# 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