15 February 2005 AST 2010: Chapter 10 1

The Giant PlanetsThe Giant Planets

15 February 2005 AST 2010: Chapter 10 2

Jupiter

Saturn

Uranus

Neptune

The Jovian Worlds

15 February 2005 AST 2010: Chapter 10 3

Exploration First spacecrafts: Pioneer 10 (1972) & 11

(1973)• Can we navigate through the asteroid belt?

• What are the radiation hazards near the planets?

Pioneer 10 flew by Jupiter in 1973 and flew out the solar system

Pioneer 11 flew by Jupiter in 1974 and was deflected towards Saturn which it reached in 1979

15 February 2005 AST 2010: Chapter 10 4

Exploration Voyager 1 & 2 (launched 1977)

• Highly productive Missions• Carried 11 scientific instruments

including cameras and spectrometers, devices for measuring magnetospheres

Voyager 1• Reached Jupiter (1979) and Saturn (1980)• Used gravity assist towards Saturn

Voyager 2 • Reached Jupiter four month later than Voyager 1• Reached Saturn (1981), Uranus (1986), Neptune (1989)

Multiple Flybys possible thanks to approximate alignment of the planets• Such an alignment occurs once in 175 years

Voyager 2 2

15 February 2005 AST 2010: Chapter 10 5

Exploration Galileo space probe

• Launched 1989

• Reached Jupiter December 1995

• Deployed a small entry probe for a direct study of Jupiter’s atmosphere

• Sept. 2003, probe sent into Jupiter’s atmosphere to end its mission

Cassini • Launched 1997

• Reached Saturn in 2004, now in orbit

• Deployed entry probe for Titan in Jan. 2005

Galileo Space Probe

15 February 2005 AST 2010: Chapter 10 7

Galileo – Jupiter Entry Probe Mass 339 kg Plunged at shallow angle into

Jupiter at speed of 50 km/s Slowed down by friction against

Jupiter atmosphere• Temperature of its shield reached

15,000°C

Speed dropped to 2500 km/h Deployed parachute for actual entry in the

atmosphere Transmitted data to orbiter, for retransmission to

Earth

15 February 2005 AST 2010: Chapter 10 8

Some Results of the Galileo Mission

The discovery of a satellite (Dactyl) of an asteroid (Ida) Jovian wind speeds in excess of 600 km/hour (400 mph)

were detected Far less water was detected in Jupiter's atmosphere than

estimated from earlier Voyager observations and from models of the Comet Shoemaker-Levy 9 impact

Far less lightning activity (about 10% of that found in an equal area on Earth) than anticipated. The individual lightning events, however, are about ten times stronger on Jupiter than the Earth

Helium abundance in Jupiter is very nearly the same as its abundance in the Sun (24% compared to 25%)

Extensive resurfacing on Io due to continuing volcanic activity since the 1979 Voyagers fly-bys

Evidence for liquid water ocean under Europa's ice

15 February 2005 AST 2010: Chapter 10 9

Huygens Probe Dropped by Cassini Orbiter

Basic Facts of Jovian Planets Large distances from the sun Long periods Jupiter and Saturn similar in composition and internal

structure Uranus and Neptune smaller and differ in

composition and structure

Basics Properties of the Jovian Planets

Planet Distance (AU)

Period (years)

Diameter (km)

Mass (Earth=1)

Density (g/cm3)

Rotation (hours)

Jupiter 5.2 11.9 142800 318 1.3 9.9

Saturn 9.5 29.5 120540 95 0.7 10.7

Uranus 19.2 84.1 51200 14 1.2 17.2

Neptune30.1 164.8 49500 17 1.6 16.1

15 February 2005 AST 2010: Chapter 10 11

Appearance Only the upper atmosphere of the giant

planets is visible to us• Astronomers believed that their interiors are

composed primarily of hydrogen and helium The uppermost clouds of Jupite and Saturn

are composed of ammonia (NH3) crystals Neptune’s upper clouds are made of methane

(CH4) Uranus has no obvious clouds, only deep and

featureless haze

15 February 2005 AST 2010: Chapter 10 12

Rotation How does one determine the rotation rate of

the giants? For Jupiter:

• 1st option: use dynamic surface features (storms) However the cloud rotation may have nothing to do with

the rotation of the mantle and core…

• 2nd option: look at periodic variations of radio waves associated with the magnetic field produced deep inside the planet

This gave rotation period of 9 h 56 m

The same technique is used to measure the rotation of other giant planets:• Saturn has 10 h 40 m• Uranus and Neptune have about 17 hours

Jupiter is tilted by 3o

• No seasons to speak of

Saturn is tilted by 27o

• Long seasons

Neptune is tilted by 27o

• Long seasons

Uranus is Tilted by 98o

• Practically orbiting on its side

• Rings and satellites follow same pattern

• 21 year seasons!!!

• Why this odd tilt? A giant impact in the past could be the cause

Seasons on the Giants

Seasons on Uranus

15 February 2005 AST 2010: Chapter 10 15

Giant Planets – Giant Pressure

Giant planets composed mainly of hydrogen (H) and helium (He)

But because of its enormous size, the H and He in the center of Jupiter are compressed enormously•Estimated pressure: 100 million bars.

•Central density of 31 g/cm3

Earth by contrast has 4 million bars and 17 g/cm3 in its center

Giant Planet implies giant pressure!!

15 February 2005 AST 2010: Chapter 10 16

Consequences of the Pressure Few thousand km below the surface,

hydrogen is in a liquid state Still deeper, the liquid is further

compressed and begins to act like a metal•On Jupiter, part of the interior is metallic

hydrogen! Saturn is less massive

•Most of its interior is liquid, but not metallic Neptune and Uranus are probably too

small to liquefy hydrogen

15 February 2005 AST 2010: Chapter 10 17

More about Composition The planets also have a core composed

of heavier materials•Possibly the original rock/ice bodies that

formed before gas were abundantly captured by the planets

15 February 2005 AST 2010: Chapter 10 18

Internal Heat Source (1) Because of their large sizes, all the giant planets are

believed to be strongly heated during their formation• Jupiter was the hottest

• Some of the primordial heat still remains

Giant planets may also generate energy internally by slowly contracting • Even a small amount of shrinkage can generate significant

heat This probably raises the temperature of the core and

atmosphere above the temperature due to the Sun’s heat

Jupiter has the largest internal source of energy• 4x1017 W

• A cross between a planet (like earth) and a star

• Internal heat probably primordial heat

15 February 2005 AST 2010: Chapter 10 19

Magnetic Fields

All four giant planets have strong magnetic fields and associated magnetospheres

The magnetospheres are large• They extend for millions of km in space

Jupiter’s field was discovered in the late 1950’s • Radio waves detected from Jupiter

• Electrons circulating in the magnetosphere produce the radio waves by a process called synchrotron emission

Magnetic fields of Saturn, Uranus, and Neptune discovered by flyby spacecraft

15 February 2005 AST 2010: Chapter 10 20

Magnetospheres Jupiter’s magnetic field is not aligned

with its axis of rotation •It is tipped by 10o

Uranus and Neptune have tilts of 60o and 55o

Saturn’s field is perfectly aligned with its axis of rotation

15 February 2005 AST 2010: Chapter 10 21

Atmospheres of the Giant Planets

The part of the planets accessible to direction observation

Dramatic examples of weather patterns•Storms on these planets can be larger than

Earth!

15 February 2005 AST 2010: Chapter 10 22

Atmospheric Composition Methane (CH4) and ammonia (NH3) were first

believed to be the primary constituents of the atmospheres

We know today that hydrogen and helium are actually the dominant gases• First based on far-infrared measurements by

Voyager

• Less helium in Saturn’s atmosphere Precipitation of helium? Energy source of Saturn?

• Best measurements of composition by Galileo spacecraft (1995) upon entry to atmosphere

15 February 2005 AST 2010: Chapter 10 23

Clouds and Atmospheric Structure

Jupiter’s clouds are spectacular in color and size• Color: orange, red, brown• Fast motion

Saturn is more “subdued”• Clouds have nearly

uniform butterscotch hue

15 February 2005 AST 2010: Chapter 10 24

Atmospheric Structure of the Jovian Planets

15 February 2005 AST 2010: Chapter 10 25

Winds and Weather Many regions of high/low pressures

• Air flow between these regions sets up wind patterns distorted by the fast rotation of the planet

Wind speeds measurable by tracking cloud patterns

Differences with Earth• Giant planets spin much faster than Earth

Rapid rotation smears out air circulation into horizontal (east-west) patterns parallel to the equator

• No solid surface No friction or loss of energy – this is why tropical storms

on Earth eventually die out…

• Internal heat contributes as much energy to the atmosphere as sunlight (except for Uranus)

15 February 2005 AST 2010: Chapter 10 26

Winds on The Giant Planets Winds on Uranus and Neptune are

rather similar to those on Jupiter and Saturn•True on Uranus in spite of the 98O tilt

15 February 2005 AST 2010: Chapter 10 27

Storms Omni-present on the giant planets Superimposed on the regular circulation

patterns Large oval-shaped high-pressure regions on

both Jupiter and Neptune Most famous: Jupiter’s Great Red Spot

• In the southern hemisphere• 30,000 km long (when Voyager

flew by)• Present since first seen 300 years

ago• Changes in size, but never

disappears

From Voyager 1 in 1979

15 February 2005 AST 2010: Chapter 10 28

Great Red Spot Counterclockwise rotation with rotation

period of 6 days Similar disturbances formed

in the 1930s on Jupiter• Smaller circles near the red

spot Cause unknown Long-lived because of

• absence of ground• their size

Expected lifetime: centuries

15 February 2005 AST 2010: Chapter 10 29

Neptune Great Dark Spot First seen in 1989 by

Voyager About 10,000-km long 17-day period Had disappeared (faded?)

in mid 1990s New dark spot seen in

Nov. 1994• Faded by 1995

Do storms form and disappear quicker on Neptune?