Earth Formation and Early History

Initial Formation

• 4.6 Billion Years Ago: Earth starts to accrete from a stellar nursery cloud where the Sun also forms.
• 4.5 Billion Years Ago: Earth becomes a significant planetary object, clearing its orbit of debris, and begins to form a primordial atmosphere.
  • Initial atmosphere composed of light volatiles like hydrogen and helium, which were lost, retaining nitrogen, CO2, methane, ammonia, etc.

Moon Formation

• 4.4 Billion Years Ago: Accretion slows, significant impacts occur.
  • A Mars-sized object, Theia, collides with Earth creating the Moon.
  • Resulting in Earth having a larger metal core; the Moon forms without a metal core.

Evidence from Ancient Rocks

• 4.375 Billion Years Ago: Oldest known zircon crystals found in Jack Hills, Australia.
  • Indicate that Earth's surface had cooled to temperatures allowing liquid water shortly after Moon formation.

Formation of Oceans and Early Atmosphere

• 4.3 Billion Years Ago: Oceans form, atmosphere composed mainly of nitrogen, CO2, methane, ammonia.
  • No large continents; Earth appears as a water planet.

Continental Formation

• 4.2 Billion Years Ago: Island chains and proto-continents start to form.
  • Geologic processes convert crustal basaltic rock to lighter silicates.

Late Heavy Bombardment

• 4.1 to 3.8 Billion Years Ago: Earth undergoes increased bombardment by comets and asteroids.
  • Evidence from lunar samples indicates clustering of impact events during this time.

Possible Causes of Late Heavy Bombardment

• Theories suggest gravitational influences from the outer planets, particularly a 2:1 resonance between Jupiter and Saturn.
• This resonance may have disturbed Neptune’s orbit, causing it to scatter Kuiper belt objects inward, leading to increased bombardment.

Impact on Earth's Environment

• Simulations show varying impacts of different-sized objects.
  • Small impacts could cause localized effects.
  • Larger impacts, possibly causing oceans to boil and reset surface conditions.

Post-Bombardment Conditions

• Despite bombardment, Earth retains oceans and conditions eventually become favorable for life, setting the stage for future developments.