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.