1. Lesson in Characteristics of Science
Origin of Moon
Lesson in Characteristics of Science

2. Today’s Goals Overall Goal: Use Supporting Evidence
Decipher history of moon from features
Moon rocks to support formation theory
Facts and Synthesis:
Moon properties, compared to Earth
Moon formation theory – link to Earth, Solar System

3. Today’s Plan Moon Features Formation Theories of the Moon
Failed Theories
Fission
Capture
Co-Accretion
Small impacts
Current Best Theory
Giant Impactor

4. Earth’s Moon How did our Moon form?

5. A few data to ponder ….
Lower density – “lighter” - relative to planets
Less iron than whole Earth, more aluminum and titanium
Moon’s chemical signature ~ Earth’s mantle
The moon is enriched in Titanium, Aluminum, and related elements and depleted in iron and volatile elements (and compounds - DRY!)
The moon (~3.3) has a low density compared to earth (~5.5)
Earth/moon has anomalously large amount of angular momentum compared to other planets - conservation of momentum. Angular momentum is the measure of rotation in a system.
The combined momentum of these two closely related bodies must stay the same over time. If moon closer earlier, then earth must have been rotating faster (shorter days).
Momentum = mass x velocity
Angular momentum = mass x velocity in a circle
Earth’s high because earth + moon + impact = higher momentum
Old highlands (cratered), young basalt “seas” billion years ago.
Formation of the Moons's geosphere
(fig p179 McSween) The moon too developed a magma ocean during its formation due to the same sources of heat, impact, internal compression, radiogenic decay and gravitational tides. We know this because when the first moon rocks were brought back to Earth during the Apollo program they were composed almost entirely of one mineral -- Feldspar. The only way to form such a rock (called Anorthosite) is to precipitate crystals from a molten lava and then separate the cystals according to differences in density -- a process known as fractional crystallization. This would happen if feldspar crystals intially crystallized with other minerals but then floated to the surface of a large magma ocean. The denser minerals would sink and the less dense minerals would float. Eventually, when the magma ocean cooled and solidified the crustal surface would be dominated by feldspar and the rock would be Anorthosite. This separating out of minerals of different density is known to occur in magma chambers within the continetal crust on Earth, but the planetary scale of this process on the moon is mind-boggling.
(fig p178 Mc Sween) Estimates of the thickness of the Anothosite crustal layer on the moon indicate that it is ~20 km thick. To separate out a 20 km thick layer of feldspar from a molten magma requires a magma ocean that was at least 1000 km thick. So far measurement of radiometric ages of the Anorthosites indicates that the lunar crust solidified 4400 Ma ago, only 200 Ma after the formation of the solar system.

6. How Did the Moon Form? A few more data to ponder
Does not orbit in equatorial plane of Earth, or ecliptic
Earth/Moon - high angular momentum
How Did the Moon Form?

7. Moon Facts Before Moon landings we knew:
Shape of Moon, Mass, Diameter, Density, Distance
Heavily Cratered, Other Surface Features – Approx. Age
Orbital Information, Angular Momentum of System

8. Age from Cratering
Cratering history, number of impacts as function of time
Assume rate of impacts during past few billion years decreasing at known rate, estimate how long since last ‘erasure’ activity
Get age
About 4-5 billion years
Crater Clavius, photograph from Celestron.com, courtesy Arpad Kovacsy

9. Models/Theories Make Predictions
Let’s look at the most popular theories for the formation of the Moon PRIOR to our visiting the Moon.
Each makes predictions about what we should observe.
Three biggest theories: daughter, wife, sister

10. Theory: Fission
Young Earth spun so fast that a bulge occurred at the equator.
Over time a chunk from Earth's mantle was flung into orbit.
Predict: Amount of angular momentum needed to fling piece off and can compare to current angular momentum of system
Current E-M system doesn’t have enough angular momentum.

11. Fission Predict: Moon should orbit the Earth at its equator
BUT: It doesn’t

12. Fission Predict: Moon rocks same composition as Earth mantle rocks
Probably not the method of formation: angular momentum not sufficient and orbital orientation is incorrect.

13. Theory: Capture
An “asteroid” gets captured by the Earth in the early solar system
Problem: How does it slow down and get into orbit?
Predict: Expect different composition

14. Theory: Co-Accretion/Condensation
Moon and Earth form at same time from same material
Predict: Expect same density
Predict: Same overall composition of Earth and Moon (i.e. iron, water, gases)
BUT: We already know not same density

15. Theory: Colliding Planetesimals
Moon forms from bits of Earth blasted off by small impacts
Problem: Small impacts don’t send enough material into space. Most recaptured by Earth’s gravitational field before can form Moon.
Predict: Same overall composition of Earth’s surface/mantle and Moon

16. After Lunar Landing: Composition of Moon Rocks
No gases
Evidence of heating
Oxygen isotopes same as Earth, different from other parts solar system
Density of surface rocks like average density of whole moon
Little iron

17. After Landing
Find: Moon’s surface is lots like Earth’s mantle, even the ratio of oxygen isotopes
Supports fission
Negates capture
Negates co-formation
Supports small impacts

18. Formation of Moon
Theory best able to explain based on moon rock evidence
Moon rocks almost as old as Earth
Moon rocks made up of nearly identical material as crust and mantle
Moon rocks lack all volatiles
Moon rocks show evidence of heating

19. Fission? Or Small Impacts?
So many other problems with fission that it seems unlikely
Small impacts just don’t have the energy to get fragments into Earth’s orbit.

20. New Theory: Giant Impactor
Mars sized body (½ diameter, (1/10 mass)
Impact causes mantle material to be ejected
Re-coalesce to form Moon
Alastair Cameron, Harvard University

21. Giant Impactor Predictions:
Depending on when impact occurred - composition could be very similar to mantle but not to core
Oxygen ratios can be same if impactor big enough to modify overall for system
Extreme heating of rocks means all volatiles gone
Giant impactor can have enough energy to get particles to orbit earth
Giant impactor means Moon doesn’t have to orbit at equator

22. Likely?
Is it likely that a huge Mars-sized chunk was around to whack into Earth to make this happen?
Where is the Moon’s iron?
Let’s go back to age of Moon…
What was going on in the solar system?
What was Earth like?

23. Timing We know Moon about 4 Billion years old Lots of impacts then
So - Mars sized impactor not out of the question

24. Earth Formation
Very hot initially: heat of formation, internal radioactive decay, collisions
Molten at first, then cools & solidifies
No atmosphere

25. Iron on Molten Earth? Know structure of Earth by: Iron Core NOW - how?
Overall density vs. surface density
Seismic waves
Iron Core NOW - how?
Differentiation: denser materials sink to center
Can only happen if Earth is molten or liquid
So - for Moon to have no iron - impact would have had to happen AFTER differentiation
Allows us to know when Earth’s iron core formed.

26. Confirmation: Particle Simulation
Canup & Asphaug 2001, SwRI

27. Summary: Impactor Theory Evidence
No volatiles in moon rocks - heating, from impact
Moon rocks like Earth mantle rocks – impact occurs after most of iron differentiated
Oxygen isotopes (asteroids not like Earth, but Moon is like Earth)
Earth tilt, Moon orbit geometry

28. Characteristics of Science Summary
Moon features tell us about Earth’s formation history
Evidence suggests Moon formed by Mars sized impactor

29. Assumed Prior Knowledge
Theories must be falsifiable, testable, make predictions that can be tested
Formation of the Solar System (nebular hypothesis, planets in a plane, oddities to explain with impacts)
Formation of the Earth - Geologic history of the Earth, including higher incidence of impacts in the past, settling of iron core in molten earth, erosion processes, fossil record
Radioactive Dating, half-lives, log plots
Density, Volume calculations
Energy conservation, especially with impacts

30. Test Questions
Describe some of the evidence gleaned from moon rocks that disproves the capture theory
When was the last time people were on the moon?
Did the Moon have volcanic activity? How do we know?
Make a graph (with appropriate labels) estimating the number of impacts as a function of time in the history of the solar system.

31. More test questions
Did the giant impact that formed the moon happen before or after the differentiation of the Earth? What evidence supports that? (be sure you describe the what differentiation is in your answer)
Were large impacts more or less likely in the past? Describe the evidence.
Calculate the average density of the moon.
If avg density of iron is 8000 kg/m^3 and surface rocks on earth have avg density of 2500, which are moon rocks more like? Explain how this fits in to the impactor theory of the formation of the moon.

32. Other Moon Resources http://www.lpi.usra.edu/mymoon/
Look for Moon Formation and Processes Powerpoint:

33. Moon Origin http://www.psi.edu/projects/moon/moon.html

35. Moon Facts 0.27 Earth 0.012 Earth 1/50 Earth 0.61 Earth
orbit: 384,400 km
diameter: 3476 km
mass: x 10^22 kg
volume:
density:3340 kg/m3
0.27 Earth
0.012 Earth
1/50 Earth
0.61 Earth

36. Earth Moon 2160 miles (3476 km) diameter 7 degree tilt (~no seasons)
Surface temps C to –153 C (224 F to –243 F)
No atmosphere
No liquid water … Ice at poles in shadows?
7930 miles (12,756.3 km) diameter
23 degree axis tilt (seasons!)
Surface temps –73 to 48 C (-100 to 120F)
Thick atmosphere, mild greenhouse effect
Liquid water – lots! - at surface
antwrp.gsfc.nasa.gov

37. Earth Moon No Magnetic Field Small Moon Quakes Small, Offset Core
No Magnetic Field
Small Moon Quakes
Small, Offset Core

38. Gory Details: Moon Rock Composition
Moon relatively depleted in volatile elements and enriched in refractory elements. Highly depleted in siderophile elements… b/c scavenged by earlier differentiation..

39. Formation of the Earth Formed in protoplanetary disk
All planets in single plane
Think hula hoop, or Saturn's rings
Formed via accretion
Planetesimals to protoplanets
Lots of heat

40. Structure of Earth Today

41. Molten Earth?
Molten early Earth explains why oldest rocks on Earth are not as old as oldest meteorites or as old as Sun
Molten early Earth also explains why no atmosphere present initially - boils atmosphere away
Although, didn't form with many volatiles, including water

42. Theory of Moon Formation
Large impact with Mars size object
Explains heating
Explains lack of volatiles
After Earth differentiated
Explains why moon similar to crust and mantle
Before 4.4 million years ago
When impacts were much more frequent
Crater counts provide another estimate of age of moon

43. Lunar Interior Structure
A cross section view of the Moon’s internal structure. The outer, rocky crust averages about 60 km thick. Most of the Moon is the mantle, which is also rocky but of somewhat different composition than the crust (more magnesium, less iron and aluminum). There is probably also a small, iron rich core, a few hundred km in size at the center of the Moon.
Copyrighted, LPI