7.2 Structure of Planets

The solar system features two primary kinds of planets: rocky terrestrial planets and gas-rich jovian planets.
These distinct compositions indicate formation under different conditions, with varying dominant elements.

Characteristics:
- Primarily composed of hydrogen (75\%) and helium (25\%), similar to the Sun.
- Often called "gas planets," but due to immense size, hydrogen in their interiors is compressed into a liquid state, making them "liquid planets."
- Heavier elements (rock, metal, ice) sink to form dense cores due to gravity, which are inferred through studies of the planets' gravity.
- Jupiter and Saturn: Largest, nearly solar chemical makeup.
- Uranus and Neptune: Smaller than Jupiter and Saturn, with proportionally smaller atmospheres as they were less efficient at attracting hydrogen and helium gas. Still possess cores of rock, metal, and ice.
- Their hydrogen-dominated composition, where nearly all oxygen combines with hydrogen to form water (H_2O), results in a chemical state known as reduced chemistry.

Characteristics:
- Much smaller than the giant planets.
- Composed predominantly of rocks (silicates like silicon and oxygen) and metals (most commonly iron).
- Elements are less common in the universe overall compared to hydrogen and helium.
- Densities: Vary based on metal proportion (Mercury has highest, Moon has lowest).
- Bulk Composition (Earth, Venus, Mars): Approximately one-third iron-nickel or iron-sulfur combinations, two-thirds silicates.
- Their composition, dominated by oxygen compounds (e.g., silicate minerals), is characteristic of oxidized chemistry.
Internal Structure and Differentiation:
- Denser metals form a central core, while lighter silicates are near the surface.
- This layered structure, called differentiation, suggests that these planets were once hot enough to melt (>1300K).
- Gravity causes heavier elements to sink and lighter minerals to float, preserving the layered structure upon cooling.

Characteristics:
- Earth's Moon: Chemically and structurally similar to terrestrial planets.
- Outer Solar System Moons (e.g., Ganymede, Callisto, Titan): Composed of half frozen water and half rocks/metals, similar to giant planet cores.
  - Many differentiated during formation, having rock/metal cores and upper layers/crusts of very cold, hard ice.
- Asteroids and Comets (and smallest moons): Mostly undifferentiated, retaining their original composition from the solar system's formation.
  - Considered "chemical fossils" because they provide insight into the early solar system.
  - Some largest asteroids (e.g., Vesta) show differentiation, or are fragments from differentiated bodies.
  - Many smaller objects are fragments or rubble piles from collisions.

Influence on a Planet's Surface Temperature:
- Distance from the Sun: The primary factor; radiant energy from the Sun weakens with the square of the distance from it.
  - Generally, farther planets are cooler (e.g., Mercury ranges from 280-430^{\circ}C, Pluto is about -220^{\circ}C).
  - Mathematically, temperatures decrease approximately in proportion to the square root of the distance from the Sun (e.g., Pluto is about 10 times colder than Mercury, based on a 100 times greater distance, \sqrt{100} = 10).
- Atmosphere (Greenhouse Effect): Plays a crucial role in trapping heat.
  - Earth: Atmospheric insulation (greenhouse effect) keeps oceans from freezing.
  - Venus: A thick atmosphere of carbon dioxide acts as a strong insulator, leading to surface temperatures hotter than Mercury (>700K).
  - Mars: A very thin atmosphere results in generally below-freezing temperatures.
Habitability: Earth is currently the only planet where surface temperatures typically lie between water's freezing and boiling points, supporting life as we know it.

Surface Modification: The crusts of terrestrial planets, larger moons, and Pluto are modified by both internal and external forces.
- External Forces: Constant bombardment by space projectiles (e.g., comets, asteroids) causes impact craters. This was more intense in the early solar system but continues.
- Internal Forces: On terrestrial planets, this involves buckling/twisting crusts, mountain building, and volcanic eruptions, collectively called geological activity.
Drivers of Geological Activity:
- A hot interior is essential. Volcanism and mountain building are driven by heat escaping from within.
- Primordial Heat: Planets were initially heated at birth, powering early volcanic activity.
- Planet Size: The larger a planet or moon, the longer it retains its internal heat.
  - Larger objects (Earth, Venus) have molten interiors and continued activity for billions of years.
  - Intermediate objects (Mars) cooled more slowly than smaller ones, showing activity longer than the Moon but less than Earth/Venus.
  - Small objects (Moon, Mercury) cool off quickly and become geologically "dead" relatively early in their history (billions of years ago).