Chapter 10: Outer Planets

Jupiter

  • 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

  • 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

  • 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.

Ganymede

  • 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?

Callisto

  • 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

  • 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.

Uranus

  • 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.

Neptune

  • 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)


Summary

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


Note Created by
Is this note helpful?
Give kudos to your peers!
10
Wanna make this note your own?
Fork this Note
96 Views