Giant Planets: Hydrogen and Helium

Last: 4. Terrestrial Planets: Rock and Iron Next: 6. Formation of Solar Systems

The outer Solar System contains two planetary behemoths and two planets which are merely enormous by our standards. Jupiter & Saturn have hundreds of times the Earth's mass; both radiate more energy than they receive from the Sun, and this outflowing energy powers dramatic activity in their interiors and atmospheres. Uranus & Neptune, each about 15 times the Earth's mass, are less active. All have satellite systems and rings shaped by subtle dynamical effects over trillions of orbits.



    A Closer Look 7.1: Comparative Data for the Major Worlds   p. 154
  7.1 Jupiter   p. 153-155
  7.1b The Great Red Spot   p. 156
  7.1c Jupiter's Atmosphere   p. 156-157
  7.1d Jupiter's Interior   p. 157
  7.1e Jupiter's Magnetic Field   p. 157
  7.1g Jupiter's Amazing Satellites   p. 158-162
  7.2 Saturn   p. 162-163
  7.2a Saturn's Rings   p. 163-165
  7.2d Saturn's Moon Titan   p. 167-168
    A Closer Look 7.4: Saturn's Rings and Moons from Cassini   p. 169
  7.3 Uranus   p. 170-171
  7.3c Uranus's Interior and Magnetic Field   p. 172-173
  7.4 Neptune   p. 173
  7.4a Neptune's Atmosphere   p. 173-175
  7.4b Neptune Interior and Magnetic Field   p. 175
  7.4d Neptune's Moon Triton   p. 176-178

Overview of the Solar System

The Solar System can be divided into two zones. Bodies in the inner zone are composed mostly of rock and metal, while those in the outer zone consist largely of gas and ice.

The inner solar system   The outer solar system
Inner Solar System: Mercury, Venus, Earth, Mars and asteroids.   Outer Solar System. Left: Jupiter, Saturn, Uranus, Neptune, Pluto, a comet, and the Kuiper belt. Right: The Oort cloud.

Gas Giants & Water Worlds

Jupiter [Views of the Solar System]
  Gibbous Saturn and rings
Saturn [Views of the Solar System]
  Uranus from Voyager 2
Uranus [Views of the Solar System]
  Neptune from Voyager 2
Neptune [Views of the Solar System]

The four planets of the outer solar system come in two general types. Jupiter and Saturn are gas giants: huge spheres of hydrogen and helium with only traces of other elements. Uranus and Neptune are water worlds: somewhat smaller planets with a larger fraction of heavier elements (eg., oxygen, carbon, nitrogen) than their giant neighbors.

Gibbous Jupiter with red spot
Cassini Jupiter Portrait [Views of the Solar System]
Gibbous Saturn and rings
Saturn in Color [Views of the Solar System]
Uranus from Voyager 2
Uranus [Views of the Solar System]
Neptune from Voyager 2
Neptune [Views of the Solar System]

Internal Structure: Pressure Balance

The concept of pressure balance helps us understand the structure of giant planets (and stars), as well as the atmospheres of terrestrial planets such as Earth.

Anywhere in a planet (or star), the pressure is just the weight per unit area of the material above. On Earth at sea level, for example, the weight of a column of air extending to the top of the atmosphere and 1 square inch on a side is 14.7 lbs; thus normal atmospheric pressure is 14.7 psi (psi = `pounds per square inch'). This much pressure is called `one atmosphere' or 1 atm.

A column of water 34 ft high and 1 square inch on a side also weighs 14.7 lbs, so the total pressure 34 ft below sea level is twice what it is at the surface.

  Pressure of air and water

The pressure at the center of Jupiter is about 4×107 atm. How can a gas resist such enormous pressure?

Gas Pressure

The gas in a planet (or star) behaves somewhat like a spring. Consider a column of gas like the one shown here. If there's no gravity the gas is distributed uniformly.

With gravity pulling downward, the gas at the bottom of the column is compressed, while the gas at the top spreads out.

The compressed gas at the bottom of the column pushes upwards, resisting the weight of the gas above it. The further up the column, the less weight above and the less upward push is required.

A spring supporting its own weight does the same thing, compressing more at the bottom and less at the top.

  A column of gas as a spring

If the upward push of the compressed gas exactly balances the downward pull of gravity everywhere, the planet (or star) is in pressure balance.

Internal Structure: Jupiter and Saturn

Interior of Jupiter
The Interior of Jupiter [Views of the Solar System]
  Interior of Saturn
The Interior of Saturn [Views of the Solar System]

Both Jupiter and Saturn have outer envelopes of gas -- mostly molecular hydrogen and helium, along with hydrogen compounds like water, methane, and ammonia. At some depth the pressure becomes so high that hydrogen becomes a metal and a good conductor of electricity. At the center of each planet there's probably a core of rock and ice, about 15 times the mass of the Earth.

Both planets release large amounts of internal heat, powering convection throughout their interiors.

Question 5.1

Both Jupiter and Saturn contain metallic hydrogen. However, most of Jupiter's interior is metallic, while most of Saturn's interior is not. Why?

  1. Jupiter contains a larger fraction of hydrogen than Saturn.
  2. Jupiter is colder than Saturn.
  3. Jupiter's internal pressure is higher because of its greater mass.
  4. Saturn receives less heat from the Sun.

Internal Structure: Uranus and Neptune

Interior of Uranus
The Interior of Uranus [Views of the Solar System]
  Interior of Neptune
The Interior of Neptune [Views of the Solar System]

Uranus and Neptune have outer envelopes of hydrogen, helium, and methane; below are mantles of water in ice or liquid form, along with substantial amounts of methane and ammonia. Their central cores of rock and ice are small, about the mass of the Earth.

Curiously, Neptune has a significant heat flow from its interior, but Uranus does not. The reason for this difference is not known.

Magnetic Fields

Planetary magnetic fields

Further evidence that the outer solar system has two types of planets comes from magnetic fields. Jupiter and Saturn have well-aligned fields generated by convection and rotation in metallic hydrogen deep within their interiors. Uranus and Neptune, in contrast, have off-center fields, suggesting more localized sources in the mantles of these planets.

Satellite Systems

As of last count, Jupiter has 63 satellites, Saturn has 56, Uranus has 27, and Neptune has 13. These numbers will increase as further observations are made. At this point, however, all the large satellites have been discovered; the objects we are now finding are typically only a few kilometers in size.

Satellites divide into two groups. Regular satellites are relatively close to their planets, have circular or nearly circular orbits, and travel in the same direction as their planet spins. Irregular satellites are further from their planets, sometimes have quite elliptical orbits, and may travel in either direction.

Jupiter: The Galilean Satellites

The Galilean Satellites
The Galilean Satellites [NASA/JPL]

In order of distance from Jupiter, the Galilean Satellites are Io, Europa, Ganymede, and Callisto. Europa is slightly smaller than the Moon, while Ganymede is larger than Mercury.

The Outer Satellites of Jupiter
The Jupiter Satellite Page [IfA]
Topography and Volcanoes on Io [NASA/JPL]
Natural and False Color Views of Europa [NASA/JPL]
Ganymede Color Global [NASA/JPL]
Global Callisto in Color [NASA/JPL]

Io: Volcanic Satellite

Io's surface is extremely young, and lava flows cover much of the terrain. There are many active volcanos on Io; this caldera has pools of hot black lava and red deposits of sulfer.   A volcano on Io
Looking Into an Io Volcano [NASA/GSFC]

Europa: Rafts of Ice?

Europa - Ice Rafting View
Europa - Ice Rafting View [NASA/JPL]

Europa's icy crust has fragmented and refrozen many times, creating the complex pattern seen here. Blue areas have been covered with icy dust from a fairly recent impact crater; red areas are due to mineral contaminants.

Surfaces of Europa, Ganymede, and Callisto
Europa, Ganymede, and Callisto: Surface comparison [NASA/JPL]

The almost complete absence of craters on Europa (left) suggests a very young surface; liquid water may have filled in many craters. Ganymede's surface (middle) has many similar features, but the scattered craters implies an older terrain. The large number of craters on Callisto (right) shows that the surface is very old.

Galilean Satellites: Internal Structure

Internal structures of Io, Europa, Ganymede, and Callisto. The first three are differentiated, with metallic cores (grey) and rocky mantles (brown); Callisto, in contrast, is not. Europa and Ganymede have shells of water or ice (blue).

Why are Io and Europa more active than their larger neighbors?

  Interior of Galilean Satellites
Possible Internal Structures of the Galilean Satellites [NASA/JPL]

Resonance. I

This animation shows an example of resonance. The inner satellite makes two orbits in the same time the outer satellite makes one, so this is called a 2:1 resonance.   Animation of a 2:1 resonance

Resonance. II

Because these orbits are in a 2:1 resonance, the inner satellite gets a small tug from its outer neighbor every other time around. As a result, its initially circular orbit becomes elliptical with time.

The orbit will keep getting more and more elliptical, unless some form of friction acts to limit the orbit's ellipticity.

  Resonant excitation of eccentricity

Resonance: Jupiter's Satellites

The three inner satellites of Jupiter are in a 4:2:1 resonance; Io makes 4 orbits in the same time that Europa makes 2 and Ganymede completes 1.

As a result, Io and Europa are forced into elliptical orbits; their distances from Jupiter vary with time. Tides from Jupiter create friction which keeps their orbits roughly circular and heats their interiors.

  Animation of a 4:2:1 resonance

Titan: A Satellite of Saturn

Clouds over Titan
Clouds over Titan [NASA/JPL]
Xanadu on Titan: meandering rivers
Xanadu: Rivers Flowed onto a Sunless Sea Xanadu: Rivers Flowed onto a Sunless Sea [NASA/JPL]

Ring Systems

A HST image of Saturn
Saturn [NASA]

All four planets in the outer solar system have rings of icy or rocky particles traveling in circular orbits. Saturn's rings are evident in even a small telescope; the rings around Jupiter, Uranus and Neptune were not discovered until recently.

Saturn's Rings

Saturn's rings are only about 20 m thick. They orbit exactly over Saturn's equator.   Saturn at different inclinations
Structure in Saturn's rings
Unraveling Saturn's Rings [NASA]

Closeup of the Rings

Up close, the rings are less orderly than they seem from afar; a swarm of small lumps orbiting the planet and slowly grinding each other to dust.

  Artist's impression of rings
Dynamic Ephemeral Bodies

Ring Formation

Rings can form when a small satellite comes too close to a large planet. Within a distance known as the Roche limit - about 3 times the radius of the planet - a small satellite's gravity can't hold it together, and it is torn apart by tidal forces. The debris spread out and eventually form a ring.   Animation of ring formation

Since all four planets in the outer solar system have rings, it seems that tidal disruption of small satellites is pretty common.

Ring Structure: the Role of Resonance

The largest gap in Saturn's rings is known as the Cassini division; under good conditions it is visible in a small telescope. This gap is created by a 2:1 resonance with the satellite Mimas. Many other gaps in the rings are also associated with satellites.   Structure in Saturn's rings
Unraveling Saturn's Rings [NASA]
Last: 4. Terrestrial Planets: Rock and Iron Next: 6. Formation of Solar Systems

Joshua E. Barnes (
Last modified: September 21, 2006
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