Overview
This lecture introduces planetary geology, focusing on the differences among the terrestrial planets (Mercury, Venus, Earth, Moon, Mars), the processes shaping their surfaces, and the fundamental internal properties that drive geological activity.
The Terrestrial Worlds: Surface and Internal Differences
- Earth, Mercury, Venus, the Moon, and Mars began with similar rocky compositions but evolved very different surfaces.
- Major surface features include craters (Moon, Mercury), volcanic plains (Venus, Mars), and Earth's biosphere.
- All terrestrial planets have layered interiors: dense metallic core, rocky mantle, and outer crust.
- Differentiation (separation by density) produced these layers when planets were hot and molten.
- Mercury’s large core and Moon’s small core suggest violent past impacts.
Interior Structure and Geological Activity
- The lithosphere is the planet’s rigid outer layer; its thickness increases as planetary size decreases.
- Thin lithospheres (Earth, Venus) enable more geological activity; thick lithospheres (Moon, Mercury, Mars) inhibit it.
- Large planets become spheres as gravity pulls material into rounded shapes.
Sources and Loss of Internal Heat
- Three heating processes: accretion (impact energy), differentiation (frictional heat), and radioactive decay (element breakdown).
- Three cooling processes: convection (heat transfer via rising/falling material), conduction (heat flow by contact), and radiation (heat lost to space, mainly infrared).
- Larger planets retain internal heat longer due to greater volume-to-surface area ratio.
Magnetic Fields and Planetary Protection
- A strong global magnetic field requires: a conducting liquid core, convection, and rapid rotation.
- Earth is the only terrestrial planet with all three, protecting its atmosphere from solar wind.
- Mercury’s magnetic field is unusual; possibly from its large, partially molten core.
Four Major Geological Processes
- Impact Cratering: bowl-shaped craters from collisions; small craters are more common than large ones.
- Volcanism: molten rock (magma/lava) erupts, forming volcanic plains, shield volcanoes, and stratovolcanoes.
- Tectonics: surface deformation from internal stresses, mantle convection, and, on Earth, plate tectonics.
- Erosion: breakdown/movement of rock by wind, water, and ice, shaping and depositing new features.
Determining Surface Age and Geological Activity
- Heavily cratered surfaces are old and geologically inactive; few craters indicate recent geological resurfacing.
- Cratering rates help estimate the age and activity history of planetary surfaces.
- Moon studies show lunar highlands are older (~4.4 billion years) than volcanic maria regions.
Fundamental Factors Affecting a Planet's Geology
- Size controls internal heat retention and geological activity.
- Distance from the Sun impacts surface temperature, atmospheric retention, and the possibility of liquid water or ice.
- Rotation rate influences wind, weather, and the potential for a magnetic field.
Key Terms & Definitions
- Lithosphere — the rigid outer layer of a planet, including the crust and part of the mantle.
- Differentiation — process where denser materials sink and lighter materials rise, forming layers.
- Convection — heat transfer by movement of fluid materials (liquid or gas).
- Outgassing — release of gases from a planet’s interior, often via volcanism.
- Impact Cratering — creation of surface craters by collisions with space objects.
- Tectonics — deformation of the lithosphere due to internal forces.
- Erosion — wearing down or moving surface material by wind, water, or ice.
- Magnetosphere — region around a planet controlled by its magnetic field.
Action Items / Next Steps
- Review diagrams of layered planet interiors and compare terrestrial planets.
- Prepare for detailed study of geology and atmospheres on individual terrestrial worlds in future lectures.
- Optional: Re-read notes on the four geological processes and their examples.