DF

Terrestrial Planets: Mercury and the Moon

Introduction to Terrestrial Planets

  • Class: AST101: The Sun and its Neighbours, Professor C. Barth Netterfield

Overview of the Solar System

  • Major terrestrial planets: Mercury, Venus, Earth, Mars

  • Key terminology:

    • Astronomical Unit (AU): Standard unit of measurement in astronomy, reflecting the average distance from the Earth to the Sun.

    • Ceres: Dwarf planet located in the asteroid belt.

Formation of Terrestrial Planets

Molecular Clouds

  • Gravity pulls a molecular cloud (gas and dust) together, collapsing it.

  • Conservation of momentum causes the cloud to spin faster, flattening orbits.

  • Results in a spinning disk of gas and dust.

Planetesimals Formation

  • Tiny objects stick together to form planetesimals, which are the seeds or cores of planets.

Frostline and Material Composition

Inside and Outside the Frostline

  • Frostline: Divides regions where different materials can form planetesimals.

  • Inside the Frostline:

    • 0.2% Metals (Solid)

    • 0.4% Rocks (Solid)

    • 1.4% Hydrogen Compounds (Gas)

    • 98% Helium and Hydrogen (Gas)

  • Outside the Frostline:

    • 0.2% Metals (Solid)

    • 0.4% Rocks (Solid)

    • 1.4% Hydrogen Compounds (Solid Ice)

    • 98% Helium and Hydrogen (Gas)

Characteristics of Terrestrial Planets

  • General Traits:

    • Small size

    • Rocky composition

    • Relatively thin or non-existent atmosphere

    • Few moons

    • Composed primarily of heavy elements (rock and metal)

  • Planets Included: Mercury, Venus, Earth, Mars, and Earth's Moon.

Crater Formation on Terrestrial Bodies

Characteristics of Craters

  • Present in various sizes and overlapping on Mercury and the Moon.

  • Created when a planetesimal strikes the surface at speeds of approximately 100,000 km/hr.

  • The collision vaporizes surface rock, leading to an explosion that forms a crater.

Implications of Crater Density

  • Heavy Bombardment: A time when planetesimals were abundant, causing numerous impacts.

  • Significant decrease in impacts post-heavy bombardment (~4 billion years ago).

Understanding Planet Interiors

Methods to Probe Interior Structures

  • Drilling: Impossible due to depth limitations (max depth ~12 km vs Earth's radius of 6371 km).

  • Seismology: Utilizes earthquake vibrations to understand planetary interiors.

    • P Waves: Compressional waves that can travel through solids and liquids.

    • S Waves: Shear waves that cannot travel through liquids, indicating liquid core presence.

Internal Composition of Planets

  • Layers defined by density: Low-density rock, medium-density rock, high-density iron and nickel core.

  • Differentiation: Process whereby lighter materials rise to the surface and heavier materials sink to the core.

Heat Sources in Planets

Why are Planet Interiors Hot?

  • Accretion: Generated heat during planet formation.

  • Differentiation: Significant in early formation.

  • Radioactive Decay: Currently the primary source of internal heat.

Cooling Processes in Planets

Methods of Cooling

  1. Convection: Hot mantle rock rises, cooler rock sinks, transferring heat from the core.

  2. Conduction: Slow heat transfer through the rigid crust.

  3. Radiation: Infrared light escapes the surface, cooling the planet.

Influences of Planet Size

  • Small planets cool faster due to higher surface area-to-volume ratios.

    • Formula impact: As radius doubles, volume increases by 8 times while surface area increases by only 4 times.

Surface Changes on Moons and Planets

Cratered Surfaces

  • Areas of smooth surfaces (Mares) result from earlier molten lava flooding over cratered surfaces post-heavy bombardment.

  • Both Mercury and Earth's Moon have similar surface histories.

Recent Surface Changes

  • Exploration of other moons (e.g., Enceladus) to assess surface age and activity.

Conclusion

  • Terrestrial planets share similarities in composition and structure, with variations influenced by their history, formation processes, and proximity to the Sun.