After the Big Bang occurred, the universe was a boiling soup of dust and gas particles. A similar cloud of dust surrounded our very own sun until around 4.6 billion years ago, when a nebula exploded in its vicinity. The explosion seems to have disturbed this cloud and the particles coalesced together, due to electrostatic and gravitational forces, to form pebbles of matter.
The pebbles further interacted and accumulated under the spell of gravity to form small planets called planetesimals. These entities swept up more gas and formed nascent planets until their growth stopped as the remaining gas was blown out of the solar system by violent solar winds.
Who would’ve expected that a cloud of gas and dust would act as the ingredients in the formation of titanic planets we now know, their faithful moons and most importantly, delicious hot dogs! Amazement is an understatement to describe such a sequence of events!
No, but seriously… how did our moon come into existence?
There isn’t a single verified theory, as a number of strong theories compete with each other to this day. Let’s go through a few of them and see what we can learn.
Giant Impact Hypothesis
This is the prevailing theory that is currently supported by the scientific community. According to this theory, a Mars-sized protoplanet – a planet that never survived long enough to gain full planetary status – crashed into Earth a 100 million years after it was born. The protoplanet, often called the “impactor”, was named Theia, after the Greek goddess of sight and heavenly light.
Advocates of this theory suggest that Theia and Earth participated in a head-on collision. That destructive collision vaporized chunks of young Earth’s crust, as well as the impactor, sending them into space. The moon eventually sprung into existence as the resulting debris swirled into our orbit and accumulated due to gravity.
There’s a lot of evidence to support this hypothesis, such as the eerie similarity between the composition of Earth’s crust and the moon’s lunar rocks. From a different perspective, the lack of iron in the moon’s composition can be explained by the fact that the iron inside the Earth’s core was already well established when this collision occurred. The debris was a mixture of the iron-depleted rocky mantle blown out of Earth and the impactor’s core.
The material that formed came from the Earth’s crust, leaving the planet’s core untouched. This scenario has been successfully simulated on computers. The lack of iron also explains the moon’s inferior density in comparison to the Earth. A collision at a glancing angle wouldn’t allow for this, but a head-on collision seems likely to be the cause of having such a similar chemical composition.
This theory suggests that the moon formed at the same time as its parent planet. More specifically, it argues that gravity aggregated two clouds of matter in the early solar system at a separated distance at the same time. This might explain the similar composition and the moon’s present location. However, it still doesn’t account for the moon’s inferior density, especially when you consider the fact that both of the celestial bodies started out with the same heavy elements at their core.
Another possibility is that, similar to the formation of Mars’ moons, our moon was just a celestial body whizzing by and Earth probably snagged it into its orbit with the aid of gravity. It could be possible for a rocky body to have formed in a remote corner, flew towards us, got drawn into our orbit and has stuck with us forever.
Although this theory explains the disparity in chemical composition, it is highly improbable that a random orbiting chunk would be so conspicuously spherical. Another limitation is that random orbital paths rarely line up with the ellipse of their parent planet, unlike the moon which is in sync with Earth.
The last two theories leave a lot of unanswered questions. The giant impact theory also explains the rotational dynamics of the Earth-Moon system and why we have a 24-hour day. It suggests that the colossal impact established an angular momentum so vigorous that we started out with a 5-hour day!
Over the millennia, the moon steadily receded from us, consequently reducing Earth’s rotational velocity, resulting in the current 24-hour day. Scientists figured this out by looking at the moon’s current orbit and working out how rapidly the angular momentum of this system has been transferred by the tidal forces between the interlocked bodies, and then working backwards.
Thus, the giant impact theory remains the best model that fits the scientific evidence for how the moon was formed.
That being said, all theories do agree that the moon’s cavernous craters were formed around 3-4 billion years ago when it was violently struck by a plethora of asteroids, an event known as the Late Heavy Bombardment.
The moon has long been a fanatical source of fascination and delight for astronomers and artists alike. The hubbub of everyday life often makes us callous towards its constant presence, blinding us to the extraordinary enticement of gravity, by which it literally hangs in space. Perhaps in the near future it will become a frequent escape, a sort of parenthesis, as it remains the only other celestial body mankind ever set foot on.
- Planetary Science Institute
- University of California, Davis
- Harvard University
- University of Arizona
- University of California, Los Angeles