Moon: The Origins

Earth is the only planet having one moon. But have you ever wondered how this circular natural satellite was formed? What all theories might be behind it ?

The Origin:

The Origin of the moon has been one of the most inflaming questions to the astronomical world. Many theories have been proposed regarding this matter, some of them being very widely accepted but none completely sure. In this blog, I will try to present all the various accepted theories regarding the formation of this astronomical wonder.

Giant Impact Hypothesis:

As is not uncommon in science, the new Apollo data, which was originally intended to test existing theories instead inspired a new one. In the mid-1970s, researchers proposed the Giant Impact Hypothesis. The new impact scenario envisioned that at the end of its formation, Earth collided with another planet-sized body. This produced a great deal of debris in Earth's orbit, which in turn coalesced into the Moon. The impacting planet would later be named “Theia,” after the Greek goddess who was the mother of the Moon.

The new theory seemed to reconcile multiple lines of evidence. If the material that formed the Moon originated from the outer layers of Earth and Theia, rather than from their cores, an iron-poor Moon would naturally result. A giant impact that struck Earth obliquely could also account for Earth’s rapid initial spin. Finally, the enormous impact energy associated with such an event would vaporize a substantial portion of the ejecta, accounting for the Moon's lack of volatile materials. The scientific community was initially sceptical of this new model.

But at the same time, work on other competing models proved increasingly unsatisfying. The energy dissipation needed to capture an intact Moon during a close fly-by seemed implausible, if not impossible. Models of the Moon’s co-formation alongside Earth struggled to explain why the Moon would have obtained a vastly different proportion of iron. Additionally, the current angular momentum of the Earth-Moon system was too low to be explained by a rotationally unstable Earth that flung off enough material to form the Moon. Although at first, researchers carried out little quantitative work on the giant impact model, it eventually emerged as the most promising idea during a mid-1980s conference on lunar origin, largely due to the weaknesses of competing theories.

But could a giant impact really produce the Moon? The answer to this question was not obvious. From basic physics, scientists know that ejecta launched from a spherical planet either entirely escape or fall back to the planet's surface. It does enter into a stable orbit around the planet. However, a large enough impact — one by a body about the size of the planet itself — distorts the shape of the planet, altering its gravitational interactions with the ejecta.

Additionally, partially vaporized material can be accelerated as gases escape, modifying the material's trajectory. However, assessing the impact of such effects required a new generation of computer simulations at a scale never before modelled. With then-available technology, such simulations were extremely challenging for computers, but researchers were able to demonstrate that giant impacts could produce orbiting ejecta that might assemble itself into the Moon. But thanks to vast computational improvements, by the early 2000s, researchers identified what would later become known as the "canonical" impact theory: a low-velocity collision at about a 45-degree angle by Theia, which had a mass similar to that of Mars. Such an impact produces an iron-depleted disk of material massive enough to form the Moon and leads to a five-hour day on Earth. But over billions of years, tidal interactions then transfer angular momentum to the Moon, which drags the Moon outward while simultaneously slowing down the spin of Earth. This fits well with both Earth's current 24-hour day, as well as the present orbital distance of the Moon.


There are also some other popular theories like:

Co-formation theory:

Moons can also form at the same time as their parent planet. Under such an explanation, gravity would have caused material in the early solar system to draw together at the same time as gravity-bound particles together to form Earth. Such a moon would have a very similar composition to the planet and would explain the moon's present location. However, although Earth and the moon share much of the same material, the moon is much less dense than our planet, which would likely not be the case if both started with the same heavy elements at their core.

In 2012, researcher Robin Canup, of the Southwest Research Institute in Texas, proposed that Earth and the moon formed at the same time when two massive objects five times the size of Mars crashed into each other.

"After colliding, the two similar-sized bodies then re-collided, forming an early Earth surrounded by a disk of material that combined to form the moon," NASA said. "The re-collision and subsequent merger left the two bodies with the similar chemical compositions seen today.

Capture theory:

Perhaps Earth's gravity snagged a passing body, as happened with other moons in the solar system, such as the Martian moons of Phobos and Deimos. Under the capture theory, a rocky body formed elsewhere in the solar system could have been drawn into orbit around Earth. The capture theory would explain the differences in the composition of Earth and its moon. However, such orbiters are often oddly shaped, rather than being spherical bodies like the moon. Their paths don't tend to line up with the ecliptic of their parent planet, also unlike the moon.

Although the co-formation theory and the capture theory both explain some elements of the existence of the moon, they leave many questions unanswered. At present, the giant impact hypothesis seems to cover many of these questions, making it the best model to fit the scientific evidence for how the moon was created.


Apollo mission sparked many new theories about the formation of the moon and the legacy is still being continued by the various missions being carried out on the moon. With the current technology, however, the research is limited but the possibilities are endless. Do comment and tell what your favourite theory was and don't forget to share this to your friends to increase their knowledge as well.

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