Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 2:30

Lecture 37: The Gas Giants:
Jupiter & Saturn

Key Ideas:

• Gas Giants

Cloud Features:

• Colored Belts & Zones
• Strong Cyclonic Storms

Atmosphere & Internal Structure:

• Thick Hydrogen & Helium Atmospheres
• Radiate more energy than they get from the Sun
• Rocky cores & deep metallic Hydrogen mantles

Jupiter at a Glance

Orbital Data

Semi-Major Axis: a = 5.2 AU

Orbital Period: P = 11.9 years

Eccentricity: e = 0.048

Inclination: i = 1.3°

Planetary Data

Equatorial Radius: R = 71,492 km (11.21 R_Earth)

Mass: M = 317.7 M_Earth

Rotation Period: 9^h 50^m

Axis Tilt: 3.1°

63 Moons & a faint ring system

Spacecraft to Jupiter

• Pioneer 10 & 11 (1973 & 1974)
• Voyager 1 & 2 (1979)
• Ulysses (1992)
• Cassini (2001), enroute to Saturn

Orbiters:

• Galileo (Dec 1995 until Sept 2003)
• Dropped an atmospheric probe in Dec 1995

Saturn at a Glance

Orbital Data:

Semi-Major Axis: a = 9.5 AU

Orbital Period: P = 29.5 years

Eccentricity: e = 0.056

Inclination: i = 2.5°

Planetary Data:

Equatorial Radius: R = 60,268 km (9.44 R_Earth)

Mass: M = 95.2 M_Earth

Rotation Period: 10^h 14^m

Axis Tilt: 26.7°

60 Moons & a spectacular ring system

Spacecraft Visits to Saturn

• Pioneer 11 (Sept 1979)
• Voyager 1 (Nov 1980)
• Voyager 2 (Aug 1981)

Orbiters:

• Cassini (launched Oct 1997, went into orbit July 2004)
• Huygens Titan probe carried by Cassini. Safely landed on Titan in January 2005

Jupiter & Saturn are Gas Giants

• Deep, heavy hydrogen/helium atmosphere
• Rock & ice core

Rotate rapidly as measured from their magnetic fields:

• Jupiter Rotates once every 9h 50m
• Saturn Rotates once every 10h 14m

This rapid rotation noticably flattens the planets at the poles:

• Jupiter: 6.5% flattening
• Saturn: 10% flattening

Atmospheres of Jupiter & Saturn

	Jupiter		Saturn
Hydrogen (H2)	86%		>93%
Helium (He)	14%		>5%
Methane (CH4)	0.1%		0.2%
Water Vapor (H2O)	0.1%		0.1%
Ammonia (NH3)	0.02%		0.01%

These are examples of Reducing Atmospheres, which are atmospheres dominated by Hydrogen Chemistry (CH₄, H₂O, NH₃).

By comparison, Terrestrial planets Oxidizing Atmospheres, dominated by Oxygen chemistry (CO₂, H₂O, N₂).

The Clouds of Jupiter

• Crystals of ammonia, methane, & water ices
• Average temperature is 100-140 K

The atmosphere of Jupiter is divided into East-West latitudinal bands:

• Dark Belts
• Bright Zones

Belts & Zones

• Gaps in the higher clouds (belts), looking into darker, slightly warmer & chemically more complex regions below.
• Colors due to complex organics & polysulfides.

Zones: low pressure, low temperature

• Regions of cold, high ice clouds
• Lighter in color due to a lack of complex organics & polysulfides

Cyclonic Storms

• Wind speeds are up to 400 km/hr (compare to a hurricane at 120 km/hr)
• Winds blow either east-to-west or west-to-east, alternating between belts/zones.

Results in strong cyclonic storms:

• Get both low-pressure systems (cyclonic) and high-pressure systems (anticyclonic).
• Range in size up to ~3 Earth diameters!

The Great Red Spot

• Immense high-pressure system more than 40,000 km across (3 earth diameters)
• Outer parts take about 4-6 days to rotate once counterclockwise (anticyclonic)
• Varies in strength over time, sometimes deep red, other times nearly fading out.
• It was first seen in 1665 (>300 years ago!)
• May be a persistent atmospheric instability.

The Atmosphere of Saturn

• Saturn is farther from the Sun, and colder.
• Makes the bands less chemically complex, and so more subtle & less colorful than Jupiter's.
• West-to-East winds are very strong, up to ~1800 km/hr, faster than on any other planet.
• Fewer and shorter-lived cyclonic storms.
• Occasional very powerful storms are seen.

Internal Energy

Energy Source:

• They are slowly contracting under their own weight.
• Slow gravitational contraction releases gravitational energy, heating the deep interior.

This internal heat helps power the weather seen on Jupiter and Saturn.

By contrast, the weather on the Earth, Venus, and Mars is powered by solar energy (internal "geothermal" heat is insignificant compared to sunlight).

Interior of the Gas Giants

• Just below the clouds, get mushy, hot liquid hydrogen region.
• Very deep, a pressures above 4 Million atmospheres, get region of liquid metallic hydrogen (protons swimming in a sea of free electrons)
• Circulation currents in the metallic hydrogen regions generate huge magnetic fields.

At the bottom is a massive rock & ice core of 10-15 M_Earth & ~1.5 R_Earth.

Magnetic Fields

• Jupiter has the strongest planetary magnetic field in the Solar System.

• How we measure the rotation speed of the gas planets
• Winds in the upper atmospheres make it hard to measure the rotation rate from watching clound features.

• Jupiter is one of the brightest radio sources in the sky

Saturn vs. Jupiter

• Saturn is less dense (0.7 g/cc, less than the density of water!)
• Saturn has more rotational flattening at the poles (~10%) because of its lower density.
• Saturn has a smaller metallic hydrogen mantle because its lower overall mass means lower internal pressures.
• Saturn has less surface Helium than Jupiter (~1/4), probably due to "rainout" in the deep interior.