The Gas Giants Astronomy 311 Professor Lee Carkner Lecture 16

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

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”

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

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

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 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”

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)

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

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

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

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

Internal Structure of Jupiter

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

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

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 Most of interior mass is liquid metallic hydrogen Most of interior mass is composed of fluid ices

Next Time Read 11.1-11.4, 11.6

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

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