Terrestrial Planet Formation and Geological Evolution

Inside the Terrestrial Worlds

After formation, molten planets differentiate into three distinct zones:

• Core: Made of metals.
• Mantle: Made of dense rock.
• Crust: Made of less dense rock.
• Lithosphere: The rigid, outer layer consisting of the crust and part of the mantle that does not deform easily.

Cooling the Terrestrial Worlds

Planets cool through three primary mechanisms:

• Conduction: Heat flowing on the microscopic level.
• Convection: Heat flowing on the macroscopic level (bulk motions).
• Radiation: Energy transferred by photons.

Note: The larger the planet, the longer it takes to cool off.

Magnetic Fields

Three ingredients are required for a planetary-scale magnetic field:

• Electrical conductor
• Convection (warm interior)
• Rotation

Shaping Planetary Surfaces

Major geological processes that shape planetary surfaces include:

• Impact cratering: Excavation of the surface by asteroids or comets.
• Volcanism: Eruption of lava from the interior.
• Tectonics: Disruption of the lithosphere by internal stresses.
• Erosion: Wearing down by wind, water, or ice.

Impact Cratering

• Objects hit the planet at 10–70 km/s.
• Solid rock is vaporized.
• A crater is excavated.
• Matter is ejected in all directions.
• Craters are circular; large craters often feature a central peak.

Counting Craters to Determine Surface Age

The cratering rate has decreased as the Solar System has aged. Consequently, the older the surface, the more craters are present.

Volcanism

Underground molten rock, called magma, breaks through cracks in the lithosphere, sometimes gently and sometimes violently. This process releases trapped gases, known as volcanic outgassing, including H₂O, CO₂, and N₂.

Tectonics

Convection cells in the mantle cause:

• Compression in the lithosphere: Produces mountains.
• Extension in the lithosphere: Produces valleys.
• Plate Tectonics: Specifically observed on Earth.

Erosion

The movement of rock by ice, liquid, or gas:

• Valleys shaped by glaciers.
• Canyons carved by rivers.
• Sand blown by wind.

How Planetary Properties Affect Each Process

• Impact cratering: The number of impacts is similar for all planets, but larger planets erase more craters.
• Volcanism & tectonics: Requires interior heat, which is retained longer by large planets.
• Erosion: Requires an atmosphere, which depends on large size for volcanic outgassing, moderate distance from the Sun, and fast rotation for wind.

Effects of an Atmosphere on a Planet

• Greenhouse effect: Makes the planetary surface warmer than it would be otherwise.
• Scattering and absorption of light: Absorbs high-energy radiation from the Sun; scattering of optical light brightens the daytime sky.
• Pressure: Can allow water to exist as a liquid at the right temperature.
• Wind and weather: Promotes erosion of the planetary surface.
• Auroras: Interaction with the solar wind when magnetic fields are present.