How Was the Moon Formed

How was our Moon formed? Since when is our planet's only companion in the cosmos, been giving it company? Read this article to get the whole scoop on the matter.

Moon is an integral part of our lives. It sets the rhythm of tides, around the world. Besides that, it serves as a guiding light, after Sun retires for the day. Besides being the inspiration for poets since ancient times, the Moon makes for great company on a lonely night. Have you ever wondered, how our planet's only satellite was formed. Since when has it been accompanying our Earth, in her sojourns around the Sun?

This article endeavors to answer this question, on the basis of research in astronomy and space exploration, done till date.

The Apollo missions that landed on the Moon, brought back lunar rock and for the first time, scientists got an idea, about its composition. To understand the origin of this satellite, you must first understand what it is made of. 

Then, from the composition, constitution, and its similarities with the composition of Earth, one could decide which of the many hypotheses, regarding its formation, make sense. The theory that finally won over was the 'Giant Impact' theory, which describes most peculiarities regarding the Moon's origin, quite convincingly. 


Man on Moon during Nasa's Apollo 11 Mission (Image Credit: NASA)

The Giant Impact Theory 

According to this theory, the Moon was created from the debris ejected by an impact of a huge object (of the size of today's Mars) on our planet, about 4.53 billion years ago, when a molten Earth was still cooling down. Hence, its formation is quite intimately connected with the creation of Earth. Let us understand how our only natural satellite was formed from collision of the young Earth, with this huge Mars-sized object (named Theia after the Greek Goddess who gave birth to Moon Goddess Selena, in Greek Mythology).

Theia is Created
As the protoplanetary disk, which orbited the young Sun (called Solar Nebula), condensed under gravity, many planetoids formed in the vicinity of the Sun, which revolved around it. These small planetoids, collided and coalesced together to form planets like Earth later. 

The giant impact theory proponents say that Earth had a companion planet around it, placed at a Lagrangian Point (L5 or L4 supposedly). (A Lagrangian point is a position in the Sun-Earth gravitational system, where the gravitational pull between both of them, is nullified. The result being that any comparatively less massive object placed at that Lagrangian point is stationary with respect to the Earth and the Sun. There are 5 such Lagrangian points in the Sun-Earth system.) So, the hypothetical planet Theia is supposed to have been created at this L5 or L4 point, revolving along with Earth, around the Sun. During this period, Earth had no natural satellites. It was without a Moon. Theia was in almost the same orbit as Earth, except that it was 60º, out of phase from earth. That is, it either led or followed the Earth in its orbit around the Sun.

Orbit of Theia Destabilized
The path of every planet orbiting the Sun is affected by the gravitational pull of other planets in the Solar system. So many times, planets, including Earth stray away from their normal paths. This means that they have eccentric orbits.

Similarly, Theia's orbit was also affected by Earth and the other planets around. As a planet, it was still growing in size through accretion of matter. Its earlier non-perturbed orbit kept it a safe distance away from Earth. However, as its mass increased through accretion, over a period of 20 to 30 million years, its orbit was destabilized and became increasingly erratic, taking it dangerously close to Earth. 

Theia Collided with Earth, Creating the Moon
So finally one day, 4.53 billion years ago, the increasingly eccentric orbit of Theia lead to its inelastic collision with Earth, at a very oblique angle. Imagine an object having a size of planet Mars (which is roughly three-fourth of the size of Earth), colliding with our planet. It is like millions of atomic bombs exploding at the same moment on Earth. However, by cosmic proportions, it was a moderate impact. 

As the two planets collided, they stuck together. This led to generation of an immense amount of heat, which led to the melting and sinking of Theia's iron core into Earth's core, sending a significant amount of Theia's and Earth's crust, as well as mantle into orbit around them. 

It was almost half of this molten debris, ejected from the collision that accreted and coalesced to become our Moon, in about a month after collision. The rest of the material may have been in orbit around the Earth, in small chunks. However, it was eventually cleaned up by Earth's gravitational pull. The reason we can talk quantitatively and put time bounds on this whole process is that, it has been studied through computer simulations that recreate all the conditions of collisions virtually. Through computer simulations, all these astrophysical models can be treated quantitatively and precise numerical analysis can be done. The Earth also got a boost in its angular momentum and mass from the collision. So this 'giant impact' theory is our best answer to the puzzle of Moon formation. 


Earth rise photographed by the NASA's Apollo 11 Mission(Credit: NASA)


Evidence for Giant Impact Theory 

Reasons why this is the best accepted theory, are as follows:
Identical Oxygen Isotope Ratios: The rocks brought back by the Apollo missions show the same oxygen isotope, present in the Moon's composition, as found on Earth.
Moon was Once Molten: The surface impact features of the Moon are testimony to the fact that it was once molten in the past. The Giant Impact theory gives us a molten Moon, which agrees with this evidence.
Moon's Smaller and Low Metallic Core: There is evidence which indirectly shows that Moon has a very small core with less metal content, compared to other terrestrial planets. Many simulations of the 'Giant impact theory' predict a Moon created from mantles of the two impacting objects, leaving it with a low metallic core.
Still, some facts remain which cannot be explained by this theory. However, this theory still remains the strongest contender so far. Maybe the theory is missing some more details, which could explain all of it. Any such huge event is very complicated in all its details. May be, with time and new evidences coming in, the picture of the Giant Impact hypothesis will be changed in some form, but the gist would still remain the same. 

Reconstructing the past from indirect evidence is a tough job and takes years of research. The giant impact hypothesis, is currently our best guess.