Moon Lesson 3

Formation of the Moon

More detail on the lunar composition• The Moon’s bulk composition is similar to the

Earth’s but not identical.• The average density of the Moon is virtually

the same as the Earth’s lithosphere density. (3.3 gm/cm3)

• Both the Earth and Moon have basaltic rocks that are dominated by iron and magnesium-rich silicates (such as olivine and pyroxene).

• Rocks on the Earth have volatile chemicals (chemicals that evaporate readily) such as H2O and CO2 locked up in the minerals. The Moon rocks have virtually none.

• The isotopic ratio of 016, O17 and O18 are identical between the Earth and the Moon. No other objects in the Solar System have this isotopic ratio of Oxygen.

Isotopes.

• The most abundant form of oxygen in O16. It has 8 protons and 8 neutrons in its nucleus (99.762% of oxygen).

• O17 has 8 protons and 9 neutrons

• O18 has 8 protons and 10 neutrons.

• Is it possible to have an isotope of oxygen that has 7 protons and 10 neutrons?

.

1 2

0%0%

1. Yes, it is also called O17

2. No, that would be nitrogen, N17

• Rocks on the Earth have volatile compounds (compounds that evaporate readily) such as H2O and CO2 locked up in the minerals. The Moon rocks have virtually none.

• The isotopic ratio of 016, O17 and O18 are identical between the Earth and the Moon. No other objects in the Solar System have this isotopic ratio of Oxygen.

• The Moon is virtually deplete of Cobalt and Nickel• The Moon’s orbit is tilted 5 degrees from the Earth-

Sun plane. This means, on average, it is tilted 23.5 degrees compared to the Earth’s Equator.

Moon’s orbit is tilted by 5o

On average it is tilted 23.5o from Earth’s equator

There are four models for the Moon’s origin

• The Moon formed elsewhere in the Solar System and was captured by the Earth’s gravity. (Capture Model)

• When the Earth first formed it was rotating very rapidly and it flung the Moon out of its surface. (Fission Model)

• The Moon formed in orbit around the Earth and accumulated mass just like the Earth did. (Co-accretion Model)

• The Earth was struck by a large body and the Moon formed from the material that was thrown out into orbit around Earth (Collision Model)

Observables

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

Capture Model

• Here the Moon formed elsewhere in the Solar System but ventured to close to the Earth and was captured by the Earth’s gravity.

• Is there any refuting data form the observable list?

Observables

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

Which observable is a problem for the capture model?

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1 2 3 4 5 6 7

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1. #12. #23. #34. #45. #56. None of them7. All of them

Observables for Capture Model

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

These are problems ( √ )

√

Maybe a problem

Maybe a problem

Capture has an added difficulty.

A.

Capture has an added difficulty.

B.

Capture has an added difficulty.

C.

• What would happen to the Moon’s velocity as it approached the Earth? (Going from A. to B.)

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1 2 3

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1. It would speed up2. It would slow down3. It would remain the

same

Reality.

A.

Moon speeds up.

B.

Moon is still free.

C.

Capture

• In order to capture the Moon, the Moon has to slow up as it approaches the Earth.

• This requires an incredible amount of drag (frictional force) which is extremely unlikely.

Fission Model

• First proposed by George Darwin (Charles’ son). It explained the Pacific Ocean Basin.

Fission Model

• In this model very early Earth was spinning very rapidly. (1 day = 1.4 hours)

• This is fast enough to launch material into orbit around the Earth.

• Where on the Earth’s surface is the Earth’s rotational speed the fastest?

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1 2 3

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1. At the poles2. At the Equator3. Lexington, KY .. Go

Cats!

Observables for Fission model

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

Which observable is a problem for the Fission model?

.

1 2 3 4 5 6 7

0% 0% 0% 0%0%0%0%

1. #12. #23. #34. #45. #56. None of them7. All of them

Observables for Fission model

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds √#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit √

These are problems ( √ )

Added problem

• The Earth is spinning way too fast. • It will slow after the Moon leaves because the

orbiting Moon will take away some of the Earth’s rotational angular momentum.

• This will drop the day to about 2.5 hours.• We saw that the Moon has slowed the Earth’s

rotation from 6.5 hours to 24 hours.• There is no good way to get rid of the rest of

the Earth’s rotational angular momentum.

Co-accretion Model

• Here the Earth and Moon were in orbit early on when they were very small.

• The Earth was larger and received more material from the Solar System than the Moon, but they grew at the same time.

Observables for Co-accretion model

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

Which observable is a problem for the Co-accretion model?

.

1 2 3 4 5 6 7

0% 0% 0% 0%0%0%0%

1. #12. #23. #34. #45. #56. None of them7. All of them

Observables for Co-accretion model

#1. Average density difference √#2. Oxygen isotopes are the same#3. No volatile compounds √#4. Cobalt and Nickel difference √#5. Tilt of the Moon’s orbit

These are problems ( √ )

Collision Model

• Very early on in the Earth’s history, after it had partially differentiated, but before it had fully formed the Earth was impacted by a planet about the size of Mars. (10% the Earth’s mass)

• This impact tore off material from the outside of the Earth and flung it into space. The rest of the planet (including the core) merged with the Earth.

• A small percentage of the ejected material went into orbit around the Earth and formed the Moon.

• In the early Solar System, small chunks of rock grew bigger by collecting smaller pieces of debris.

• Toward the end of the formation time there were many planet size objects left.

http://www.flickr.com/photos/thane/3134316459/

Observables for Collision model

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

Which observable is a problem for the Collision model?

.

1 2 3 4 5 6 7

0% 0% 0% 0%0%0%0%

1. #12. #23. #34. #45. #56. None of them7. All of them

Observables for Collision model

#1. Average density difference#2. Oxygen isotopes are the same#3. No volatile compounds#4. Cobalt and Nickel difference#5. Tilt of the Moon’s orbit

These are problems ( √ )

Conclusion

• Currently, the collision model for the origin of the Moon is the model that best matches the known observables.

• For now it is the model that is most widely accepted.

• That could always change in the future as we get more information from the Moon.