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Giant Planets. Jupiter, Saturn, Uranus, Neptune. Size Comparison. Jupiter: 318 Earth-masses, Saturn: 95, Uranus: 14.5, Neptune: 17.2. Two subclasses: Jupiter-Saturn and Uranus-Neptune. Terrestrial planets low mass high density slow rotators (  24 hours) few satellites

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Giant Planets

Jupiter, Saturn, Uranus, Neptune


Size Comparison

Jupiter: 318 Earth-masses, Saturn: 95, Uranus: 14.5, Neptune: 17.2

Two subclasses: Jupiter-Saturn and Uranus-Neptune


Terrestrial planets

    • low mass
    • high density
    • slow rotators ( 24 hours)
    • few satellites
    • close to Sun ( 1.6 AU)
    • Thin atmospheres
    • Weak or no magnetic field
  • Giant planets
    • high mass
    • low density
    • rapid rotators (18 hours)
    • many satellites
    • far from Sun ( 5 AU)
    • Thick atmospheres
    • Strong magnetic field

Jupiter, Saturn, Uranus, and Neptune have all been visited by the Voyager space probes.

    • Galileo was crashed into Jupiter
    • Cassini is now at Saturn
  • Jupiter, Saturn, Uranus, and Neptune are all massive bodies
    • formed in outer part of pre-solar nebula where ices condense
    • growth by accretion and coalescence
  • Giant planets are gaseous/fluid bodies
    • supported by balance between pressure and gravity: Hydrostatic equilibrium

Solar Nebula Composition

98% of the nebula was in the form of gaseous H2 and He.

2% consisted of H2O, CH4, and NH3, (ices), and even smaller amounts of rocks and metals (olivines, pyroxenes, iron, nickel, etc.).

Jupiter is closest to “solar” composition

Saturn less H2 and He

Uranus and Neptune mostly ices



The physical size of a planet depends on both its mass and its chemical composition.


Atmospheres of Jovian Planets

  • Jupiter and Saturn
        • For both planets, methane and frozen ammonia (NH3) crystals are common.
        • For Saturn, the NH3 extends over a greater depth and is harder to see through, giving the planet a uniformly hazy appearance
  • Uranus and Neptune
    • For both planets only methane (CH4) in  atmospheres; NH3 completely frozen out.
    • Uranus and Neptune have a greenish-blue appearance because methane absorbs red light.
  • Rapid differential rotation of giant planets stretches clouds into bands.


  • Jupiter -- reds and browns (ammonia, sulfur compounds, methane)
  • Saturn -- reds and yellows (ammonia, sulfur compounds, methane)
  • Uranus -- blues and greens (mostly from methane gas)
  • Neptune -- blue (from methane gas)

Giant Planet Magnetic Fields

  • At very high pressures inside Jupiter and Saturn hydrogen begins to act like a liquid metal This provides an electrically conducting fluid in which a magnetic field is generated.
  • In Uranus and Neptune the magnetic fields are generated by convection of water, ammonia and methane.
  • Jupiter and Saturn have very strong magnetic fields which are closely aligned with the planet's spin axis
  • The magnetic fields of Uranus and Neptune are weaker, irregular and highly tilted with respect to the planet's rotation axis.

Jupiter’s Great Red Spot

The Great Red Spot is a huge storm measuring 12,000 by 25,000 km (7,500 by 15,500 miles), which is big enough to hold two Earths side by side. While Jupiter's cloud patterns can change within hours or days like on Earth, the Spot has lasted for over 300 years.


Jupiter’s Internal Sources of Energy

Jupiter radiates 1.6 times a much energy as falls on it from the Sun. Thus, Jupiter has an internal heat source.

It is thought that much of this heat is residual heat left over from the original collapse of the primordial nebula to form the Solar System, but some may come from slow contraction.

This internal heat source is presumably responsible for driving the complex weather pattern in its atmosphere, unlike the Earth where the primary heat source driving the weather is the Sun.




Spacecraft Encounters:

Pioneer 11 (1979); Voyager 1 & 2 (1980, 1981)Cassini - Huygens (2004)

Mean Distance from the Sun:

9.539 AU

Length of Year:

29.46 Earth years

Rotation Period:

10.66 hours

Mean Orbital Velocity:

9.64 km/s (6 mi/s)

Inclination of Axis:

26.73 degrees


120,536 km (74,901 mi)

Number of Observed Satellites:


Comparisons With Earth:


9.4 X Earth's

Average Distance from the Sun:

9.5 X Earth's


95 X Earth's


0.13 X Earth's

Saturn Facts

Now 34


Saturn’s High-Velocity Winds

There are extremely high velocity winds in the atmosphere of Saturn. Unlike the case for Jupiter, the variations in wind speeds are not strongly correlated with the positions of the belts and bands. The wind speeds in the atmosphere of Saturn have been measured to be as high as 1800 km/hr, which is about 4 times the highest speeds in the atmosphere of Jupiter.

Saturn’s Surface

The surface of Saturn bears many similarities with the surface of Jupiter, but the color contrast is generally less. This is thought to be due to Saturn being colder than Jupiter (further from the Sun, but also smaller with less internal heat), so it has different chemical reactions in its atmosphere, leading to different coloration.

There are large anticyclonic cells on the surface, apparently driven by the planet's internal heat source, but none are as large as the Great Red Spot on Jupiter, and they are not as abundant as on Jupiter.



1997 launch – 2004 Saturn arrival

The Cassini Orbiter's mission consists of delivering a probe (called Huygens, provided by ESA) to Titan, and then remaining in orbit around Saturn for detailed studies of the planet and its rings and satellites.


JPL technicians reposition and level the Cassini orbiter in the Payload Hazardous Servicing Facility at the Kennedy Space Center in July 1997, after stacking the craft's upper equipment module on the propulsion module.



at Saturn

The international Cassini-Huygens mission successfully entered orbit around Saturn at 9:12 p.m. PDT on June 30, 2004. This begins a four-year study of the giant planet, its majestic rings and 34 known moons.


Cassini will make 74 unique orbits around the planet, using close flybys of Saturn's largest moon Titan for gravity assists and science data acquistion. Because of the size of Titan, the flybys will allow for major changes in orbital paths, allowing engineers to minimize fuel use while maximizing science data collection.

  • Highlights of the Saturn Tour
  • 74 Orbits of Saturn
  • 44 Close flybys of Titan
  • 8 close "targeted" flybys of other satellites:
    • 3 close flybys of Enceladus
    • Phoebe
    • Hyperion
    • Dione
    • Rhea
    • Iapetus
  • 30 additional satellite flybys at distances less than 100,000 kilometers(about 62,100 miles)
  • Many Saturn and Ring occultation opportunities
  • One "Titan 180 degree" transfer
  • One high inclination sequence


This false-colour composite was created with images taken during the NASA/ESA/ASI Cassini spacecraft's closest fly-by of Titan on 16 April 2005.



This mosaic of three frames provides unprecedented detail of the high ridge area including the flow down into a major river channel from different sources.


Saturn’s Rings

Nine days before it entered orbit, Cassini spacecraft captured this exquisite natural color view of Saturn's rings.

The brightest part of the rings, curving from the upper right to the lower left in the image, is the B ring.

Saturn's rings are made primarily of water ice. Since pure water ice is white, it is believed that different colors in the rings reflect different amounts of contamination by other materials such as rock or carbon compounds.


Saturn’s South Pole

These images of Saturn's south pole, taken by two different instruments on Cassini, show the hurricane-like storm swirling there and features in the clouds at various depths surrounding the pole. Different wavelengths reveal the height of the clouds, which span tens of kilometers in altitude.


Dione, Tethys and Pandora

A 330-kilometer-wide (205 mile) impact basin can be seen near the bottom right on Dione (at left). Ithaca Chasma and the region imaged during the Cassini spacecraft's Sept. 24, 2005, flyby can be seen on Tethys (middle).

Tethys is on the far side of the rings in this view; Dione and Pandora are much nearer to the Cassini spacecraft. Dione is 1,126 kilometers (700 miles) across. Tethys is 1,071 kilometers (665 miles) across and Pandora is 84 kilometers (52 miles) across.



Saturn's moon Rhea is an alien ice world, but its cratered surface looks in some ways similar to our own Moon, or the planet Mercury. Rhea's icy exterior would quickly melt if this moon were brought as close to the Sun as Mercury. Rhea is 1,528 kilometers (949 miles) across.



Saturn's moon Enceladus is only 505 kilometers (314 miles) across, small enough to fit within the length of the United Kingdom, as illustrated here. The intriguing icy moon also could fit comfortably within the states of Arizona or Colorado.



Uranus is the 3rd of the Gas Giant planets, and the first planet discovered in "modern" times (1781).

It is barely visible from the Earth without a telescope, which explains why it was not known as a planet to the ancients, and why it had been observed various times after the telescope had been invented without the observers realizing that it was a planet and not a star.

Documented sightings go back to at least 1690 when Flamsteed catalogued it as a star.



Sir William Hershel (1781)

Uranus Facts

Spacecraft Encounter(s):

Voyager 2 (1986)

Mean distance from Sun:

19.19 AU (2.871 billion km/1.784 billion mi)

Length of year:

84.01 Earth years

Rotation period:

17.24 hours

Mean orbital velocity:

6.81 km/s (4.2 m/s)

Inclination of axis:



51,118 km

Number of Observed Satellites:



4.0 x Earth's

Mean Distance from Sun:

19.2 x Earth's


14.5 x Earth's


0.22 x Earth's

The picture on the right uses false colors and contrast enhancement to bring out subtle details in the polar region of Uranus.



Uranus has a relatively featureless appearance at visible wavelengths. Even from Voyager 2 at a distance of 80,000 km there were few distinguishable features.

This is believed to be due to Uranus being further from the Sun than Jupiter and Saturn, which means its temperature is lower (only 58 degrees Kelvin in the upper atmosphere).

This decreases the likelihood of chemical reactions making the colorful compounds that give the surface features on Jupiter and Saturn.

In addition, the upper atmosphere is thought to have a high-level petrochemical haze that obscures features lower in the atmosphere.


Neptune is the outermost of the four gas giants

  • Because of its distance from the Sun, Neptune's atmosphere is a frigid -225° C (-373° F)
  • Neptune, like Jupiter and Saturn but unlike Uranus, has an internal heat source and produces 2.7 times more heat than it absorbs.
  • The blue-green color of the planet is due to the presence of methane in the atmosphere. The atmosphere consists mostly of hydrogen, helium and methane.
  • Until the Voyager encounter in 1989, the rings surrounding Neptune were thought to be arcs. We now know that the rings completely circle the planet, but the thickness of each ring varies along its length. 

Neptune Facts



Galle, Challis, Adams, & Le Verrier (1846)

Spacecraft Encounter(s):

Voyager 2

Mean Distance from the Sun:

30.06 AU (4.497 billion km/2.794 billion mi)

Length of year:

165 years

Rotation period:

16.11 hours

Mean orbital velocity:

5.43 km/s (3.3 mi/s)


49,528 km/30,775 mi

Inclination of axis:


Number of observed satellites:


Comparisons with Earth:


3.883 x Earth's

Average distance from Sun:

30.06 x Earth's


17.14 x Earth's


0.31 x Earth's

Neptune Facts


Neptune has a great storm in the southern hemisphere called the “Great Dark Spot" that is about half the size of Jupiter's Great Red Spot and so is roughly the same diameter as the Earth, and at least one other smaller storm spot has been detected as well.

  • Like the other gas giants, there are rapid winds confined to bands of latitude, including one band that is moving the Great Dark Spot westward at over 1,100 km (or 700 miles) per hour. Indeed, Neptune has the fastest planetary winds in the Solar System, reaching as fast as 2,000 km (over 1,200 miles) per hour.

Neptune’s Great Dark Spot