1. Explorations of the Universe How Did the Earth and Moon Form?

2. 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

3. A Classic Early Piece of Space Art: Chesley Bonestell, 1953

4. The Early Earth Cools

5. The Oceans Form

6. 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

7. A Modern Idea of Early Earth

8. How Did the Moon Form? Pre-1985 Ideas Fission Co-Creation Capture

9. 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

10. Co-Creation Moon should orbit in Earth’s equatorial plane Moon is less dense and different in chemistry than Earth

11. 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

12. 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

13. 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.

14. As Usual, In Any Area of Science, Gary Larson Gets There First

15. A Great Time for Space Artists

16. This is a more or less literal rendition of an early computer simulation

17. Computer Simulations by H.J. Melosh (University of Arizona)

20. Earth would have been as hot as the Sun for about 10,000 years

21. The First Moonrise