1. The Gas Giants
Astronomy 311
Professor Lee Carkner
Lecture 16

2. Gas Giant Basics Sometimes called Jovian or Outer planets
Does not include Pluto
The gas giants have no real surface, most of the volume of the planet is gas and/or liquid

3. Gas and Ice
When planetary scientists say “gas”, they mean hydrogen and helium
When planetary scientists say “ice”, they mean water, methane, and ammonia (volatiles)
Gas giants should really be called “fluid giants”

4. Missions to the Outer Planets
1972 Pioneer 10 --
first spacecraft to the outer planets
1973 Pioneer 11 --
1977 Voyager 1 -- Jupiter/Saturn flyby
1977 Voyager 2 –
1989 Galileo -- Jupiter orbiter and probe
1997 Cassini --
arrived 2004, currently taking data

5. 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

6. Voyager 2’s Grand Tour

7. 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

8. Densities The gas giants have very low densities
Saturn is less dense than water (it floats)
The gas giants have a low density because they are made up of light elements (gas and ice)
Jupiter and Saturn are mostly hydrogen (and helium)
Uranus and Neptune are mostly “ices” (water, methane and ammonia)
Sometimes called “ice giants”

9. Atmospheric Features Clouds Bands Storms
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)

10. Continuous Boost Normal space probes coast with no boost
Spaceship speed would keep increasing
Rate of change of speed is acceleration, a
Solution: turn around halfway and decelerate to a stop

11. Time to get to Planet Find time to get half way there and double it
But we only are boosting half way, so the distance is ½d
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

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

13. Structure Degree of oblateness depends on mass distribution
Models indicate that gas giants have a small, dense, rock-ice core
Must have conducting liquid mantles

14. Internal Structure of Jupiter

15. Moons
Number of satellites ( number large than larger than ~10 km given in parenthesis)
Jupiter – 64 (16)
Saturn – 62 (26)
Uranus – 27 (27)
Neptune – 13 (13)
Including the very small ones, about 170 total

16. Moon Properties The satellites are very diverse:
They formed from icy planetesimals
The satellites are very diverse:
Io is the most active world in the solar system and is covered with volcanoes
Europa may have a liquid water ocean

17. 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

18. Differences Between the Gas Giants
Most of interior mass is liquid metallic hydrogen
Most of interior mass is composed of fluid ices

19. Next Time
Read , 11.6

20. Summary Size: ~4-11 times Earth diameter Mass: ~15-318 Earth masses
Interior: rocky core, mantle of hydrogen or ice
Atmosphere: mostly hydrogen and helium
Clouds: water, methane and ammonia
also have large, long-lived storm systems and oppositely moving bands

21. 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