4.1 Internal structure and composition of terrestrial planets

Terrestrial planets have layered interiors with crust, mantle, and core. These structures formed through differentiation, where denser materials sank to the center. Understanding these layers helps explain planetary features and processes.

Each planet's internal structure varies due to size, composition, and formation history. Earth has active plate tectonics and a strong magnetic field, while Mars lacks both. These differences shape surface features and geological activity.

Internal Layers of Terrestrial Planets

Composition and Structure

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• Terrestrial planets have a layered internal structure consisting of a crust, mantle, and core, which formed through the process of planetary differentiation
• The crust is the thin, solid, outermost layer composed primarily of silicate rocks
  • It is the coolest and least dense layer of a terrestrial planet
• The mantle is the thick, solid layer beneath the crust, composed of denser silicate rocks
  • It is hotter than the crust and can experience convection currents
• The core is the innermost layer of a terrestrial planet, composed primarily of iron and nickel
  • It is the densest and hottest layer

Variations Among Terrestrial Planets

• The thickness and composition of each layer can vary among terrestrial planets due to differences in their formation history, size, and distance from the Sun
• The core can be further divided into a liquid outer core and a solid inner core, depending on the planet's size and thermal history
  • Earth has a distinct liquid outer core and solid inner core
  • Mars has a proportionally smaller core and lacks a liquid outer core
• Variations in internal structure influence surface features and processes
  • Active plate tectonics on Earth due to convection in the mantle
  • Lack of plate tectonics on Mars and Venus due to differences in mantle dynamics

Terrestrial Planet Structures: A Comparison

Earth

• Earth has a distinct crust, mantle, liquid outer core, and solid inner core
• Active plate tectonics driven by convection in the mantle
• Strong magnetic field generated by convection in the liquid outer core

Venus

• Venus has a similar internal structure to Earth but lacks plate tectonics
• Weaker magnetic field, possibly due to slower rotation and lack of mantle convection
• High surface temperatures and thick atmosphere due to greenhouse effect

Mars

• Mars has a thinner crust, a mantle, and a proportionally smaller core than Earth
• Lacks active plate tectonics and has no global magnetic field, suggesting a solid core
• Evidence of past volcanic and tectonic activity, but currently geologically less active

Mercury and the Moon

• Mercury has a very thin crust, a mantle, and a disproportionately large core relative to its size
  • Likely due to its proximity to the Sun during formation and the loss of lighter elements
• The Moon has a crust, mantle, and a small, partially molten core
  • Its internal structure is less differentiated compared to larger terrestrial planets
• Both Mercury and the Moon lack plate tectonics and have heavily cratered surfaces

Determining Internal Composition

Seismic Waves

• Seismic waves generated by earthquakes or impacts provide information about the internal structure and composition of a planet
• Velocity and behavior of seismic waves change as they travel through different layers
  • P-waves (primary waves) travel through solids, liquids, and gases
  • S-waves (secondary waves) only travel through solids
• Analysis of seismic wave propagation helps map the boundaries between layers

Gravity and Magnetic Field Measurements

• Gravity measurements can reveal variations in the density of a planet's interior
  • Higher density regions exert a stronger gravitational pull
• Magnetic field measurements indicate the presence and characteristics of a planet's core
  • Magnetic fields are typically generated by convection in a liquid outer core
• Combining gravity and magnetic data helps constrain internal structure and composition

Meteorites and Numerical Models

• Meteorites, particularly those originating from Mars and the Moon, provide direct samples of the composition of these bodies
  • Analysis of meteorites helps infer the internal structure and composition
• Numerical models and experiments simulating planetary interiors under high pressure and temperature conditions
  • Models help predict the behavior of materials and constrain possible internal structures
• Combining meteorite data with numerical models improves understanding of planetary interiors

Differentiation and Internal Structure

The Process of Differentiation

• Planetary differentiation is the process by which a planet's interior separates into distinct layers based on density
  • Denser materials (iron, nickel) sink towards the center to form the core
  • Lighter materials (silicates) rise towards the surface to form the mantle and crust
• Differentiation occurs during the early stages of a planet's formation
  • Driven by heating from accretion, radioactive decay, and gravitational compression
• As the planet heats up, materials begin to melt and separate based on their density

Factors Influencing Differentiation

• The extent of differentiation depends on factors such as the planet's size, composition, and thermal history
• Larger planets generally experience more extensive differentiation
  • Greater heat retention and higher internal pressures facilitate melting and separation
• Composition of the planet determines the types of materials that differentiate
  • Terrestrial planets are primarily composed of silicates and metals
• Thermal history, including the rate of cooling and heat sources, affects the duration and extent of differentiation

Implications of Differentiation

• Differentiation plays a crucial role in establishing the distinct layers of a terrestrial planet's interior
• Determines the distribution of elements and minerals within the planet
  • Concentration of dense elements (iron, nickel) in the core
  • Enrichment of lighter elements (silicon, oxygen) in the mantle and crust
• Influences the development of other planetary features
  • Volcanism and tectonics are linked to mantle dynamics and heat transfer
  • Magnetic fields are generated by convection in the liquid outer core
• Understanding differentiation helps explain the observed variations in the internal structures of terrestrial planets