Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 35: Gas Giants: Jupiter and Saturn

Key Ideas:

Jupiter and Saturn are the largest Planets: Gas Giants; Substantial Moon Systems
Cloud Features: Colored belts and zones; Strong Cyclonic Storms
Atmosphere and Internal Structure: Thick Hydrogen and Helium Atmospheres; Radiate more energy than they get from the Sun; Rocky cores and metallic Hydrogen mantles

Spacecraft to Jupiter

Fly-bys:: Pioneer 10 and 11 (1973 and 1974); Voyager 1 and 2 (1979); Ulysses (1992); Cassini (2001, while enroute to Saturn)
Orbiters:: Galileo (arrived Dec 1995, deliberately crashed into Jupiter in 2003); Dropped an atmospheric probe in Dec 1995

Spacecraft to Saturn

Fly-bys:: Pioneer 11 (Sept 1979); Voyager 1 (Nov 1980); Voyager 2 (Aug 1981)
Cassini Orbiters:: Launched Oct 1997; Arrived July 2004; Huygens Titan probe landed January 2005

Jupiter and Saturn are Gas Giants

No solid surfaces: Deep, heavy hydrogen and helium atmospheres; Rock and ice cores
Rapidly rotating, measured from magnetic fields: Jupiter: 9hours 50min; Saturn: 10hours 14min
This rapid rotation noticeably flattens them at the poles: Jupiter: 6.5% flattening; Saturn: 10% flattening

Moon Systems

Jupiter: 63 moons; 4 Giant Moons; 59 small moons (<200km in diameter, irregular, rock/ice or mostly ice)
Saturn: 62 moons; 1 Giant Moons; 61 small moons (20-1500km, below 300km are irregular, above 300km are spherical, rock/ice or mostly ice)

Atmospheres

Jupiter Atmospheric Probe

Released by the Galileo mission in December 1995
Survived 200km into the atmosphere, until crushed by high pressure
Measured relatively constant wind speed and the chemistry of the atmosphere: Surprisingly little water; High abundance of Xe, Ar, Kr

Clouds of Jupiter

What we see are the tops of the clouds: Crystals of ammonia, methane, and water ices; Average temperatures is 100 - 140 K
Atmosphere is divided into latitudinal bands:: Dark Belts; Bright Zones; There are also Cyclonic and Anti-Cyclonic storms that appear between the belts and zones

Belts and Zones of Jupiter

Belts: high pressure and low temperature: Gaps in high clouds, view lower atmosphere; Colors due to complex organics and polysulfides
Zones: low pressure, low temperature: Regions of cold, high ice clouds

Cyclonic Storms

Strong winds at the belt/zone boundaries:: Wind speeds up to 400 km/hr; Blow east-to-west or west-to-east, alternating between belts/zones
Results in strong cyclonic storms:: Get both low-pressure (cyclonic) and high-pressure (anticyclonic) systems
Range in size up to ~3 Earth diameters
Some storms persist for centuries
The Great Red Spot is the largest and most complex of Jupiter's storms: Immmense high-pressure system more than 3 Earth diameters across; Rotates every 6 days counterclockwise (anticyclonic); Varies in strength over time, sometimes deep red, other times nearly fading away; It was first noticed in 1665 (>300 years ago!)

Atmosphere of Saturn

The atmosphere is divided into dark bands and bright zones like Jupiter's
Differences from Jupiter are: Saturn is farther from the Sun, so colder; This makes the bands less chemically complex and so more subtle and less colorful; West-to-East winds are very strong, up to ~1800 km/hour (faster than on any other planet); Fewer and shorter-lived cyclonic storms

Internal Energy

Jupiter and Saturn radiate more energy at infrared wavelengths than they receive from the Sun
Energy source:: They are slowly contracting under their own weight; Slow gravitational contraction releases gravitational energy, heating the interior; This heat help power their weather
By contrast, the weather on Venus, Earth, and Mars is powered by solar energy (internal heat is insignificant in comparison)

Gas Giant Interiors

Atmosphere gets thicker the deeper you go: Just below the clouds there is a region of hot, liquid Hydrogen; Very deep there is liquid metallic Hydrogen where circulation currents generate huge magnetic fields
At the center is a massive rock and ice core:: Jupiter: 10-15 M_Earth; Saturn: 1.5 M_Earth

Magnetic Fields

Circulating currents in the metallic hydrogen "mantle" generates powerful magnetic fields: Jupiter has the strongest magnetic field in the Solar System; Magnetic field corotates with the interoir of the planet; This is how we measure rotation speed; Winds make it hard to do this with cloud features

Saturn vs. Jupiter

Saturn is different from Jupiter in a number of respects:: Saturn is less dense; Saturn is more rotationally flattened because of its lower density; Saturn has a smaller metallic Hydrogen mantle because its lower overall mass means lower internal pressures

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.