Overview
This lecture covers the formation of planets, asteroids, and comets in the early solar system, focusing on the processes and conditions that produced terrestrial and Jovian planets, as well as how planetary migration shaped the solar system's current structure.
The Solar Nebula and Initial Conditions
- The solar system formed from a cold, low-pressure solar nebula composed mostly of hydrogen, helium, and small amounts of heavier elements.
- Dust grains and ice crystals mixed with gas served as building blocks for planets.
- The solar nebula collapsed and flattened into a rotating disk about 200 AU in diameter.
Condensation and the Snow Line
- Substances with high condensation temperatures (rock and metal) solidified in the hot, inner nebula; others remained gaseous.
- The snow line (or frost line) marks the distance from the Sun where temperatures drop enough for water to condense into ice (~170 K).
- Inside the snow line, only rocks and metals could form solids; beyond it, ices were also available, providing more material for planet formation.
Formation of Terrestrial and Jovian Planets
- Terrestrial planets formed inside the snow line from accretion of rock and metal particles, resulting in small, dense planets.
- Jovian planets formed beyond the snow line where abundant ices, metals, and rocks allowed for larger cores that could capture hydrogen and helium gas.
- The core accretion model explains how Jovian planets' large icy-rocky cores attracted thick atmospheres.
Accretion and Differentiation
- Accretion is the process by which dust and ice particles stuck together, first by electrostatic forces and later by gravity, forming planetesimals.
- Collisions and mergers grew planetesimals into protoplanets.
- Heat from impacts and radioactive decay caused melting, leading to chemical differentiation: heavy metals sank to planet cores, lighter materials formed mantles/crusts.
Planetary Migration and Solar System Shaping
- Gravitational interactions caused Jupiter and other Jovian planets to migrate inward and then outward, affecting the masses and positions of other bodies.
- Jupiter’s migration explains Mars’ low mass and the mix of rocky and icy objects in the asteroid belt (Grand Tack model).
- Later migration (Nice model) moved Neptune outward, scattered icy planetesimals to form the Kuiper Belt and Oort Cloud, and triggered the Late Heavy Bombardment.
Solar System Overview and Evolution
- The solar system evolved from a collapsing gas cloud to a disk, forming a protosun and planetary seeds.
- Planet formation sequences and migrations explain current terrestrial and Jovian planet positions and distributions of small body populations.
Key Terms & Definitions
- Solar Nebula — The cloud of gas and dust from which the solar system formed.
- Condensation Temperature — The temperature below which a substance changes from gas to solid.
- Snow Line (Frost Line) — The distance from the Sun where water condenses into ice.
- Accretion — The process of particles sticking together to form larger bodies.
- Planetesimal — A small, solid body from which a planet formed.
- Core Accretion Model — Theory that planet cores form first and then accumulate gas.
- Differentiation — The sinking of heavier elements to a planet’s center during melting.
- Grand Tack Model — The theory describing Jupiter’s inward and outward migration.
- Nice Model — The theory explaining later outward migration of Jovian planets and solar system restructuring.
- Oort Cloud — A distant, spherical shell of icy objects surrounding the solar system.
- Kuiper Belt — A region beyond Neptune populated with small, icy bodies.
- Late Heavy Bombardment — A period of intense impacts in the inner solar system.
Action Items / Next Steps
- Review upcoming lectures on terrestrial and Jovian planet geology and characteristics.
- Study diagrams of solar nebula, condensation curves, and planetary migration for visual reinforcement.