GS7: Story of formation of Our Moon
Updated: Jun 11, 2020
By now, I believe, you all are familiar with how every matter that constitutes our planet or our body or, even, our space came into existence. We started our journey from the Big Bang which happened 13.7 billion years ago (Ga), on our way we talked how stars are element factory out there, we also discussed about how our solar system and planet formed around 4.6 billion years ago. Now there is one thing left for us to discuss which will lead us one step closer to completion of our discussion about the Earth and the Universe. It is our Moon.
Image credit: NASA (Near-Side of the Moon)
In the early days of the Solar System, planets were bombarded by several solid objects, such as fragments of planetesimals, falling from the space on a planet, called Meteorites. Heavy bombardment in the early days of the Solar System created huge numbers of craters on the surface of planets. These bombardments also contributed to the heating of the planet (as already discussed).
How did the Earth have such a huge moon relative to its size?
This question has put scientists to recheck their calculation and has allowed them to reconsider their hypothesis. After biggest relative size ratio of Pluto and its moon Charon, the Earth and the moon stand second when compared to the other planets in our solar system. All other planets have much smaller natural satellites compared to their size. Charon is so big in relation to its planet, Pluto, that they are considered two body systems.
During the initial phase of the Moon, for billions of years, it had active volcanoes on its surface. The dark areas that we see on the moon’s surface at night are nothing but the massive basaltic flow, which is considered as the largest volcanic flow in the Earth-Moon system. These huge dark basalt flows exist exclusively on the near-side (the one facing the Earth) compared to its far-side (the one away from the Earth) as testified by the presence of few craters on the far-side. However, the reason for the distinctiveness is not greatly agreed in the scientific community.
Do you know that we can only see the one face from the Earth?
Yes, this is true. That's what the term far-side and near-side implies. This relationship of the same side of the Moon facing the Earth at all times is called Synchronous rotation. This occurs because the moon's rotation on its axis being the same as the period of the moon's orbit around the Earth. Because of the tilt greater than 5 degrees of the Moon's orbit around the Earth and its wobbling motion, luckily we are able to observe extra 4.5 degrees on both the sides. This results in the observation of 59% of the surface of the Moon from the Earth. Isn't it interesting? I found this interesting video on YouTube, do watch it for a clear understanding.
What's hidden behind the unseen face of the Moon?
Perhaps, not the Optimus Prime or his team waiting for the retaliation. Well, this is confirmed by the Soviet space mission Luna 3 in 1959. During this mission, numerous photographs were taken of the far side of the moon. These photographs justified the lack of dark pools of basalt as seen on the near side of the moon. Well, there is still a debate in the lunar scientific community for this anomalous behaviour. One hypothesis-turned-into-theory suggests that the larger amount of radiogenic heat-producing elements are present in the near-side mantle and the thicker crust inhibits eruptions in the far-side, so preferred eruptions occurred on the near-side of the moon. Therefore, larger crater and greater basaltic flows observed.
Image credit: NASA (Far-Side of the Moon)
Do we have a rock sample from the Moon?
Yes, we have. There are three sources from where we get the Moon rocks on the Earth:
United States Apollo Mission- First successful landing of the humans on the moon.
Soviet Luna Programme- Robotic Spacecraft mission by the Soviet Union.
Lunar Meteorites falling on the surface of the Earth (collected from Antarctica, the Sahara Desert and Oman).
But, how did the moon formed?
Probably we, still, are left with this question after this thorough discussion. When these rock samples were analyzed and dated with the help of radioactive dating technique, their age ranged from 3.16 Ga to 4.44 Ga. A scientist named Robin Canup, created a program which illustrated the formation of moon by the impact of large asteroid, called the Giant Impactor, on the Earth. Her program helped in concluding that at about 4.53 Ga, a Mars-sized protoplanet crashed into the newborn Earth. The energy of the impact was so in the process that the colliding body disintegrated and melted, along with a large part of the Earth’s Mantle (by then very thin crust existed). A ring of debris, which created a cloud of heated material, formed around the remaining, molten Earth. Due to the Earth’s gravitational pull, this molten debris which was thrown into space could not escape far away and started orbiting around the Earth. Also, few were attracted back to the Earth but those which could not quickly coalesced to form the Moon. Since the entire outside of the Earth was shocked and liquefied by the impact, we get the oldest known rocks on Earth circa 4.0 billion years old. This has happened before the oldest known rocks on the Moon were formed. As from the moon samples we have come to know the oldest rock dates back to 4.44 billion years. Therefore, the impact occurred after the formation of planets, 4.56 billion years ago, and before 4.44 billion years ago.
However, this is not a universal theory for the formation of Moons in the solar system for other planets. Some may have been formed independently when the comets or protoplanets were captured by larger planet’s gravity. However, the probability of this happening is very low. After the careful calculations, scientists arrived at the conclusion that no asteroid captured by the Earth could have the orbits around the planet that the moon has.
Name of this Mars-sized protoplanet was Theia.
The composition of Moon’s rock will be discussed in the upcoming articles but for now, it will be helpful for you to know that a large part of Moon’s rock is largely composed of Anorthosite, Gabbro and Basalt. What we see as the milky-white surface during nights is nothing but the anorthosite and the dark rocks in the crater (if you have telescopic eyes, you can actually see them) are basalts. To know more about the rock composition.
Before going, take a look at this brief video of NASA- LRO.
References and other readings:
1. John D. Winter- Principle of Igneous and Metamorphic Petrology
2. Stephen Marshak- Essentials of Geology
3. Linda T. Elkins-Tanton-The Earth and the Moon