11.2 Giant Planets

1. Introduction to Jovian Planets

The four giant (or jovian) planets are Jupiter, Saturn, Uranus, and Neptune.
They are compared based on similarities and differences, relating properties to their differing masses and distances from the Sun.

2. Basic Physical Characteristics and Rotation

Distance and Orbital Period:
- Jupiter: 5.2 AU from the Sun, takes 11.9 years to orbit.
- Saturn: 9.5 AU from the Sun, takes 29.5 years to orbit.
- Uranus: 19.2 AU from the Sun, takes 84.1 years to orbit.
- Neptune: 30.0 AU from the Sun, takes 164.8 years to orbit.
Physical Properties:
- Jupiter:
  - Diameter: $142,800$ km.
  - Mass: $318$ times Earth's mass.
  - Average Density: $1.3$ g/cm $^3$ (lower than terrestrial planets).
  - Volume: Can fit ~ $1,300$ Earths.
- Saturn:
  - Diameter: $120,540$ km.
  - Mass: $95$ times Earth's mass.
  - Average Density: $0.7$ g/cm $^3$ (lowest of any planet, would float in water).
- Uranus:
  - Diameter: $51,200$ km.
  - Mass: $14$ times Earth's mass.
  - Average Density: $1.3$ g/cm $^3$ . This higher density indicates a different composition than gas giants like Jupiter and Saturn.
- Neptune:
  - Diameter: $49,500$ km.
  - Mass: $17$ times Earth's mass.
  - Average Density: $1.6$ g/cm $^3$ . Also indicating a different composition.
Rotation Rates (of the interior, determined by magnetic fields):
- Jupiter: 9 hours 56 minutes (shortest day of any planet).
- Saturn: 10 hours 40 minutes.
- Uranus: ~17 hours.
- Neptune: ~16.1 hours.
Planets the size of Uranus and Neptune (ice giants) are common as exoplanets.

3. General Appearance and Seasons

Visible Atmospheres: We only see the uppermost clouds, primarily composed of hydrogen and helium gas.
- Jupiter & Saturn: Uppermost clouds are composed of ammonia crystals.
- Neptune: Upper clouds are made of methane.
- Uranus: No obvious cloud layer, only a deep and featureless haze; appears blue-green due to methane absorbing red light.
Jupiter's Appearance: Colorful and dynamic, with distinct cloud patterns.
**Seasons on Giant Planets (Spin Axis Tilt): ** * Jupiter: Spin axis tilted by only $3^ ext{o}$ , resulting in virtually no seasons.
- Saturn: Spin axis inclined at $27^ ext{o}$ , experiences seasons.
- Neptune: Spin axis inclined at $29^ ext{o}$ , experiences similar but slower seasons than Saturn.
- Uranus: Most unusual spin axis tilt of $98^ ext{o}$ . It orbits on its side, creating dramatic seasons.
  - When Voyager 2 arrived, its south pole faced the Sun, experiencing a 21-year sunlit summer, while the northern hemisphere was in darkness.
  - Over its 84-year orbit, different poles and equator receive varying sunlight. A platform at the south pole would experience 42 years of light and 42 years of darkness.
  - This extreme tilt might be due to a collision with a large planetary body early in the solar system's formation.

4. Composition and Structure of Jupiter, Saturn, Uranus, and Neptune

Overall Composition:
- Jupiter and Saturn: Primarily hydrogen and helium. Calculations confirm these light gases are consistent with their observed masses and densities.
- Uranus and Neptune: Heavier element cores dominate. Composed largely of compounds of carbon, nitrogen, and oxygen (referred to as "ices"), and silicates/iron ("rock").
**Internal Structures (from center outward): ** * All Four Planets (common feature): Have a core composed of "rock and ice" (iron, silicon, oxygen for rock; carbon, nitrogen, oxygen with hydrogen for ice). * These materials are under immense pressure and temperature, existing in exotic forms. * Cores formed as original rock-and-ice bodies before gas capture.
- **Jupiter & Saturn (Gas Giants): ** * Core: Similar rock-and-ice cores, but constitute only a few percent of total mass.
  - Liquid Metallic Hydrogen Layer: Above the core, hydrogen becomes liquid metallic due to extreme pressure (>100 million bars in Jupiter's center).
    - In this state, electrons are not bound to nuclei, making it a good electrical conductor.
    - Jupiter has a large volume of liquid metallic hydrogen; Saturn has a smaller volume due to less mass.
  - Liquid Hydrogen Layer: Above the metallic layer, hydrogen is liquid but not metallic.
  - Gaseous Hydrogen/Helium Atmosphere: Outermost layer, gradually transitioning from liquid.
- **Uranus & Neptune (Ice Giants): ** * Core: Rock-and-ice cores make up most of their mass (unlike Jupiter/Saturn).
  - Icy Mantle: Above a small rocky core, they have a thick layer of a liquid mixture of water, methane, and ammonia (the "ice" materials).
  - These planets are too small to achieve pressures necessary to liquefy hydrogen into its metallic state.
  - They were unable to attract massive quantities of hydrogen and helium during formation, hence their different composition.

5. Internal Heat Sources of the Giant Planets

Sources of Heat:
- Primordial Heat: Heat retained from their formation due to the collapse of surrounding material onto their cores. Larger planets like Jupiter retained more.
- Slow Contraction: Giant gaseous planets can generate heat by slowly shrinking, releasing gravitational energy.
- Differentiation (Helium Rain): In Saturn, helium separates from hydrogen and sinks, releasing gravitational energy.
**Comparison of Heat Sources: ** * Jupiter: * Largest internal energy source, $4 imes 10^{17}$ watts (equivalent to 4 million billion 100-watt lightbulbs). * This energy is comparable to the solar energy it absorbs. * Most of its internal heat is primordial, from its formation 4.5 billion years ago. * Its atmosphere is heated by both the Sun and its powerful internal source.
- Saturn:
  - Internal energy source about half that of Jupiter ( $2 imes 10^{17}$ watts).
  - Produces twice as much energy per kilogram as Jupiter (due to its smaller mass).
  - Primordial heat is less significant; its primary heat source is the separation of helium from hydrogen (helium rain) in its interior, which sinks toward the core and releases gravitational energy.
- Neptune: Has a small internal energy source.
- Uranus: Does not emit a measurable amount of internal heat.
- Atmospheric Temperature: Despite Neptune's greater distance, Uranus and Neptune have almost the same atmospheric temperature due to Neptune's internal heat source and Uranus's lack thereof.
- The difference in internal heat sources for Uranus and Neptune is not well understood.

6. Discovery and Characteristics of the Giant Planets’ Magnetic Fields

Generation Mechanism: Strong magnetic fields are generated by electric currents within their rapidly spinning interiors, specifically in layers of electrically conducting fluid (e.g., liquid metallic hydrogen in Jupiter/Saturn, ionized water/ammonia mantle in Uranus/Neptune).
Magnetospheres: Regions around a planet where its own magnetic field dominates over the interplanetary magnetic field. These are the largest features of the giant planets, extending millions of kilometers into space.
Discovery of Jupiter's Magnetic Field:
- Discovered in the late 1950s through observations of radio waves.
- Jupiter emitted radio waves that were more intense at longer wavelengths (opposite of thermal radiation).
- This behavior is characteristic of synchrotron radiation, produced when high-speed electrons are accelerated by a magnetic field.
- This indicated a strong magnetic field and vast numbers of charged atomic particles spiraling around magnetic field lines.
- Similar to Earth's Van Allen belts, but on a larger scale.
**Characteristics of Magnetic Fields: ** * Jupiter: Has the strongest magnetic field. Its axis is tipped by about $10^ ext{o}$ relative to its rotation axis.
- Saturn: Magnetic field operates similarly but is not as strong. Uniquely, its field is perfectly aligned with its rotation axis.
- Uranus & Neptune: Magnetic fields discovered by spacecraft (Voyager 2) are not as strong as Jupiter's.
  - They have even greater magnetic tilts: Uranus at $60^ ext{o}$ and Neptune at $55^ ext{o}$ , relative to their rotation axes.
Significance: Studying the magnetospheres of giant planets and Earth provides nearby analogs for more energetic and challenging cosmic processes observed in distant objects like remnants of dead stars and quasars.
Sources of Charged Particles: Can come from the Sun or the planet's neighborhood (e.g., Io, one of Jupiter's moons, blasts charged particles into Jupiter's magnetosphere via volcanic eruptions).
The reasons for the distinct magnetic tilts among the giant planets are not fully understood.