The Gas Giants Astronomy 311 Professor Lee Carkner Lecture 16

Upcoming Events  Thursday, October 21  Nelson Lecture in Astronomy Kepler Mission’s Search for Planets Around Other Stars: Finding Other Earths”  Dr. Jason Steffen, “Kepler Mission’s Search for Planets Around Other Stars: Finding Other Earths”  Saturday, October 23  Planetarium Open House  See planetarium show and look through telescopes  10 points extra credit on observing project for each

Gas Giant Basics   Sometimes called Jovian or Outer planets  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 2 --Jupiter/Saturn/Uranus/ Neptune flyby   1997 Cassini -- Saturn orbiter and Titan probe (Huygens)  arrived 2004, currently taking data

Voyager 2   Took advantage of favorable alignment of outer planets to visit them all   Used gravity of each planet to change orbit to get to the next   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   Jupiter and Saturn about 10 Earth diameters   Uranus and Neptune about 4 Earth diameters   The gas giants contain 99% of the mass of the solar system (not including the Sun)   Volume and mass go as radius cubed

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)

Continuous Boost   Rate of change of speed is called the acceleration, a   Solution: turn around halfway and decelerate to a stop

Time to get to Planet  Find time to get half way there and double it  In general, distance equals ½at 2  As a simple estimation, we will assume a direct boost at opposition  In reality we would have to take into account the orbital velocities of the planets

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   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

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