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 Jupiter flyby   1973 Pioneer 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:  Mass:  Orbital Radius: 5.20  Saturn  Diameter: 9.42  Mass:  Orbital Radius: 9.54  Uranus  Diameter: 4.01  Mass:  Orbital Radius:  Neptune  Diameter: 3.88  Mass:  Orbital Radius:  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/m 3 )  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)

Odyssey to Jupiter Earth Jupiter at Conjunction Spacecraft Least Energy Orbit Jupiter at Opposition Spaceship Direct Boost

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 , 11.6

Summary  Size: ~4-11 times Earth diameter  Mass: ~ 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