The Moon Guards Her Secrets Part III

Origins Unknown: More Lunar Formation Theories

The only thing that's certain about the formation of our Moon is that we really don't know exactly how it came to be. Last week, we looked at two theories, the Capture Theory and the later Giant Impact Hypothesis.

Photo credit NASA

But the Impact Hypothesis didn't explain everything. In 1959, the Russian Luna 3 craft first photographed the far side of the Moon. Scientists were, again, shocked. The Dark Side looked completely different from the face we'd always seen. It was covered in craters, but uniformly. There were no great lava patterns, no Mares. No faces on the Moon. Why?

Scientists later discovered the answer was that the Dark Side of the Moon has a much thicker crust--30 miles thicker! There had been no molten lava seas created by asteroids because the magma couldn't penetrate the extremely thick crust.

But what did that mean? How could one side of the Moon have a crust so much thicker than the opposite half?

Theorists went to work again, and they came up with something really astounding that might explain the difference. Maybe the Moon had once been twin moons.

When Theia crashed into Earth 4.5 billion years ago, enough Earth debris was thrown into space to have two moons form instead of one. As the twin moons circled the Earth, their orbits gradually brought them closer and closer together, until they finally, slowly, mashed together in slow motion to form one body, half of which gained a much thicker crust. Viola!

Not all scientists bought into this new hypothesis. They took issue with some of the theories. For one, the chemical composition on the two hemispheres of the Moon is not different enough to indication it was the product of two separate bodies that melded together. They thought the Second-Moon Theory was a little too far-fetched, and looked for other explanations.

They found a possible answer orbiting another distant star.

Nearly 400 light years away from Earth, a rocky planet orbits its star a little too closely. The gravity of the sun causes the planet to always face the star, just as the Moon always faces Earth. The result is that the sun side of the planet is hot and molten, but the far side is cold and solid.

Photo credit NASA

At the time of Theia's impact, the surface of the Earth would have been very hot and molten because of the release of energy, so the side of the Moon facing the Earth would also be hot and the side facing space would be cool. At that point in the life of the solar system, simulations show that the Moon orbited much, much closer to Earth--at only around 20,000-30,000 kilometers (12,500-18,600 miles) away compared to today's 384,000 kilometers (238,600 miles), or about 15 times further away than when it first formed.

With the Earth that close just after a major impact, it was hot--estimated at about 4,500 F (2500 C). All that heat and gravity would have had the effect of a mini-sun, creating some major influences on the newly formed Moon. This caused either clouds of mineral vapor to rise up on the boiling surface and then fall to the surface on the cool side away from Earth, or mineral feldspar containing aluminum and calcium to flow to the cooler side and condense.

These theories would only work if tidal lock had been in place from the time of formation of the Moon, which may have happened as quickly as 100 days (or sooner) after the impact with Theia. This "migration" of minerals to the cool side of the Moon would have built up over time until the crust on the far side became much thicker than the near side of the Moon.

There are many theories, but in truth, the origins of our Moon are still not known. Today, billions of years later, the surface of the Moon is cool. But it's not dead, as we once thought. The Moon still has a dynamic core...and a few more surprises awaiting its visitors.

Next week:

The Moon Guards Her Secrets
Part VI: How the Moon Created Life on Earth

Resources:
How the Universe Works: Secret History of the Moon, Science Channel S4/E6 (2015)
AirSpaceMag.Com: The Second-Moon Theory
Slate.com/BadAstronomy: Why Do We Have a Two-Faced Moon?
CuriousAstroCornell.edu: How Close Was the Moon to the Earth when it Formed?