Origin of Moon
Lesson in Characteristics of Science
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
Today’s Plan
• Moon Features
• Formation Theories of the Moon
– Failed Theories
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Fission
Capture
Co-Accretion
Small impacts
– Current Best Theory
• Giant Impactor
Earth’s Moon
• How did our Moon form?
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
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?
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
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
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
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.
• http://www.youtube.com/watch?v=qiYKonNE
c-c&feature=related
Fission
• Predict: Moon should orbit the Earth at
its equator
• BUT: It doesn’t
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.
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
Theory: CoAccretion/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
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
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
After Landing
• Find: Moon’s surface is lots like Earth’s
mantle, even the ratio of oxygen isotopes
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Supports fission
Negates capture
Negates co-formation
Supports small impacts
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
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.
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
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
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?
Timing
• We know Moon about 4 Billion years old
• Lots of impacts then
• So - Mars sized impactor not out of the
question
Earth Formation
• Very hot initially: heat of formation,
internal radioactive decay, collisions
• Molten at first, then cools & solidifies
• No atmosphere
Iron on Molten Earth?
• Know structure of Earth by:
– Overall density vs. surface density
– Seismic waves
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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.
Confirmation: Particle
Simulation
Canup & Asphaug 2001, SwRI
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
Characteristics of Science
Summary
• Moon features tell us about Earth’s
formation history
• Evidence suggests Moon formed by Mars
sized impactor
Assumed Prior Knowledge
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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
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.
More test questions
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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.
Other Moon Resources
• http://www.lpi.usra.edu/mymoon/
• Look for Moon Formation and Processes
Powerpoint:
http://www.lpi.usra.edu/education/resources
/s_system/moon.shtml
Moon Origin
• http://www.psi.edu/projects/moon/moon.ht
ml
• http://www.swri.org/3pubs/ttoday/spring99/
moon.htm
• http://www.swri.org/3pubs/ttoday/fall01/Mo
on.htm
QuickTime™ and a BMP decompressor are needed to see this picture.
Moon Facts
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orbit: 384,400 km
diameter: 3476 km
mass: 7.35 x 10^22 kg
volume:
density:3340 kg/m3
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0.27 Earth
0.012 Earth
1/50 Earth
0.61 Earth
Mass
4 diameter
Density
,Vsphere   


Volume
3  2
3
Earth
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
Moon
2160 miles (3476 km)
diameter
7 degree tilt (~no seasons)
Surface temps - 107 C to –
153 C (224 F to –243 F)
No atmosphere
No liquid water … Ice at
poles in shadows?
Earth
Moon
No Magnetic Field
Small Moon Quakes
Small, Offset Core
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..
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
Structure of Earth Today
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
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