Why does Earth’s Moon stand out/what is different about it compared to other planet’s satelittes within the solar system
Size: The Moon is unusually large relative to the planet it orbits, which is Earth.
Density: It has a notably low density compared to what you’d expect for a satellite of a terrestrial planet like Earth.
Core Size: The Moon’s core is tiny, only 2-4% of its total mass, whereas Earth’s core makes up 30% of its mass.
Angular Momentum: The Moon-Earth system has unusually high angular momentum
Why is the origin of the Moon a very active area of research today?
Becuase it is hard to explain how a planet like Earth could be partnered with a satellite like the Moon considering the moon’s unusual characteristics
What 3 hypotheses were thrown around before 1970, when there was little evidence to support any one?
1. Fission hypothesis: Moon broke off from a rapidly spinning Earth.
2. Condensation hypothesis: Earth and Moon formed contemporaneously (at same time) from the same material.
3. Capture hypothesis: Moon formed as an independent planetary body that was later ‘captured’ by Earth during a close pass.
What was one of the main objectives of the Apollo space program?
to resolve the question of lunar origin (test the hypothesis about the origin of Earth’s Moon)
What did Apollo space program do in effort to solve the question of lunar origin
- set up instruments on Moon’s surface to gather data about its interior (determine ‘interior zoning’ which “interior zoning” refers to determining how the Moon is structured beneath its surface)
- collected rock samples from the Moon’s surface
with these things, they brought back to Earth raw data and material needed to answer the question of lunar origin
Which of the 3 proposed hypotheses at the time of the Apollo Space program were proved right by the raw data and material (interior zoning and surface samples) collected by Apollo?
None of them! The evidence gathered by Apollo killed all 3 hypotheses
What is the fission hypothesis and why was it rejected?
Who proposed it and when?
What is the fission hypothesis?
The fission hypothesis was that Earth spun so fast in the early days of its formation that a buldge developed and soon a chunk just broke off and was thrown into orbit (the moon was a part of Earth that broke off because it was spinning so fast
Why was it rejected?
this would require that Earth rotated once every 2.5 hours instead of every 24 hours, which was difficult to reconcile with its current rotational rate
Who and when?
George Darwin (son of Charles Darwin). Proposed more than 100 years ago
What is the Condensation Hypothesis ? Why was it rejected (2 points that needed to support it that were not satisfied by evidence)
What is the condensation hypothesis?
both bodies formed at about the same time form exactly the same parental ‘cloud’ of dust and gas.
Why was it rejected?
2 points were needed to support this hypothesis that were not satisfied by evidence:
- they must have the same chemical composition - this was poven untrue - Moon has a tiny metallic core compared to Earth
- Moon must orbit Earth exactly on an equatorial plane - this is not the case.
(If the Moon had formed alongside Earth from the same material, it would likely inherit the same angular momentum and should orbit along Earth’s equator. However, this is not the case. The Moon’s orbit is inclined by about 5 degrees relative to the Earth’s equator)
What is the capture hypothesis? Why wasit deemed unlikely even as it was proposed?
What is the capture hypothesis?
Moon formed as an independent planetary body that was later ‘captured’ by Earth during a close pass.
Why was it deemed unlikely from the start and eventually rejected?
1. the probability for the exact gravitational and dynamic conditions needed for an object the size of the Moon to fall into orbit about Earth is extremely unlikely.
2. Some chemicals (especially oxygen isotopes) were so similar between Earth and Moon that there had to be some ‘genetic relationship’
3. Every attempt to model a capture via computer program failed miserably
What is the giant impact hypothesis?
- When was it proposed?
- what does it state?
The Giant Impact Hypothesis was proposed in 1976/77 as a new explanation for the Moon’s formation.
- This theory suggests that a (hypothetical) Mars-sized planet, which we name Theia, struck Earth in a glancing blow about 4.5 billion years ago.
- This collision(s) released an immense amount of kinetic energy, causing both Earth and Theia to melt almost instantly.
- We say collision(s) possibly plural because computer simulations strongly suggest it would have been 2 mild collisions resulting from a glancing blow
- This gave an increased angular momentum to Earth (increased spin rate)
- The metal core of the impactor (theia - mars-sized body) separated and dropped into earth, thus giving Earth a large metal core and remarkably high density
- To some degree, the molten material of both bodies mized, and formed debris in space just above earth
- Over a relatively short period of time, some of the debris fell back to Earth but most of it collected into a single mass to become the Moon
What is kinetic energy?
What does it mean that kinetic energy increases with the square of the velocity.
Energy a body has by virtue of it’s motion.
Kinetic energy increases with the square of the velocity
What is the normal travelling speed of a 10km in diameter asteroid. If it smashed into earth, what would be the effect?
What would be the likely result of even a glancing blow to earth by a planet-sized body?
if a 10km in diameter asteroid was travelling at the normal 15 to 20km/s, smashing into earth would result in temperatures hotter than the sun’s surface. This is because of the kinetic energy.
Even a glancing blow from a planet sized body would result in so much kinetic energy being transformed to heat that both the impactor and Earth would melt in an instant.
What is one of the challenges to a model like the grand impact hypothesis?
One of the challenges of grand impact hypothesis has been to explain why lunar rocks are so similar in composition to Earth rocks rather than retaining a large portion of what was originally Theia
As more precise measurements were made, what 3 constraints emerged (evidence/things we know about the moon that a hypothesis of lunar origin would have to account for?)
Is the grand impact hypothesis still viable considering these constraints combined with the ones we already knew (low density, high angular momentum) ?
Explain how each of the 3 points interacts with the grand impact hypothesis. Do they challenge it, support it?
- Tungsten (W) isotopes tell us that the Moon started life at least 30 million years after the start of the solar system (a long time after a moon-sized body would have formed by accretion)
- The oldest moon rocks were formed when a ‘magma ocean’ cooled, so the Moon must have started out with a large enough input energy to be largely melted (moon must have been very hot, like an ocean of molten rock)
- The isotopes of oxygen in lunar samples have almost exactly ssame proportions as Earth rocks. No other object in the Solar System mathes Earth composition like this (this must mean that Earth and Theia compositions were well mixed)
**Considering this, it still seems an impact event is required, as stated by the grand impact hypothesis. **
- in the Giant Impact Hypothesis, the Moon forms from the debris of the collision between Earth and Theia. This could have happened after the initial formation of the planets in the Solar System, so the 30-million-year gap is not necessarily a contradiction.
- If a giant collision occurred between Earth and Theia, the resulting debris would have been extremely hot from the energy of the impact, and the Moon would have formed from this molten material. So, the evidence of a “magma ocean” aligns with the idea that the Moon was formed from the hot, melted debris of the impact, which then cooled to become the Moon we know today.
- This is a key piece of evidence for the hypothesis. If the Moon had formed from completely different material (like if Theia were a totally foreign body that didn’t interact much with Earth), the Moon’s isotopes would be different. However, the similar isotope ratios suggest that Earth and Theia mixed very well during the impact. This would mean that the Moon formed from material that was a blend of both Earth and Theia, which explains why the Moon’s isotopes closely match Earth’s.
What two new models with different views about the impact event that led to the Moon’s origin were published in 2012? (these are the current hypotheses)
One model suggests a larger impact with a planet nearly as massive as Earth, with lower velocity.
* In this model Earth and Theia mix over 26 hours
The second involves a smaller, high-velocity impact combined with rapid Earth rotation.
* in order to spit out enough material to form the Moon, Earth has to be spinning really fast (rotation every 2 hours)
* therefore, this model is a combination of the Fission hypothesis (proposed by Darwin) and the Grant Impact hypothesis
* recall, Darwin model rejected becuase Earth spinning that fast was not compatible with current rotation
* this new model solves this by suggesting a mechanism for sloweing down the rapidly spinning Earth - by tidal interaction with the Sun
Describe what is depicted in this image
this image illustrates a low velocity impact with a planet (Theia) being roughly Earth sized, a simulation for this model which was proposed in 2012
What is another recent idea of the Moon’s origin?
That Earth may have once had 2 Moons
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Chapter 1: Scientific Method and Big Bang46
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Chapter 2: Time and Space28
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Chapter 3: Nebular Hypothesis49
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Unit 2 Introduction12
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Chapter 4: A Star Called The Sun51
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Chapter 5: Geometry of the Solar System58
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Unit 3 Introduction1
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Chapter 6: Collision17
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Chapter 7: Earth137
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Chapter 8: The Moon103
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Chapter 9: Mercury5
