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•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

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

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 ageAbout 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=qiYKonNEc-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: Co-Accretion/Condensati

on• 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– 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

• 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• 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• 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.html

• http://www.swri.org/3pubs/ttoday/spring99/moon.htm

• http://www.swri.org/3pubs/ttoday/fall01/Moon.htm

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Moon Facts

• orbit: 384,400 km• diameter: 3476 km• mass: 7.35 x 10^22 kg

• volume:• density:3340 kg/m3

• 0.27 Earth• 0.012 Earth• 1/50 Earth• 0.61 Earth

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Density =Mass

Volume,Vsphere =

4

3π

diameter

2

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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

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?

Moon

Earth Moon

No Magnetic FieldSmall Moon QuakesSmall, 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