14.5 Planetary Evolution

Planetary Geological Activity and Atmospheres

Stages of Terrestrial Planet Evolution

Rocky planets don't just form and sit there. They go through a series of stages that transform them from hot, undifferentiated masses into the diverse worlds we observe today. These stages overlap and interact, but here's the general sequence:

Differentiation happens early, while the planet is still molten or partially molten. Denser materials (iron and nickel) sink toward the center under gravity, while lighter silicates rise toward the surface. This creates the layered structure you see in all terrestrial planets: a metallic core, a silicate mantle, and a thin crust made of basalts and granites.

Heating drives everything that follows. Two main sources fuel a planet's internal heat:

• Accretional heating from the gravitational energy released when material slammed together during formation
• Radioactive decay of elements like uranium, thorium, and potassium, which continues generating heat long after formation ends

This internal heat is what powers volcanism and mantle convection.

Volcanism occurs when mantle material melts and erupts onto the surface. This builds features like shield volcanoes (Olympus Mons on Mars) and vast lava plains. Volcanism also drives outgassing, which releases gases trapped in the interior, including carbon dioxide, water vapor, and nitrogen. These gases form the planet's early atmosphere.

Tectonics results from convection currents circulating in the mantle. On Earth, this produces plate tectonics, where crustal plates move, collide, and subduct beneath one another. That process builds mountain ranges (the Himalayas), opens rift valleys, and recycles crust back into the mantle. Earth is the only planet in our solar system with active plate tectonics.

Crater formation comes from asteroid and comet impacts, which leave circular depressions often with raised rims and central peaks. Impact bombardment was especially intense during the first ~700 million years of solar system history (the Late Heavy Bombardment). Heavily cratered surfaces, like those on Mercury and the Moon, indicate a world where geological activity has largely stopped, since there are no resurfacing processes to erase the craters.

Erosion and weathering gradually wear down surface features through wind (aeolian erosion), water (fluvial erosion), and chemical reactions. This stage requires an atmosphere, and ideally liquid water, to be significant. On Earth, erosion has carved river valleys, canyons like the Grand Canyon, and rounded ancient mountain ranges over millions of years.

Factors in Planetary Surface Elevation

Why does Mars have a volcano 22 km tall while Earth's tallest mountains top out around 9 km? Several factors control how extreme a planet's topography can get.

• Planetary mass and gravity: Higher gravity compresses surface features, producing a smoother overall surface (Earth, Venus). Lower gravity allows taller mountains relative to the planet's size, which is why Mars can support Olympus Mons despite being a much smaller world.
• Geological activity: Active tectonics create dramatic elevation differences. Earth's plate collisions build mountain ranges like the Andes, while rifting opens low valleys like the East African Rift. Planets without recent tectonic activity, like Mars, have more static topography shaped by ancient processes.
• Volcanism: Volcanic eruptions build up elevation over time. Shield volcanoes like Mauna Loa on Earth and the massive Tharsis volcanic region on Mars are prime examples. On Mars, the lack of plate tectonics means a volcano can sit over a hotspot for billions of years, growing far larger than anything on Earth.
• Impact cratering: Large impacts excavate deep basins (Hellas Basin on Mars is about 7 km deep) and raise crater rims. Smaller impacts add surface roughness, as seen across the lunar highlands.
• Erosion and sediment deposition: Weathering breaks down high elevations over time (the Appalachian Mountains were once as tall as the Himalayas). Sediment deposition fills low-lying areas, like the Mississippi River Delta, gradually flattening the landscape.

Atmospheric Development of Rocky Planets

Earth, Venus, and Mars all started with similar raw materials, yet they ended up with wildly different atmospheres. Understanding why comes down to four key factors.

Initial composition: All three planets likely captured thin hydrogen-helium envelopes from the solar nebula early on, but these were quickly lost. The more lasting atmospheres came from volcanic outgassing, which released carbon dioxide, water vapor, and nitrogen from the planets' interiors.

Planetary mass and gravity: Earth and Venus are massive enough to hold onto heavier atmospheric gases through gravitational attraction. Mars, with only about 11% of Earth's mass, couldn't retain its atmosphere nearly as well. Today, Mars's atmospheric pressure is just 0.6% of Earth's.

Distance from the Sun: A planet's distance affects how much solar energy heats its atmosphere, which in turn affects how fast gas molecules move and whether they can escape to space. Venus's proximity to the Sun contributed to a runaway greenhouse effect, pushing surface temperatures to around 460°C.

Magnetic field: Earth's active core dynamo generates a strong magnetic field that deflects the solar wind, shielding the atmosphere from being stripped away. Mars lost its global magnetic field early in its history, and Venus also lacks a strong one. Without that protection, the solar wind gradually erodes a planet's atmosphere over billions of years.

These factors combined to produce three very different outcomes:

1. Earth: Photosynthetic cyanobacteria transformed the atmosphere by producing oxygen, while carbon dioxide was sequestered into carbonate rocks and the ocean. The result is a nitrogen-oxygen atmosphere that supports complex life.
2. Venus: A runaway greenhouse effect trapped heat, evaporated surface water, and left behind a dense atmosphere of carbon dioxide with clouds of sulfuric acid. Surface pressure is about 90 times Earth's.
3. Mars: Low mass and the absence of a magnetic field allowed the solar wind to strip away most of the atmosphere. What remains is a thin, carbon dioxide-rich atmosphere far too thin to support liquid water on the surface today.

Planetary Dynamics and Internal Processes

A few additional concepts tie together the evolution described above:

• Planetary migration: Planets don't always stay where they formed. Gravitational interactions with other planets and the protoplanetary disk can shift orbits inward or outward over time. This migration affects which materials a planet encounters and can reshape the architecture of an entire solar system.
• Mantle convection: Hot material deep in the mantle rises, cools near the surface, and sinks back down. This circulation is the engine behind plate tectonics on Earth and drives volcanic activity on other terrestrial worlds.
• Planetary magnetic fields: Generated by the motion of electrically conducting liquid (usually molten iron) in a planet's outer core. This process is called a dynamo. The resulting magnetic field protects the atmosphere from solar wind erosion, making it a critical factor in long-term atmospheric retention.
• Atmospheric escape: Gas molecules at the top of the atmosphere can reach escape velocity and be lost to space. The rate of escape depends on planetary mass (stronger gravity holds gas better), solar radiation intensity (more energy means faster-moving molecules), and magnetic field strength (shields against solar wind stripping).