The Moon Guards Her Secrets: Part II: Origin Unknown

May Moon Month continues.

Last week, the introduction to the series presented some of the mysteries and dangers our Moon poses to man and the machines who venture there. This week, we take a look at several theories on how the Moon might have formed. Only the universe knows for sure.

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Photo credit NASA

Centuries ago, man looked at the Moon and saw great plains of basalt stretched across the surface like vast pools. They believed the Moon had seas. That's why many of the features on the Moon are known by "Mare," a derivative of the Latin word "Maria" which is the plural form of "Sea." Mare Imbrium. Mare Frigoris. Mare Serenitatis. Perhaps the best known is Mare Tranquillitatis--or the Sea of Tranquility--the Apollo 11 landing site.

We now know that those "seas" were the result of major asteroid bombardment 4 billion years ago, when molten lava oozed across the lunar surface forming massive pools and creating the familiar patterns we recognize today.

From last week, we know that the Earth-to-Moon dimensions are like nothing else in our solar system. The Moon is 27% the size of the Earth--making it between 1/4 and 1/3 as large as our planet, with a diameter of 3,474 kilometers. For comparison, the diameter of the planet Mars is 6,792 kms, making the Moon more than half the size of our neighboring planet. It is the fifth largest moon in our solar system, behind Ganymede, Calisto and Io which all orbit Jupiter, and Titan, which orbits Saturn. With Jupiter and Saturn being respectively 1,120% and 945% larger than the Earth, the difference in size comparisons of these planets to their moons and Earth's to its sole Moon is eye-opening.

Another oddity about our Moon is that the view never changes on Earth, because the Moon perpetually has one side facing us. The Earth's strong gravity tidally locks the Moon into its slow rotation. Because of this, mankind wouldn't get its first glimpse of the Moon's mysterious dark side for millennium.

Lunar Formation Theories

So how did our very strange Moon come to be? There are many theories, though science has yet to prove the true origins of Earth's massive companion.

Our solar system was created from gas, dust and rock particles that formed a disk around what would become our Sun about 4.6 billion years ago. Once the Sun ignited into a nuclear furnace from the mounting pressure, the rocky debris surrounding it began to form into clumps. These clumps later became planets--but a lot more planets that we have in the solar system today. Some were thrown out of the solar system by the immense influence of the gas giants, others collided with or merged with other bodies. The asteroid belt between Mars and Jupiter may be the remains of another planet that was destroyed.

As the planets formed, smaller clots of matter began to shape into moons around them. We once believed that was how our Moon formed, too.

Now...not so much.

There are several theories about how the Moon was created. As mentioned before, the oddity of the Earth-to-Moon ratio is completely out of proportion to other moons in our solar system. That led some scientists to believe the Moon may have started out as a small body or a moon of Venus that was later caught up in the web of our planet's immense gravity. This is called the Capture Theory.

Seemed the most simple explanation. Until science looked a little deeper.

In the 1970s, astronauts collected samples of Moon rocks from the lunar surface. Among the samples was a white rock that once made up a magma sea, anthracite. Once the samples were analyzed, the scientific world was stunned by the findings.

Anthricite was identical to Earth's crust!

That didn't make sense. If the Moon was a captured body, its makeup shouldn't be identical to Earth's. They took a closer look at the samples, this time focusing on the water content. The samples were very dry. The lack of moisture indicated they'd been subjected to tremendous heat. There was only one way that such enormous heat could be generated.

A violent collision with another body.

Photo credit NASA

A startling new theory was born. Around 4.3 billion years ago, the Earth may have impacted with another planet about the size of Mars, which scientists dubbed Theia. The glancing blow destroyed Theia and blew off huge volumes of material that encircled the Earth in a ring of fiery debris. If people had existed back then, it would have appeared as a great glowing arc across the heavens. What a sight that would have been!

From this burning debris, a ProtoMoon formed--very fast! It probably pulled together into a circular lump in less than a single year.

Now scientist's thought everything made sense. The Moon had the same makeup as the Earth because it was chunks blasted off the Earth by the impact with Theia. The Giant Impact Hypothesis now replaced the Capture Theory as the most viable explanation of the formation of the Moon.

But wait...

The Moon had even more surprises in store.

Next week:

The Moon Guards Her Secrets
Part III Origins Unknown: More Lunar Formation Theories

Resources:
How the Universe Works: Secret History of the Moon, Science Channel S4/E6 (2015)
UniverseToday.com: The Planets in our Solar System in Order of Size
AirSpaceMag.Com: The Second-Moon Theory
Slate.com/BadAstronomy: Why Do We Have a Two-Faced Moon?