Explorations of the Universe
How Did the Earth and Moon Form?
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
A Classic Early Piece of Space
Art: Chesley Bonestell, 1953
The Early Earth Cools
The Oceans Form
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
A Modern Idea of Early Earth
How Did the Moon Form?
Pre-1985 Ideas
• Fission
• Co-Creation
• Capture
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
Co-Creation
• Moon should orbit in Earth’s equatorial
plane
• Moon is less dense and different in
chemistry than Earth
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
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
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.
As Usual, In
Any Area of
Science, Gary
Larson Gets
There First
A Great Time for Space Artists
This is a
more or less
literal
rendition of
an early
computer
simulation
Computer Simulations by H.J.
Melosh (University of Arizona)
Earth
would have
been as hot
as the Sun
for about
10,000
years
The First Moonrise