Chapter 10: Outer Planets


  • Is the largest and most massive planet in our solar system.
  • Radius is derived from angular size and distance.
  • 11 times the radius of the earth, the we know the volume and it can hold 1400 Earths.
  • Mass measured from the orbits of its many moons
  • M ==2 d3/ p2
  • Average density == 1.33 gm/cm3, just slightly more than water. This tells us that it is mostly hydrogen.
  • Rotation takes only 9.9 hours even though it is so large \rightarrow very high speed \rightarrow causes Jupiter to have a bulging equator.

Jupiter’s Interior Structure

  • We cannot measure Jupiter’s seismic waves.
  • We derive the interior structure using computer calculations (we try to compute what Jupiter is like and then try to compare it with our observations).
  • Use the average density of 1.33 gm/cm3 to estimate the composition.
  • Use the balance between weight and pressure.
  • Results
  • A thick atmosphere of hydrogen gas and molecules containing a lot of hydrogen.
  • Below the atmosphere is a deep ocean of hydrogen compressed to a liquid.
  • Below that is another deep ocean of hydrogen compressed to a liquid metal capable of conducting electricity.
  • At the centre is a core of rock and metal with a mass of about 18 MEarth.
  • The core temperature == 30,000 K is about 5 times hotter than the Sun’s surface temperature.
  • Heat - comes from Jupiter’s formation;
  • From slowly shrinking \rightarrow energy of gravity (gets transferred into energy in the core)
  • Jupiter’s atmosphere is very different from Earth’s:
  • Made of hydrogen and hydrogen molecules not O2 and N2.
  • Much thicker.
  • Much faster rotation- jet streams of cloud belts, not swirls.
  • Receives more energy from the interior than from sunlight.
  • No solid land below the atmosphere, just liquid.
  • Unique feature - Great Red Spot
  • A hurricane that doesn’t go away
  • Has existed for centuries because it was never cut off from its course of energy.

Jupiter’s Magnetic Field

  • In Ch 6 we learned that Earth has a magnetic field created by the dynamo of its molten metallic core.

Jupiter’s Rings

  • We have observed Jupiter for centuries from earth, but we never saw its rings.
  • In 1979 the spacecraft Voyager 1 found the rings by taking pictures from behind Jupiter looking for lightening.
  • The rings are probably from dust blasted from tiny moons by impact.

Jupiter’s Moons

  • Galileo discovered the 4 large moons of Jupiter in 1609 - the Galilean moons Io, Europa, Ganymede, Callisto.
  • We have discovered a total of 69 moons.
  • They form a miniature “solar system” with Jupiter acting like the Sun - recall in we learned that Galileo


  • Io is the Galilean moon closest to Jupiter
  • We observe that its surface is bright and has no impact craters \rightarrow Io must be resurfaced continually.
  • Constant volcanic eruptions fill in any craters created by impacts.
  • Therefore, Io’s interior must still be very hot, but it is about the size of our Moon?

Why is Io still so hot?

  • Io orbits close to Jupiter, the most massive planet in the solar system.
  • Therefore, Io experiences powerful tides.
  • Io is a member of a large family of moons that interact with each other.
  • These interactions prevent it from having a synchronized spin so it cannot escape from tidal heating as our moon has (so since it cannot avoid the tidal heating like our moon can, it is a more complicated environment, more powerful tides are combing to do this)

Io’s Colour

  • We have measured that Io’s lava is rich in sulfur, which is common in earth’s lava too
  • Sulfur is commonly yellow, but it changes colour depending on its temperature explaining the appearance of Io.


  • Europa is the smallest Galilean moon, and the second closest to Jupiter
  • Its surface is white with long cracks and no large craters.
  • White surface \rightarrow water ice like our arctic.
  • No craters \rightarrow constant resurfacing by liquid water from below the surface.
  • Heating by tides from Jupiter’s gravity. (the stretching from the tides from the moon’s gravity provides enough heat to melt ice. Energy from tides).
  • The existence of liquid water on a moon so far from the sun was a surprise.
  • Shows that heating from tides can be as significant as sunlight.
  • The liquid water makes Europa a place where life could exist.


  • The largest moon in the solar system, even larger than Mercury.
  • Its average density is only 1.94 g/cm3 \rightarrow mixture of metal, rock, and water/ice.
  • Its surface has white craters \rightarrow ice just below the dark surface.
  • It surface has cratered and new regions
  • It has a magnetic field \rightarrow molten core?


  • Average density == 1.85 g/cm3 \rightarrow also a mixture of metal, rock, and water/ice.
  • Its surface is heavily cratered showing no signs of shifting.
  • Callisto’s gravitational pull on our space crafts indicates it does not have a differentiated core and mantle structure.

Smaller Moons

  • Jupiter’s 65 moons are:
  • Much smaller than the Galileans moons.
  • Much more random orbits.
  • Are not spherical.
  • This suggests they have probably been captured by Jupiter’s strong gravity rather than having been born with Jupiter, like the Galilean moons.


  • Saturn is the second largest planet
  • Mass == 95 MEarth
  • Radius == 9.5 Reath, vol. could hold 860 Earths
  • Average density is only 0.7 g/cm3, which is less than water \rightarrow mostly hydrogen.

Saturn’s Rings

  • Galileo observed that Saturn was not round, but his telescope was not good enough to see it’s rings
  • The width of the rings is about 100,000 km, but they are only about 100 m thick - stars can be seen through them.
  • The great width and tiny thickness tells us that the rings are not a solid structure.
  • Instead they are swarms of trillions of separate particles orbiting Saturn , each particle follows Kepler’s 3rd Law.
  • The particles in the rings range in size from a few meters down to mm.
  • Studying the ring’s reflected sunlight \rightarrow they are mostly ice with a mixture of rock.
  • A number of gaps exist in the rings which are caused by the gravity of some of Saturn’s moons.

What Causes Ring’s

  • When we thought that Saturn was the only planet…
  • Then we discovered that Jupiter has rings.
  • Possible important factors:
  • Each Jovian planet has many moons made of rock and ice. Collisions between moons can break off rock and ice creating the rings
  • The strong gravity of the Jovian planets will pull the debris apart if it comes too close to the planet - == Roche limit.

Saturn’s Moons

  • Like Jupiter, Saturn has many moons:
  • 1 large moon - Titan.
  • 62 small moons.
  • Titan is slightly larger than Mercury , and it has the densest atmosphere of any moon
  • Even denser than Earth’s atmosphere
  • Titan is so cold at 9.6 AU from the sun that is gravity can hold an atmosphere

Titan’s Atmosphere and Surface

  • Our spacecraft have found:
  • Titan’s atmosphere is mostly nitrogen like Earth’s.
  • Its clouds are hydrogen molecules not water.
  • Its surface has rivers and lakes, not water but liquid hydrogen molecules like methane == CH4 and ethane == C2H6
  • The probe Huygens landed on Titan surface showing water-ice “rocks” as small as 10 cm.

Small Moons of Saturn

  • The small moons of Saturn also have very intriguing properties.
  • Some have odd surface features
  • Enceladus has eruptions of liquid water from its surface caused by tidal heating from Saturn \rightarrow possible location for life from tidal energy, not sunlight.


  • The planets Mercury - Saturn are visible without a telescope, and they have always been known.
  • Uranus was discovered using a telescope in 1781 by the musician/astronomer William Herschel.

Physical Properties of Uranus

  • Diameter - angular size x distance == 4 x Earth
  • Mass from its moons’ orbits == 14.5 x Earth.
  • Average density 1.27gm/cm3 \rightarrow almost the same as Jupiter ( I think that’s what it said)
  • But Uranus’ mass is lower than Jupiter, so it is less compressed \rightarrow less H gas and more water and hydrogen molecules.

Atmosphere of Uranus

  • Uranus appears very blue, quite different than Jupiter or Saturn.
  • (The colour of Uranus and Neptune are very blue unlike the other planets).
  • The blue colour is cause by Uranus’ very cold temperature == 76 K == -197 degrees Celsius.
  • As a result, certain types of gas freeze out of the atmosphere and become ice particles ( in both Neptune and Uranus).
  • Methane (CH4) gas remains, which is a very efficient absorber of red light
  • Turns out methane is a very good absorber of red light, so what’s left is reflected which is mostly blue.

The Tilt of Uranus’ Rotation Axis

  • One very strange property of Uranus is the tilt of its rotation axis to its orbit: 97.86 degrees.
  • Most of the moons of Uranus orbit aligned over its equator, so they too are tilted.
  • The rings of Uranus are also aligned over the equator and share the same tilt.
  • The extreme tilt of the rotation axis causes very extreme sunlight and seasons.


  • Was discovered in 1846 when it was noticed that Uranus was deviating from its unexpected orbit.
  • The deviation was used to predict the location of an unknown planet.
  • Observing that location of the solar system found Neptune.
  • Today a similar each is being done searching for planet X.

Neptune's Physical Properties

  • The mass, radius, average density and appearance of Neptune are very similar to Uranus (only based on calculations)


  • Saturn the most, Uranus not so much, Neptune not so much

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