The Gas Giants

Astronomy 311Professor Lee

CarknerLecture 16

Upcoming Events Thursday, October 21

Nelson Lecture in Astronomy Dr. Jason Steffen, “Kepler Mission’s Search for Kepler Mission’s Search for

Planets Around Other Stars: Finding Other Earths”Planets Around Other Stars: Finding Other Earths” Saturday, October 23Saturday, October 23

Planetarium Open HousePlanetarium Open House See planetarium show and look through telescopesSee planetarium show and look through telescopes

10 points extra credit on observing project for 10 points extra credit on observing project for eacheach

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 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

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: 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:

30.06Numbers 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/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)

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 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

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 -- 16 Saturn -- 26 Uranus -- 27 Neptune -- 13

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 11.1-11.4, 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