Download - Explorations of the Universe
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Explorations of the Universe
How Did the Earth and Moon Form?
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Ideas About the Early Earth Have Run Hot and Cold (Literally)
• To 1900: Early Earth hot. Only way to explain its internal heat
• 1900-1950: Radioactivity can explain internal heat, but concept of hot formation lingers
• 1950-1980: Earth need not have formed hot• Modern: Hot Early Earth was right after all
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A Classic Early Piece of Space Art: Chesley Bonestell, 1953
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The Early Earth Cools
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The Oceans Form
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Cold Earth - Hot Earth (Again)• If Earth accreted, need not have been hot• Depends on how fast heat radiated away
compared to impact rate• As planets get bigger, their gravity causes
higher-velocity impacts• Also impact ejecta buries hot rocks• Early Earth was hot - had magma ocean
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A Modern Idea of Early Earth
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How Did the Moon Form?Pre-1985 Ideas
• Fission• Co-Creation• Capture
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Fission
• Early Earth spun rapidly, became unstable, broke in two.
• Moon should orbit in Earth’s equatorial plane
• Can’t simply throw something from surface into orbit - it either falls back or escapes
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Co-Creation
• Moon should orbit in Earth’s equatorial plane
• Moon is less dense and different in chemistry than Earth
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Capture
• Can explain why Moon orbits close to ecliptic plane.
• Can account for why Moon differs in density and chemistry from Earth
• Requires extremely stringent conditions to happen
• Seems too unlikely
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A New Hypothesis: Mega-Impact
• In computer simulations of solar system formation, we don’t get nine big planets
• First stage: hundreds of Moon-Mars size planets
• Small planets collide to make bigger ones• Can explain numerous Solar System
anomalies
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A New Hypothesis: Mega-Impact
• Can explain why Moon orbits close to ecliptic plane.
• Can account for why Moon differs in density and chemistry from Earth
• A capture requires extremely precise conditions - a collision takes no skill at all.
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As Usual, In Any Area of
Science, Gary Larson Gets There First
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A Great Time for Space Artists
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This is a more or less
literal rendition of
an early computer simulation
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Computer Simulations by H.J. Melosh (University of Arizona)
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Earth would have been as hot as the Sun for about 10,000 years
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The First Moonrise