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The Gas Giants
Astronomy 311
Professor Lee Carkner
Lecture 16
Gas Giant Basics
The 4 largest planets of the solar system
are the gas giants

Does not include Pluto

The gas giants have no real surface, all we
see is atmosphere
Missions to the Outer Planets
1972 Pioneer 10 -- Jupiter flyby

1973 Pioneer 11 -- Jupiter/Saturn flyby
1977 Voyager 1 -- Jupiter/Saturn flyby

1989 Galileo -- Jupiter orbiter and probe
1997 Cassini -- Saturn orbiter and Titan
probe (Huygens)
arrived 2004, currently taking data
Voyager 2
Launched August 20, 1977

Only mission to Uranus or Neptune

Most successful space probe ever
launched
Is still taking data on the edge of the solar
system
Voyager 2’s Grand Tour
Gas Giant Facts
Jupiter
Diameter: 11.21
Mass: 317.83
Orbital Radius: 5.20
Saturn
Diameter: 9.42
Mass: 95.16
Orbital Radius: 9.54
Uranus
Diameter: 4.01
Mass: 14.50
Orbital Radius: 19.19
Neptune
Diameter: 3.88
Mass: 17.20
Orbital Radius: 30.06
Numbers are relative to the Earth
Orbits
Orbital radii for the outer planets range
from ~5-30 AU

The outer planets cover a much larger
region

Gas giants don’t move much in the sky
from our viewpoint
Sizes of the Gas Giants
Gas giants are all very large compared to
the terrestrial planets

Jupiter a little larger

Uranus a little larger
The gas giants contain 99% of the mass of
the solar system (not including the Sun)

Volume and mass go as radius cubed
Densities
The gas giants have very low densities
Average density (kg/m3)
Jupiter:
Saturn:
Uranus:
Neptune:
For comparison Earth’s density is 5515
Saturn is less dense than water (it floats)
Composition
The gas giants have a low density because they
are made up mostly of hydrogen and helium, the
2 lightest elements

Hydrogen is the lightest and most abundant element in
the universe (1 proton + 1 electron)
Total composition


~5% heavier elements (methane, ammonia, water vapor
etc.)
Atmospheric Features
Clouds

small amounts of trace elements may produce
the colors
Bands

adjacent bands move in opposite directions
Storms

some are very long lived (e.g. Great Red Spot)
Jupiter at Conjunction
Spacecraft
Least
Energy Orbit
Earth
Spaceship
Direct
Boost
Jupiter at Opposition
Odyssey to Jupiter
Structure

Degree of oblateness depends on mass
distribution

Models indicate that gas giants have a
small, dense, rock-ice core

Must have conducting liquid interiors
Internal Structure of Jupiter
Moons

Number of satellites (larger than ~10 km)
Jupiter -Saturn -Uranus -Neptune --
Range in size from Ganymede (larger then
Mercury) to small pieces of rock
Including the very small ones, about 170 total
Moon Properties
Satellites tend to be composed of rock
and ice

Many have rocky cores and icy surfaces

Io is very active and covered with
volcanoes

Europa may have a liquid water ocean
Jupiter’s Satellites
Saturn’s Satellites
Rings
All of the gas giants have rings of small
particles

The rings of the other planets are made of
smaller darker particles and were only
discovered by spacecraft

This is the region where the tidal force from the
planet is greater than the gravitational force
holding the object together
Differences Between the Gas
Giants

Uranus and Neptune are smaller,
cooler and have less distinct cloud
features

Next Time
Read 11.1-11.4, 11.6
Summary
Size: ~4-11 times Earth diameter
Mass: ~15-318 Earth masses
Composition: mostly hydrogen and
helium
Atmosphere: clouds of methane and
ammonia
also have large, long-lived storm systems
and oppositely moving bands
Summary: The Jovian Systems
The gas giants have extensive satellite
systems
Many moons have icy exteriors with rocky
cores
Some are very large (~size of Earth’s
Moon)
All of the outer planets (not just Saturn
have ring systems)
rings composed of small particles
Ring properties different for each planet