Earth and Solar System Formation Flashcards

Nebular Hypothesis

• Origin: Describes the formation of the solar system from a primordial dust cloud.

• Steps:

  • Supernova: Initiates the formation of a primordial dust cloud.

  • Condensation: Primordial dust condenses into a disk-shaped nebular cloud rotating counter-clockwise.

  • Proto-Sun and Planets: These begin to form within the rotating cloud.

  • Accretion: Planetesimals accrete, leading to differentiation of planets and moons approximately 4.6 billion years ago.

  • Solar System Takes Shape: The solar system evolves into its current configuration.

Evidence Supporting Nebular Hypothesis

• Planetary and Lunar Revolution: Planets and moons revolve counter-clockwise around the sun (not random).

• Axial Rotation: Most planets and moons rotate on their axes counter-clockwise.

• Orbital Alignment: Planetary orbits align along the sun’s equatorial plane (not random).

• Observations: Hubble Telescope and radio astronomy confirm planetary systems forming from condensed nebular dust in places like the Orion Nebula.

Planetary Characteristics

• Terrestrial Planets: Located close to the sun.

  • Dense.

  • Small.

  • Rocky, composed of silicate minerals and metallic cores.

• Jovian Planets: Located far from the sun.

  • Low density.

  • Large.

  • Gaseous, composed mainly of hydrogen and methane.

Question

• Hydrogen Distribution: Why do terrestrial planets have minimal molecular hydrogen in their atmospheres despite the primordial dust cloud being primarily hydrogen

Asteroid Belt

• Location: Situated between the orbits of Mars and Jupiter.

• Composition: Asteroids closer to the sun contain carbon with nitrogen, oxygen, and hydrogen. Asteroids farther from the sun contain silicate rock and metallic elements like iron and nickel.

• Reasons for Existence:

  • Jupiter's Gravity: Jupiter’s mass prevents asteroids from consolidating into a planet.

  • Gravitational Balance: The asteroid belt is located at a center point between the gravitational pulls of the sun and Jupiter.

Differentiated Earth

• Compositional Zones:

  • Iron-Nickel Core: Includes a liquid outer core and a solid inner core.

  • Fe-Mg Silicate Mantle.

  • Fe-Mg-Al Silicate Crust: The oceanic and continental crust.

  • Oceans.

  • Atmosphere.

• Zoning: The Earth is compositionally zoned along a density gradient from lighter to denser materials.

Earth's Compositional Zoning

• Process:

  • Accretion of Planetesimals: Initial formation through colliding planetesimals.

  • Initial Heating: Kinetic energy from collisions and compressional heating.

  • Radioactive Decay: Additional heating from radioactive decay.

  • Iron Catastrophe: Iron melts and sinks to the core while lighter materials are displaced outwards.

    • The crust, mantle, ocean, and atmosphere form.

  • Density-Based Zoning: Earth becomes zoned by density.

    • The densest iron-nickel in the core.

    • The least dense materials in the atmosphere.

  • Timing: Occurs around 500 million years after initial accretion.

  • Convection: Convective overturn continues in the asthenosphere, mantle, and outer core.

  • Important events: You should clearly understand all the sequence of evolutionary events.

Iron Catastrophe

• Heating and Cooling: Why did the Earth heat up and then rapidly cool during differentiation?

• Heat Transfer: Heat transfer is more efficient via convection (in molten state) than conduction (in solid state).

Early Earth History

• Cratering: The moon's cratering indicates early accretion.

• Limited Evidence on Earth: Why does Earth show little evidence of early accretion history?

Degassing

• Timing: Occurred post-iron catastrophe and differentiation.

• Formation: Oceans and atmosphere formed during this process.

• Emissions: Molecular hydrogen ($H$) and helium ($He$) escape to space.

• Oxygenation: Atmosphere oxygenated later by marine algae and plants via photosynthesis, converting carbon dioxide ($CO<em>2$) to oxygen ($O</em>2$).

Evidence for Earth's Compositional Zonation

• Direct Observation: Only the crust and uppermost mantle can be directly observed.

Meteorite Studies

• Initial Composition: Meteorites provide insights into the early Earth’s composition.

• Classes:

  • Metallic Meteorites: 5-10%.

    • Composed of iron-nickel.

    • Density: $9.0-10.0 <br>ewline gm/cm^3$.

  • Chondritic Meteorites: 90-95%.

    • Composed of Fe-Mg silicate (rocky).

    • Density: $3.0 -3.3 <br>ewline gm/cm^3$.

  • Carbonaceous Chondrites: Rare but indicate precursors to life.

Density Properties of Earth

• Average Density: The Earth’s average density is $5.5 <br>ewline gm/cm^3$. Inferred from gravitational effects on orbiting satellites.

• Crust Density: Between $2.6$ and $3.0 <br>ewline gm/cm^3$ (directly measured).

• Uppermost Mantle Density: Between $3.0$ and $3.3 <br>ewline gm/cm^3$ (directly measured).

• Inferences: Density increases in the lower mantle and core consistent with meteorite data.

Earth's Magnetic Field

• Evidence: Suggests a metallic core with a liquid, convecting component around a solid part.

• Principles: Analogous to a simple electromagnet.

Seismic Wave Studies

• Application: Used to determine density and phase composition of Earth’s internal zones.

Seismic Waves

• Types:

  • Body Waves: Propagate through earth materials.

    • Primary Waves (P-waves): Compressional waves; propagate through all phases of matter.

      • Velocity: $6-7 <br>ewline km/sec$ in the lithosphere.

    • Secondary Waves (S-waves): Shear waves; propagate only through solids.

      • Velocity: $3-4 <br>ewline km/sec$.

  • Surface Waves: Restricted to Earth’s surface; cause most earthquake damage.

    • Love Waves.

    • Rayleigh Waves.

• Seismic Wave Velocity: Directly related to the density and elasticity of the material.

• Elasticity: The ability of a material to resist distortion and return to its original form. Only solids behave elastically.

• Refraction: Waves refract due to velocity changes from varying material properties.

• Acceleration: Seismic waves accelerate with increasing rigidity and compressibility. Porosity decreases with depth.

Shadow Zones

• P-wave Shadow Zone: Exists between $105°$ and $140°$ from the epicenter due to refraction at the mantle-outer core boundary (bagel-shaped).

• S-wave Shadow Zone: Exists beyond $105°$ from the epicenter due to refraction at the mantle-outer core boundary and absorption by the liquid outer core. Only one that is large.

Seismic Wave Velocity Changes

• Velocity Changes: Decrease at a depth of $100-350 <br>ewline km$ and at the mantle-core boundary.

• S-wave Absorption: Only absorbed at the mantle-core boundary, indicating a liquid outer core.

• Upper Mantle: The upper mantle is not completely liquid.

Inferred Properties of Earth's Interior

• Crust: Solid.

• Asthenosphere: Plastic; partially melted upper mantle.

• Mantle: Solid, stiffer than the asthenosphere.

• Outer Core: Liquid.

• Inner Core: Solid, composed of iron and nickel.

• Density Increase: Density increases with depth.

• Pressure Increase: Pressure increases with depth.

Lithosphere and Asthenosphere

• Low Velocity Zone: From $100-350 <br>ewline km$ depth in the upper mantle; defines the asthenosphere due to decreasing rigidity.

• Density Decrease: Density decreases in this region of the upper mantle.

• Isostatic Equilibrium: The lithosphere floats on the asthenosphere, maintaining isostatic equilibrium.

• Response to Loads: When a load (like an ice sheet) is placed on the lithosphere, it depresses isostatically. Post-glacial melt leads to isostatic rebound.