Astronomy 100 -- Gas Giants and Their Rings

JUPITER and SATURN
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Basic Properties of Jupiter and Saturn

1) How big are they?

A) Diameter: both about 10 Earth diameters, with Jupiter being the larger.
B) Mass: Jupiter = 300 Earth masses, Saturn = 100 Earth masses
C) Density: Jupiter = 1.3 grams/cc, Saturn = 0.7 grams/cc!
Saturn's density is less than water!

2) How old are they?

4.6 billion years

3) Where are they?

Jupiter is 5 A.U. (40 light-minutes) from the Sun.
Saturn is 10 A.U. (80 light-minutes) from the Sun.

4) What are they doing?

Rotation Period = 10 hours for both Jupiter and Saturn.
Revolution Period: Jupiter = 12 years -- Saturn = 30 years.

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THE INTERNAL STRUCTURE OF JUPITER AND SATURN

Composition: Mostly hydrogen (80%) and helium (20%), like the Sun.


Fix Figure 12-22

A) Small metal-rock-ice core.
This is between 1 and 20 Earth masses.

B) Layer of metallic liquid hydrogen.

Produces a strong magnetic field.

C) Layer of non-metallic liquid hydrogen.

Slow transition from liquid to gas. No liquid surface.

D) Cloud layers -- ammonia, water, ammonium hydrosufide.

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Related Links of Interest

NSSDC Jupiter Page.
The Jovian System. About Jupiter and its moons.
Project Galileo: Bringing Jupiter to Earth.
NSSDC Saturn Page.
SSI Team - Galileo Mission.
Cassini: Voyage to Saturn.

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URANUS and NEPTUNE
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Basic Properties of Uranus and Neptune

1) How big are they?

A) Diameter: both are about 4 Earth diameters.
B) Mass: both are about 15 Earth masses.
C) Density: both are about 1.5 grams/cc.
Small size and low density suggests water composition.
See Fix Figure 13-09. Internal Structure Diagram

2) How old are they?

4.6 billion years

3) Where are they?

Uranus is 19 A.U. (160 light-minutes) from the Sun.
Neptune is 30 A.U. (250 light-minutes) from the Sun.

4) What are they doing?

Rotation Period = about 16-17 hours for both.
Revolution Period: Uranus = 84 years -- Neptune = 165 years.

5) Both are "discovered" planets. Fainter than naked eye limit.

Uranus -- William Herschel - 1781

Found it while he was looking for comets.
Neptune Discovery -- Adams, Leverrier, and Galle - 1846
Predicted to exist due to perturbations of Uranus' orbit.
Observed by Galileo? Drawn in his notebook!

• These planets have a blue color due to clouds of methane ice.

Uranus does not show any cloud structure, while
Neptune seems to have a belt-zone structure.
Neptune also has the "Great Dark Spot" like Jupiter's Great Red Spot.

Both planets have a ring system.

This is true for all the Jovian planets.

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DIFFERENCES BETWEEN URANUS AND NEPTUNE

• Uranus is tipped on its side (spin axis is tilted by 90 degrees)
  This means the north and south poles will sometimes face the Sun.
  May be the reason for lack of "belt-zone" structure in atmosphere.
  

Fix Figure 13-02

• Neptune is sometimes the most distant planet from the Sun.
  This is true for 20 years of its orbit (1979-1999).

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Related Links of Interest

NSSDC Uranus Page.
Welcome to the Planets - Uranus.
Hawaiian Astronomical Society - Uranus.
NSSDC Neptune Page.
Welcome to the Planets - Neptune.
Hawaiian Astronomical Society - Neptune.

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THE RING SYSTEMS OF THE JOVIAN PLANETS

Each of the four jovian planets has a ring system.
But, the rings for each planet are very different!

"Why haven't the rings formed a moon?"

Roche Distance -- This is the distance at which the tidal forces on a object are stronger than the gravitational force holding the object together. (The difference between the gravitational pull of the planet on the near side and far side of a moon is greater than the gravitational force of the moon itself).

Typically, the Roche distance is 2.5 times the radius of the planet. So, moons that come closer than the Roche distance will be torn apart!

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The Ring System of Saturn

• The diameter is about 200,000 kilometers.
  The thickness is only about 20 meters!! An edge-on view.
  

• Rings are not solid, the inner parts of the rings revolve around Saturn faster than the outer rings.
  

• These rings are made of water ice.
  Probably pieces of rock covered by ice.

• Mostly small in size, ranging from 1 cm to 10 meters.
  

• Rings are a very complex system!
  Wide range of compositions, many "gaps", transient "spokes" occur.

• Are rings young or old? ?????

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The Rings of Jupiter

These rings were discovered in 1979.

Not easily observed like the rings of Saturn.
An Infra-red picture.

The diameter is not much greater than Jupiter.

Very close to Jupiter, so particles spiral in.

These rings are made of small dust particles.

Source of these particles:
1) Pieces of meteorites falling into Jupiter.
2) Ejecta from the volcanos on Io.

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The Uranian Ring System

Discovered in 1977 during a stellar occultation.


There are 10 separate rings, with a width between 10 and 100 kilometers.


Ring particles are "large", meters in size.

These particles are dark, not made of water ice.

Space between the rings is filled in by dust particles.

Shepard" satellites keep these rings thin!

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The Rings of Neptune

Evidence observed in mid-1980s during a stellar occultations.


Occultation suggested "arcs" rather than rings.

Existence confirmed in 1989 by Boyager 2.

There are 6 separate rings, less than 100 kilometers width.

Ring particles are very small and dark, not water ice.

Outer ring has 3 clumps of material.

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Ring Related Links of Interest

The Small Satellites and Rings of Jupiter.

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Terrestrial Planets

Outer Moons and Icy Bodies

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