Class 6: Formation of the Moon and the nature of modern impactors
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Finish off talking about the Moon
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Recap possible formation theories.
Digression into scientific method.
The success of the giant impact theory.
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The nature of modern impactors
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Comets and asteroids.

First, let’s finish the material from last class on formation of the Moon…

III : Formation of the Moon
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Possible theories…
Simultaneous formation theory
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Predicts that composition should be same.
NO!
Fission theory
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Not possible dynamically…
Capture theory
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Capture requires simultaneous interaction of 3 objects (Earth, Moon and something else)… too unlikely.

The giant impact theory
Canup
(SWRI)

The giant impact theory, cont’d
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Mars sized object hits early Earth.
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Ejects mantle material into space… forms a disk/ring orbiting around Earth
Material in ring then forms Moon.
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Probably takes only a year or so!
Moon initially formed very hot, and gravity is not large.
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Thus volatile elements and compounds are lost into space.
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Explains age, composition, lack of iron; and is very plausible given conditions in early solar system.

The nature of “modern” impactors…

I : Asteroids
Gaspra
(20 km)

Eros (33 km)

Asteroid properties
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We have already mentioned Earthcrossing asteroids (ECAs)…
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Over 1000 known.
Over 300 are “potentially dangerous”… more than 150 m diameter and on orbits that bring them “close”.
Largest is 15 km.
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Typically, they would have…
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Density of about 2000-3000 kg/m 3 (rock).
Speed of about 20-30 km/s near Earth.

Asteroids, cont’d
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Using these numbers…
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150 m diameter asteroid hits with energy of
600 1-Mton H-bombs (or up to 22,000,000
Mtons for a 5 km sized asteroid).
Later we will discuss the likelihood of such events, and what we might be able to do to prevent them.
Do you think it makes a difference if asteroids are solid or made or rubble
(with the same bulk density)?

II : Comets
Hale-Bopp

Halley (16 km)

Comet properties
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A “typical” comet has…
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Diameter = few km.
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Speed = 40-50 km/s near Earth.
Density = 500 kg/m 3 (dirty snow).
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Let’s take a very small comet (150 m)…
Energy of collision is 300 Mton. More typical is
5 km comet… 10,000,000 Mton!
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So, both asteroids and comets can be very dangerous!

Questions to think about…
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How much energy would it take to…
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Deflect an object from its path?
 What factors would it depend on?
Blow it up?
 What’s keeping it together?