The Origins of Earth and Its Moon Notes

A Fiery Birth: The Process of Accretion

Accretion is the gradual accumulation of matter through collisions and gravitational attraction.

How Did Accretion Work?

Formation of the Solar Nebula

1. The solar system began as a giant cloud of gas and dust.
2. Gravity caused the cloud to collapse, forming a rotating disk with the Sun at its center.

Planetesimal Formation

1. Within the disk, small particles collided and stuck together, forming larger bodies called planetesimals.

Growth of Protoplanets

1. As planetesimals grew, their gravitational pull increased, attracting more material.
2. Repeated collisions and mergers formed protoplanets, including early Earth.

Why Was Earth So Hot?

Kinetic Energy of Impacts

• Collisions converted kinetic energy into heat, raising Earth's temperature.

Size and Density

• Earth's larger mass and density compared to other terrestrial planets meant it experienced more energetic impacts.

Molten State

• The heat from accretion melted silicate minerals, creating a molten outer layer.

Differentiation: The Formation of Earth's Layers

Differentiation is the process by which Earth's materials separated into distinct layers based on density.

How Did Differentiation Occur?

Melting of Materials

1. Heat from accretion and radioactive decay melted silicates and iron.
2. Molten materials became free to move and recombine.

Density-Driven Separation

1. Denser materials (e.g., iron) sank to form the core.
2. Lighter materials (e.g., silicates) rose to form the mantle and crust.

Earth's Layered Structure

Core

1. Composed mainly of iron and nickel.
2. Divided into a solid inner core and a fluid outer core.

Mantle

1. Composed of dense silicates.
2. Extends from the core to about 70 km below the surface.

Crust

1. The outermost layer, composed of lighter silicates.
2. Ranges from 0 to 70 km in thickness.

The Origin of the Moon: The Giant Impact Hypothesis

Early Theories

Capture Hypothesis

1. Suggested the Moon was a passing body captured by Earth's gravity.
2. Unlikely due to the Moon's circular orbit.

Fission Hypothesis

1. Proposed the Moon was ejected from a rapidly spinning Earth.
2. Physically implausible.

Co-formation Hypothesis

1. Suggested Earth and the Moon formed together.
2. Inconsistent with the Moon's low iron content.

The Giant Impact Hypothesis

The Collision

1. A Mars-sized body (often called Theia) collided with early Earth.
2. The impact ejected a large amount of Earth's mantle material into space.

Formation of a Disk

1. The ejected material formed a disk around Earth.
2. Most of the iron from Theia's core merged with Earth's core.

Accretion of the Moon

1. The material in the disk coalesced to form the Moon.
2. This explains the Moon's low iron content and similarity to Earth's mantle.

Evidence from Apollo Missions

Moon Rocks

• Similar to Earth's mantle but poorer in metals and volatile elements.

Age of Moon Rocks

• Dated to 4.4–4.5 billion years, consistent with the timing of the giant impact.

Earth's Magnetic Field: The Magnetic Dynamo

How Is Earth's Magnetic Field Generated?

Convection in the Core

1. The outer core is composed of molten iron, a good conductor of electricity.
2. Heat from the inner core drives convection currents in the outer core.

Induction of Electric Currents

1. Moving molten iron in the presence of a magnetic field generates electric currents.
2. These currents create a self-sustaining magnetic field through a process called the magnetic dynamo.

Why Is the Magnetic Field Important?

Protection from Solar Wind

• The magnetosphere deflects high-energy charged particles from the Sun.

Formation of the Van Allen Belts

• These are regions where charged particles are trapped by Earth's magnetic field.

Auroras

• Charged particles interacting with Earth's atmosphere near the poles create auroras, such as the Aurora Borealis.

The Moon's Geological History

Early Bombardment and Crater Formation

Intense Bombardment

1. The Moon was heavily bombarded by meteorites during its first billion years.
2. This created the large craters and basins visible today.

Volcanic Activity

1. Heat from accretion and radioactive decay caused melting in the Moon's upper layers.
2. Volcanic eruptions formed the maria, or dark basaltic plains.

Why Does the Moon Lack Craters?

Erosion and Weathering

• Earth's atmosphere, water, and tectonic activity erase impact craters over time.

Geological Activity

• Plate tectonics and volcanic activity reshape Earth's surface, unlike the Moon's static crust.

Reflection and Broader Connections

1. The formation of Earth and its Moon highlights the interplay of gravity, energy, and geological processes.
2. These events set the stage for Earth's unique environment, including its magnetic field and layered structure.