Formation of the Moon and Earth's Structure

Origin of the Moon

• Moon formed ~95 million years after solar system solids condensed.
• Main theory: Earth's collision with a Mars-sized body ejected debris into orbit, forming the moon.
• Initial moon was a hot, liquid mass; cooled to form crust.
• Moon’s surface was impacted, creating the South Pole-Aitken Basin and volcanic features known as Lunar Maria.
• Late heavy bombardment (4.1 to 3.8 billion years ago) led to extensive cratering and volcanic activity.
• Moon is moving away from Earth at ~1.5 inches per year.

Earth's Composition and Tectonic Plates

• Earth has three main compositional layers: crust, mantle, and core.
  • Crust: Thin outer layer primarily made of lighter elements (rocks).
  • Mantle: Denser layer beneath the crust, rich in heavier elements, reaching a width of about 2,900 kilometers.
  • Core: Composed of liquid iron and nickel, differentiating into outer and inner cores with inner core temperatures estimated at ~5,700 degrees Celsius.
  • Mechanical layers:
  • Lithosphere: Rigid outer layer (crust + upper mantle) made of solid rock; approximately 100 kilometers thick.
  • Asthenosphere: Plastic layer beneath lithosphere that can flow slowly, facilitating plate tectonics.
  • Tectonic plates (15+ major plates) drift on the asthenosphere, with significant plates including the Pacific Plate, North American Plate, and Eurasian Plate.
  • Movement of these plates leads to geological activity like earthquakes and volcanism. Notable plate boundaries include convergent, divergent, and transform boundaries.

Mantle Temperature

• Temperature rises ~25 degrees Celsius for each millimeter drilled into the Earth, leading to extreme conditions at greater depths.
• Core heat is derived from planet formation (gravitational compression) and decay of radioactive materials such as uranium and thorium.

Unique Features of Earth

• Gravity and atmosphere provide protection from space debris with the atmosphere containing nitrogen (78%) and oxygen (21%).
• Atmosphere absorbs harmful solar radiation (especially UV rays), enabling life to thrive on Earth.
• Liquid water is a defining characteristic, unlike Venus and Mars (which lost water); about 71% of Earth's surface is covered by water.
• Earth’s magnetosphere protects against solar radiation and charged particles from solar winds, which is critical for sustaining life.
• The hydrosphere, lithosphere, and atmosphere interact constantly, cycling water through various processes (evaporation, condensation, precipitation).