Jupiter , Saturn, Uranus, Neptune - Cornell Universityastrosun2.astro.cornell.edu/academics/courses/astro202/A202_2008_Lec04.pdf · Uranus' disk is tiny: only about 2 to 4 arcsec

Lecture 4: Overview of the

Giant Planets

Astro 202Prof. Jim Bell ([email protected])

Spring 2008

But first...

Turn in Paper 1 now

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This just in from Mercury...

MESSENGER Probe

Astro 202 4

Jupiter, Saturn, Uranus, Neptune

! General properties

! Atmospheres

! Interiors

! Ring Systems

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The “Jovian” Planets

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Basic Properties of Jupiter

• Average Distance from Sun: 778,000,000 km (a=5.20 AU)

• Orbital period: 11.86 years; eccentricity: 0.048

• Period of Spin around axis: ~10 hours (varies with lat.)

• Tilt of Jovian spin axis: 3.1° (no real seasons)

• Mass: 1.9x1027 kg = 317 ME ; Radius: 71,400 km = 11 RE

• Density = 1.3 g/cm3 (mostly gaseous, small "rocky" core?)

• No "surface" like the terrestrial planets: only clouds visible

• Gravity = 24.8 m/sec2 (2.5 times Earth's)

• Cloud-top temperature: 125 K (-148°C)

• Atmosphere: Mostly H, He (plus CH4, NH3, H2O, NH4HS, ...)

• Moons: 63 presently known

• Rings: Jupiter has an elegant set of thin dark rings

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Observations

!Jupiter is a prominent object in the night sky

for many months out of each year

!Through even a small telescope, Jupiter's

clouds appear yellow, red, and brown

!And features can be seen

– Belts of red clouds

– Zones of white clouds

– Oval-shaped storms

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Space Missions

!7 robotic spacecraft have flown past Jupiter!1 more orbited Jupiter from 1995 to 2003

Mission Dates Goals and Results

Pioneer 10 1973 First Jupiter flyby; studied magnetic field, clouds

Pioneer 11 1974 Flyby; studied magnetic field, atmosphere

Voyager 1 1979 Flyby; high resolution imaging and other studies

Voyager 2 1979 Flyby; high resolution imaging and other studies

Ulysses 1992 Flyby (Sun gravity assist); solar wind & magnetic field

Galileo 1995- Orbiter; imaging, spectroscopy, magnetic fields,

2003 and atmospheric entry probe

Cassini 1999 Flyby (Saturn gravity assist); imaging, spectroscopy

New Horizons 2007 Flyby (Pluto gravity assist); imaging, spectroscopy

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Voyagers 1 and 2!

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Jupiter's Dynamic Atmosphere

!The zones, belts, and storms change with time

!The most famous storm, the Great Red Spot, has been

around at least since 1610

Jupiter rotation

Jupiter cloud motions

(rotation removed)

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Atmospheric Composition

!Determined by spectroscopy from

telescopes and spacecraft!Directly sampled by the Galileo probe

Name Symbol % Volume

Hydrogen H2 90

Helium He 4.5

Methane CH4 0.07

Ammonia NH3 0.02

Ethane C2H6 0.01

Deuterated Hydrogen HD 1.8 x 10-3

Water vapor H2O 1 x 10-4

Deuterated Methane CH3D 3 x 10-5

Phosphine PH3 1 x 10-6

Carbon monoxide CO 1 x 10-7

Germanium Tetrahydride GeH3 ? 1 x 10-7

The composition of

Jupiter is very close

to the composition of

the Sun

Galileo Probe Mission

Release Date: 7/13/95 Entry into Jupiter: 12/7/95Entry Speed: 106,000 mphProbe Data Return: 59 min 3.5 megabitsPenetration depth 200 km (124 mi)

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Belts

and

Zones

Knowledge of

the composition

& temp. allows

astronomers to

create a model to

explain the

distribution and

colors of the

belts and zones

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Atmospheric Circulation

Great Red Spot

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Jupiter's Interior

Interior structure is

inferred from models

and extrapolation of

data from the upper

cloud layers...

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Internal vs. External Heat Sources

!Energy Balance – What comes in must go out...

– Solar heat absorbed = Outgoing planetary heat

!But for Jupiter: Outgoing = 1.8 ! Incoming

!Jupiter has a strong internal heat source– Heat stored from planetary contraction?

– From some other source?

!This has a strong influence on atmospheric circulation and the "climate" of Jupiter– More in Lecture 14...

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Giant Planet

or

Failed Star?

! Calculations suggest that Jupiter is about 100 times less massive than needed to initiate hydrogen fusion reactions in its core and become a star.

! Since factors of 100 are not so large in astronomy, it is legitimate at some level to view Jupiter as either a very large planet, or as a small failed star.

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Jupiter's Magnetic Field

!The largest planetary structure in the solar

system (apparent size > the full Moon)

• Jupiter's magnetic field is

about 20,000 times stronger

than Earth's

• The solar wind is deflected

in a huge zone around the

planet

• Evidence for the magnetic

field can be seen in

telescope and spacecraft

images of auroraeAstro 202 18

Satellites &

Rings

! Galileo discovered the 4 large moons of Jupiter in 1610

– We call them the Galilean satellites! 12 other smaller moon found: 1872-1979

! Dozens more even smaller found since ‘99– Total is now 63! (http://www.dtm.ciw.edu/sheppard/satellites/)

! Jupiter also has a faint, thin ring system

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Saturn

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The Jovian Planets

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Basic Properties of Saturn

• Average Distance from Sun: 1.43 billion km (a=9.54 AU)


• Period of Spin around axis: ~10 hours (varies with lat.)

• Tilt of Saturn's spin axis: 26.7° (seasons and tilted rings)

• Mass: 5.7x1026 kg = 94 ME ; Radius: 60,270 km = 9.4 RE

• Density = 0.7 g/cm3 (mostly gaseous, small "rocky" core?)

• No "surface" like the terrestrial planets: only clouds visible



• Atmosphere: Mostly H, He (plus CH4, NH3, H2O, NH4HS, ...)


• Rings! Billions of icy moonlets all orbiting togetherAstro 202 22

Observations

!Saturn is the most distant planet that can be (easily) seen without a telescope

!Through a small telescope, Saturn's yellow clouds and spectacular rings can be seen

!Features in Saturn's atmosphere are much more subtle than in Jupiter's

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Space Missions

!3 robotic spacecraft have flown past Saturn

!Cassini has been orbiting Saturn since 2004

Mission ! Dates! Goals and Results

Pioneer 11! 1979! First Saturn flyby; studied magnetic field, clouds

Voyager 1! 1980! Flyby; high resolution imaging and other studies


Cassini! 2004-! Orbiter; imaging, spectroscopy, magnetic fields,

! ! satellite radar mapping;

! ! Huygens entry probe landed on Titan: Jan. 2005

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Features in Saturn's Atmosphere

!Zones, belts, and storms like Jupiter

!But smaller, lower contrasts, and shorter lifetimes

Saturn rotation

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Atmospheric Circulation

! Saturn has stronger equatorial winds than Jupiter

! Wind patters & "shear" different than Jupiter's

max > 1800 mph! Astro 202 26


!Determined by spectroscopy

from telescopes and spacecraft!Similar to Jupiter, but less H2O, more C–H


Hydrogen H2 94

Helium He 6

Methane CH4 8 x 10-4

Ammonia NH3 2 x 10-4

Deuterated Hydrogen HD 5 x 10-5

Deuterated Methane CH3D 2 x 10-5

Ethane C2H6 5 x 10-6

Phosphine PH3 1 x 10-6

Acetylene C2H2 2 x 10-8

Propane C3H8 1 x 10-10

The composition of

Saturn is also very

close to the compo-

sition of the Sun

}Minor trace gases

H2O?

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Saturn's Interior

!Estimated using same methods as Jupiter...

!Shape of Saturn particularly revealing:

– Saturn is very oblate (a flattened sphere)

Astro 202 28

Saturn's Interior

Like Jupiter, Saturn's

interior structure is

inferred from models &

extrapolation of data from

upper cloud layers.

• Hydrogen and helium at immense pressures

and temperatures

Astro 202 29

Saturn:


! Again, examine the energy balance

! For Saturn: Outgoing ! 1.7 ! Incoming

! About the same as Jupiter, but Saturn only receives about 25% as much sunlight

– So the heat source is unlikely to be related to the solar input...

! Saturn must have a strong internal heat source

– Heat stored from planetary contraction (gravitational)?

– Heat released from interior phase changes?

– Primordial? Radioactive? ??

! Internal heating must also have a strong influence on atmospheric circulation

Astro 202 30

The Cassini

Mission to

Saturn

Power for the

instruments on Cassini

comes from a small

nuclear reactor

(~ 72 lbs of 238Pu)

Astro 202 31

Satellites

! Dutch astronomer Christian Huygens discovered Saturn's large moon Titan in 1655

– Titan has a thick and complex atmosphere!

! 17 other smaller rocky/icy moons found: 1671-1990

! Dozens more tiny outer moons found since then

! Total is now 60! (http://www.dtm.ciw.edu/sheppard/satellites/)

(More in Lecture 5) Astro 202

Rings!• Saturn's rings are

made of millions of house-sized chunks of "dirty ice", all sharing similar orbits

• Origin? Unknown!

The rings are only about 100 m thick!

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Uranus

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The Jovian Planets

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Basic Properties of Uranus



• Period of Spin around axis: ~17 hours and retrograde

• Tilt of Uranus' spin axis: 98° (extreme seasons)


• Density = 1.3 g/cm3 (data indicate mostly "rocky/icy")

• No "surface" like the terrestrial planets: clouds/haze visible



• Atmosphere: Mostly H, He, CH4--but only ~15% of planet


• Uranus has a small set of dark, dusty ringsAstro 202 36

Discovery

!Uranus was the first planet discovered by telescope

– Uranus had been observed as early as 1690 but was catalogued as a star (it moves very slowly across the sky)

– Discovered as a planet in 1781 by the musician and amateur astronomer William Herschel (1738-1822), after a systematic search of the sky with a 7 foot refractor

– Herschel wanted to name the planet Georgium Sidus (George's planet) after his patron, King George III

– But Uranus (pronounced YUR uh nus), the Greek god of the heavens, was proposed by others and the name stuck (to the delight of 12 year old boys worldwide...)

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Observations

!Uranus' disk is tiny: only about 2 to 4 arcsec

– It can be barely visible to the naked eye, but you must have very dark skies and know exactly where to look

!Uranus appears slightly blue and relatively bland through most ground based telescopes

!More detail can be seen in higher resolution HST images, or using telescopes in the infrared

! Features in Uranus' atmosphere are very small and subtle compared to those on Jupiter or Saturn

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Space Missions

!Only one encounter: Voyager 2

Mission ! Date! Goals and Results


No plans have been

made for any new

space missions to

Uranus

Astro 202

Features in Uranus' Atmosphere

!Zones and belts weak or absent

! Smaller clouds, storms do occur

Uranus rotation

HST ImagesAstro 202 40

Strange

Seasons!

!Uranus is tilted on its

side, essentially "rolling"

around the solar system!

!This is another factor that

may influence differences

between Uranus and the

other gas giant planets

!What caused this strange

tilt? UNKNOWN

– Primordial?

– Giant Impact?

– Tides?21 year time steps

1986

late 2007

~2028

Astro 202 41



from telescopes and spacecraft!Observed (upper) composition very "solar"


Hydrogen H2 83

Helium He 15

Methane CH4 2

Water H2O ???

Carbon Monoxide CO ~1 x 10-5?

Hydrogen Disulfide H2S 1 x 10-6

Ammonia NH3 2 x 10-7

C2H4 7 x 10-7

CH3NH2 3 x 10-7

• The ratio of H2/He in

Uranus' atmosphere is

almost identical to that

in the Sun

• Much more He and

CH4 than detected on

Jupiter or Saturn

• Implies that Uranus

atmosphere still "well

mixed" (heavier He did

not differentiate?)

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Uranus' Interior: An “Ice Giant”• Slightly higher density, higher

Helium abundance, and lower

internal pressures & temperatures

argue that Uranus and Neptune have

very different interior structures

than Jupiter and Saturn

• "ice" and "rock" are much larger

fractions of Uranus and Neptune

• "ice" here means

volatiles like CH4,

NH3, H2O

• "rock" means

silicate minerals

like in the

terrestrial planets

Astro 202 43


!Again, examine the energy balance

!For Uranus: Outgoing = Incoming

!Unlike Jupiter & Saturn (& Neptune), Uranus

doesn't generate its own internal heat

!Why not???

!Lack of internal heating may explain the rarity

of clouds & other "weather" on Uranus

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Satellites

!Uranus has 5 "regular" large satellites, discovered by

telescopic observations between 1787-1948

– All are relatively dark and icy (based on spectroscopy)

! 10 smaller moons discovered by Voyager 2 (1986)

! 12 more tiny outer moons discovered since then

!Naming scheme based on Shakespeare and Pope

More details: Lecture 5

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Rings!

• Uranus has a system of

9 dark "main" rings

• These rings are also

surrounded by belts of

fine-grained dust

• Discovered in 1977 by

watching a star "blink

out" behind them

• Made of organics+ices?Astro 202 46

Neptune

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The Jovian Planets

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Basic Properties of Neptune


• Orbital period: 165 years; eccentricity: 0.01

• Period of Spin around axis: ~16 hours

• Tilt of Neptune's spin axis: 29° (seasons)


• Density = 1.6 g/cm3 (mostly "rocky/icy")

• No "surface" like the terrestrial planets: clouds/haze visible


• Cloud-top temperature: 60 K (-213°C) [same as Uranus!]

• Atmosphere: Mostly H, He, CH4


• Neptune has a set of thin, dark rings

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Discovery

! Neptune was discovered by telescope in 1846

– Galileo saw Neptune in 1613, but thought it was a star!

! Neptune's existence was predicted based on mathematical studies of small perturbations in the orbits of Uranus, Saturn, and Jupiter

– Mathematicians John C. Adams & Urbain Jean Joseph Le Verrier independently predicted Neptune's position

– Johann Gottfried Galle and Heinrich Louis d'Arrest found the planet independently at different telescopes

! Lots of nasty business about who got the credit!

LeVerrier Adams d'Arrest

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A Triumph of Newtonian physics!

"Monsieur, the planet of which you

indicated the position really exists. "

Galle to Le Verrier on 25 Sept., 1846

"I thank you for the alacrity with

which you applied my instructions.

We are thereby, thanks to you,

definitely in possession of a new

world."

Le Verrier's reply

Astro 202 51

Observations

of Neptune

!Neptune's disk is tiny: only about 2 arcsec diameter

– You need a telescope to see Neptune

– Even then, it's challenging to see any details

!Neptune appears slightly blue through telescopes

!More detail can be seen in higher resolution HST images, or using telescopes in the infrared

! Features in Neptune's atmosphere can be seen

– More subtle then those on Jupiter or Saturn

– But much more visible than those on Uranus

small telescope Keck IR HST

Astro 202 52

Space Missions

!Only one encounter: Voyager 2

Mission ! Date! Goals and Results


No plans have been

made for any new

space missions to

Neptune

Astro 202 53

Features in Neptune's

Atmosphere

!Zones & belts stronger than Uranus'

! Smaller clouds, storms apparent

!Clouds, storms changed since the 1989 Voyager flyby

Neptune rotation

HST Images

"Great Dark Spot"

1989 Voyager Images Astro 202 54



from telescopes and spacecraft!Observed (upper) composition very "solar"


Hydrogen H2 85

Helium He 13

Methane CH4 .01 to 1?

Water H2O ???

Carbon Monoxide CO ~1 x 10-6

C2H6 1.5 x 10-6

C2H2 6 x 10-8

HCN ~1 x 10-9

• The ratio of H2/He in

Neptune's atmosphere

is close to the Sun's

• Much more He than

Jupiter & Saturn, but

comparable CH4

• Neptune's atmosphere

differs from Uranus';

different internal

heating/mixing?

Astro 202 55

Neptune's Interior: Another “Ice Giant”

Remember:

• "rock" means

silicate minerals

like in the

terrestrial planets

• "ice" here means

volatiles like CH4,

NH3, H2O

• Slightly higher density, higher

Helium abundance, and lower

internal pressures & temperatures

argue that Uranus and Neptune have

very different interior structures

than Jupiter and Saturn

• "ice" and "rock" are much larger

fractions of Uranus and Neptune

+ helium

Astro 202 56


!What is Neptune's energy balance?

!For Neptune: Outgoing = 3 ! Incoming

!Neptune generates its own internal heat!– Primorial heat of formation?

– More radioactive "rocky" materials?

– But why is Uranus so different?

!May explain the greater variety and variability of clouds compared to Uranus

Astro 202 57

Satellites

of Neptune

!1 large satellite, Triton

– Also discovered in 1846

– Similar size as Earth's Moon

– But orbits backwards compared to Neptune's spin...!

– Has an atmosphere and active geysers on the surface!

!1 mid-size satellite, Nereid

– Nereid has a very eccentric orbit (e=0.75)

!6 smaller moons discovered by Voyager 2 in 1989

!5 irregular outer satellites found since then...

More details: Lecture 5

Nereid

Proteus

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Rings!

• Neptune has rings! - 4 main rings

! - 3 narrow, 1 diffuse

! - all very dark

• Discovered from Earth

by star occultations

• Once thought to be

"arcs" not complete rings

• But Voyager images

showed them to be full

rings, with "clumps"

• Made of organics+ices?

Astro 202

! Jupiter and Saturn are “gas giant” planets, made mostly of H & He in the same relative abundances as the Sun

! Uranus and Neptune are “ice giant” planets of H, He, & CH4, with gaseous outer shells

! Jupiter and Saturn have vigorous atmospheres with long-lived belts, zones, & storms

! The atmospheres of Uranus and Neptune are active, but much less so than Jupiter and Saturn

! Jupiter and Saturn’s interiors consist of liquid and metallic hydrogen, at huge temperatures and pressures

! Uranus and Neptune’s interiors are mostly “icy” and “rocky” material at very high pressure, temperature

Summary (1)

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Summary (2)

! All four giant planets have complex ring systems

! Between them, the four giant planets have at least 163 moons, including:

– Europa, which probably has a subsurface ocean

– Io, the most volcanically-active world in the solar sys.

– Titan, which has a thick, hydrocarbon-rich atmosphere

! What an incredible diversity of worlds!

Next TimeOverview of the Icy Moons, Comets, and Kuiper Belt Objects

Reading:

Skim NSS 17-24

Read PBD 5

Documents

Jupiter , Saturn, Uranus, Neptune - Cornell Universityastrosun2.astro.cornell.edu/academics/courses/astro202/A202_2008_Lec04.pdf · Uranus' disk is tiny: only about 2 to 4 arcsec