA/Goddard Space Flight Center)

The Moon is very large in relationship to Earth, too large to have stayed in orbit around Earth when the planets were first being formed. The Moon also orbits Earth at a very high angle against our equator. The Moon’s orbit is almost the same as Earth’s elliptical plane (the angle that the Earth orbits around the Sun).

With these two observations, some astronomers proposed that the Moon may have been captured by Earth’s gravity at some early point in history. If a small wandering body from our Solar System’s formation crossed Earth’s orbit’s, perhaps the gravity of the Earth captured this object and pulled it into orbit around us. Thus the Earth-Moon Capture model was born.

This theory was short-lived. As our technology advanced to give us better tools for modelling highly complex orbits, we found that the laws of physics, gravity and momentum would not line up to allow such an event to occur. There was simply too much mass between the Earth and Moon for a stable orbit to exist after a planetary capture.


The Current Theory of Earth-Moon Impact
If the Moon wasn’t around when the Earth formed and if the Earth didn’t capture the moon, where did it come from? Once geologists started analyzing rock samples retrieved from the Moon and from Moon meteorites found on Earth, they discovered something new. The ratio of atomic isotopes found on the Moon is similar to the ratios of atomic isotopes found on the Earth.

Isotopes are slight variations in basic elements that exist naturally in nature. For example, Oxygen is one element but it can have many variations. These variations are due to the number of neutron particles inside an element. Oxygen atoms will always consist of eight protons but each atom can have between 12 and 24 neutrons within it.

A Golf Ball-Sized Moon Rock
A golf ball-sized moon rock on display at Arizona State University (ASU) in Tempe, AZ. (credit: Tom Story, Arizona State University)

Don’t worry about the science behind isotopes. All you need to know is that the ratio of isotopes found in the Moon rocks seemed similar to the ratio of isotopes found in Earth’s own rocks. The only way this can occur is if Earth and the Moon were made from the same material of the same age.

Does that mean the Earth-Moon Fission model is back on the table? No, the models still showed that the moon could not have been ripped from the Earth’s surface by the force of a spinning Earth. Instead, something huge must have collided with Earth and pulled off a sizable piece of it to form the Moon.

A collision would explain the Earth’s and Moon’s similar ratios of chemical isotopes while still allowing a different density between the two bodies. Furthermore our Moon’s orbit is tilted on the same path that an unknown orbiting body would likely take if it was on a collision course for Earth. But what could have caused this impact and where is this unknown object now?


A New Planet is Born
Astronomer now believe that another planet collided with Earth around 4.5 billion years ago. In 2000 we gave this planet the name Theia, named after the mother of the Greek Goddess Selene who represented the Moon.

It was believed Theia was the size of Mars, though some simulations put it as large as 90% of the size of Earth (Mars is just over 50%). Theia and Earth may have shared the same orbit around the Sun, or the smaller Theia could have been sitting quietly in one of Earth’s Lagrange points before something pulled it out of orbit to eventually drift into Earth’s path.

Lagrange Points of Earth
Earth’s Lagrange Points. (credit: NASA/WMAP Science Team)

Lagrange points are small areas on space where the pull of gravity from Earth is equal to the pull of gravity from the sun. There are five such points around most planets and their star. It is a gravity-neutral zone where objects will remain until disturbed by an outside force.

It was thought that billions of years ago, Earth and Theia brushed into each other like stranglers jostling each other on a busy sidewalk. The resulting side-swipe of Theia against Earth peeled off some of Earth’s outer layer and flung it into orbit around Earth. Theia was blasted into smithereens and most its remaining pieces combined with the ejected material from Earth to eventually form the Moon. This is now known as the Earth-Moon Impact Model and it is the current model we use to explain the origin of the Moon.


The Search for a Lost Planet
For a while it was believed that the Moon itself was Theia – or mostly Theia. Based on numerous computer models, we believed that a glancing blow by Theia resulted in its destruction. 30% of Theia rained down on the Earth’s surface, 20% of it broke up into small pieces that orbited Earth. The rest, about half of the planet, mixed with Earth’s outer mantle that was floating in space and together they fused into the Moon. Half of the Moon was made from Earth, the other half from Theia.

But once geologists created better tools to more precisely measure the ratios of isotopes within moon rocks, we were puzzled to see that the Earth and Moon had the exact same composition of certain isotopes. This would suggest that the Moon was entirely made out of material from the Earth and did not contain any pieces of Theia. So if we can’t find traces of Theia on the Earth or Moon, where did it the alien planet disappear to?

In 2006 NASA launched twin STEREO probes (Solar Terrestrial Relations Observatory) in separate orbits around the Sun to study solar activity. In 2009 when these two probes approached the Lagrange points near Earth’s orbit, they searched the areas for any large bodies – Theia or otherwise. Unfortunately after many months of exploring millions of kilometers of space, Theia, or parts of it were no where to be seen.


The Earth and Moon is Made From Two Planets
So if Theia no longer exists and the Moon is mostly made of the Earth, what happened to the missing planet? Brand new evidence published in the journal Science on January 29, 2016 proposes a new theory as to what happened when worlds collide.

A team of geologists lead by the study’s lead author Edward Young, UCLA professor of geochemistry and cosmochemistry, came to a startling conclusion. They believed that instead of Theia side-swiping Earth, it smacked right into it.

The team re-analyzed multiple moon samples of lunar rock and compared them to rocks on Earth. Using new tools and technology, the team found that all samples between the Earth and Moon matched perfectly. Since the Earth and Moon were chemically one of the same, both had equal amounts of Theia inside of them.

Young believes that when Theia struck “pre-Earth” squarely 4.5 billion years ago, both planets blew apart and then completely reformed together into one planet. Like mixing a cup of sugar with a cup of salt and then pouring it on the table, the resulting collision between two planets created one homogenized world.

Most of the matter in space came together to form the new planet Earth, the remaining matter eventually became the Moon. This is the reason we do not see any difference in isotope ratios when comparing rocks found on the Earth and Moon. Both celestial bodies are born from the same collision. The Earth and Moon are the remains of two long-dead planets.

It is interesting to note that if Theia never collided into Earth, it may have found a stable orbit and co-existed with us. Life would have most likely formed on both Theia and Earth around the same time. If Theia did not collide with the “pre-Earth” planet, our Solar System could have been home to two planets that harbored life, radically changing the way mankind viewed the Universe.

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