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