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The Gas Giants. Astronomy 311 Professor Lee Carkner Lecture 16. Upcoming Events. Thursday, October 21 Nelson Lecture in Astronomy Dr. Jason Steffen, “ Kepler Mission’s Search for Planets Around Other Stars: Finding Other Earths” Saturday, October 23 Planetarium Open House

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The Gas Giants

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The gas giants l.jpg

The Gas Giants

Astronomy 311

Professor Lee Carkner

Lecture 16


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Upcoming Events

  • Thursday, October 21

    • Nelson Lecture in Astronomy

    • 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


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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


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Missions to the Outer Planets

  • 1972 Pioneer 10 -- Jupiter flyby

  • 1973 Pioneer 11 -- Jupiter/Saturn flyby

  • 1977 Voyager 2 --Jupiter/Saturn/Uranus/ Neptune flyby

  • 1997 Cassini -- Saturn orbiter and Titan probe (Huygens)

    • arrived 2004, currently taking data


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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


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Voyager 2’s Grand Tour


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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

Gas Giant Facts

  • Numbers are relative to the Earth


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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


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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


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Densities

  • Average density (kg/m3)

    • Jupiter:

    • Saturn:

    • Uranus:

    • Neptune:

  • For comparison Earth’s density is 5515

    • Saturn is less dense than water (it floats)


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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.)


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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)


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Continuous Boost

  • Rate of change of speed is called the acceleration, a

  • Solution: turn around halfway and decelerate to a stop


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Time to get to Planet

  • Find time to get half way there and double it

  • In general, distance equals ½at2

  • 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


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Odyssey to Jupiter

Jupiter at Conjunction

Spaceship Direct Boost

Earth

Spacecraft

Least Energy Orbit

Jupiter at Opposition


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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


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Internal Structure of Jupiter


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Moons

  • Number of satellites (larger than ~10 km)

    • Jupiter -- 16

    • Saturn -- 26

    • Uranus -- 27

    • Neptune -- 13

    • Including the very small ones, about 170 total


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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


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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


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Differences Between the Gas Giants

  • Uranus and Neptune are smaller, cooler and have less distinct cloud features


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Next Time

  • Read 11.1-11.4, 11.6


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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


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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


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