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Chapter 7Our Planetary System

Earth, as viewed by the Voyager spacecraft

7.1 Studying the Solar System

• Our goals for learning– What does the solar system look like?

– How has it evolved over time?How has it evolved over time?

– What are the major features of the Sun and planets?

– What patterns can we find?

– Can we begin to create an organized science of the planets?

Fig. 7.1cartoon

What does the solar system look like?

Actually

Voyager“family portrait”Image from 1999

Looking “down”

• We’re looking for patterns– 1 thermonuclear object– Eight major planets with

nearly circular orbits moving counterclockwise.

– Most of what we see is in a single plane extending outward from the Sun’s equator (the ecliptic)

– Thousands (millions?) of minor objects, like Pluto, which are smaller and mostly located in the outer solar system.

Planetary Science

• Old term is “comparative planetology”

• Rationale used to be “we can learn more about Earth by studying other worlds in the solar system”.

• Now: Simply understanding planetary processes

• Focus on processes yields generalized knowledge of worlds Instead of memorizing facts about a particular world.

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ScalingExercise

For ASU, the Sun to Neptune distanceequals from here tothe student union

See pages 8-10 in Chapter 1 grapefruit-sized

Localized Scaling ExerciseLocalized Scaling Exercise

We are here…We are here…

The mass of the entire The mass of the entire Solar System would fit inSolar System would fit ina large soda cup.a large soda cup.

That soda cup in the middle weighs almost as much asthe real (unscaled) Moon!

Localized Scaling ExerciseLocalized Scaling Exercise

The remainder isThe remainder isscattered across anscattered across anarea roughly thearea roughly thesize of the entiresize of the entirecampus.campus.

Week 9

• Tour of Chapter 7: Planetary Overview

• Chapter 8: Formation of the Solar System

Life Cycles• The Solar System as we see today is the last

frame of a long movie• How did it get to this point?

Life Cycles• We can say with certainty that we

are looking at a butterfly• Because we have seen the caterpillar and the pupae

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Scale Model of Sizestraditional planets

Scale Model of SizesSun and traditional planets

Scale Model of SizesScale Model of SizesAll large objectsAll large objects

Scale Model of SizesScale Model of SizesAll large objectsAll large objects

Scale Model of distancesScale Model of distancesNOTE: logarithmic scale in AUNOTE: logarithmic scale in AU

Scale Model of distancesScale Model of distancesNOTE: logarithmic scale in AUNOTE: logarithmic scale in AU

http://en.wikipedia.org/wiki/Graphical_timeline_of_our_universe

Sun

• Over 99.9% of solar system’s mass• Made mostly of H/He gas (plasma)• Converts 4 million tons of mass into energy each second

Mercury

• Made of metal and rock; very large iron core • Desolate, cratered; long, tall, steep cliffs• Very hot and very cold: 425°C (day), –170°C (night)

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Venus

• Nearly identical to Earth: Size, Composition, Density• Surface hidden by CO2 clouds with acid rain.• Runaway greenhouse effect: Hotter than Mercury: 470°C, day and night

Earth

• Only surface liquid in the Solar System• Liquid metal core with rocky surface• Proportionately large moon – a true binary planet

Earth and Moon to size scale

Mars

• Appears Earth-like, but only 6 millibars surface pressure• Small frozen metal core with rocky surface.• Giant volcanoes and canyon. Polar caps, seasons, etc.• Evidence of flowing water

• Nearest gas giant

• Mostly H/He; no solid surface

• Density increases with

JupiterJupiter

depth. May have a terrestrial core

• Many moons, rings…– Mini-solar system

• May be most common type of planet

Jupiter’s moons interesting as planets in themselves, especially the four Galilean moons

• Io (shown here): Active sulfur volcanoes all over• Europa: Confirmed liquid ocean under surface ice crust• Ganymede: Largest moon in solar system• Callisto: Cratered ice ball

Saturn

• Gas giant like Jupiter (H & He). Density < Water.• Most prominent rings• Moons include Titan w/ 1.5 bar surface atmosphere• Cassini spacecraft currently mapping the system

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• Smaller than Jupiter/Saturn; but still much larger than Earth

• Made of H/He

UranusUranus

• Made of H/He gas & hydrogen compounds (H2O, NH3, CH4)

• Rotates on side

• Moons and rings also tilted!

• Another medium-sized gas giant

• Similar chemistry to Uranus

NeptuneNeptune

• Most distant major planet

• Likely host to a captured KBO in the moon Triton

PlutoPluto

• Newly designated type for Minor Planets• Ice and rock (comet-like) composition w/ 3 moons• Large moon Charon similar in size – binary system• New Horizons mission arrives in 2016 (13 miles/second)

What are the most common patterns of the Sun and planets?

Sun and planets to scale

7.2 Patterns in the Solar System

• Our goals for learning– What features of the solar system provide clues

to how it formed?

– All orbit in the same direction

– Obey Kepler’s 3rd Law

– Angular Momentum conserved

– All created at the same time

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What features of the solar system provide clues to how it formed?

Motion of Large Bodies

• All large bodies in the solar system orbit in the samethe same direction and in nearly the same plane

• Most also rotate in that direction

Two Primary Planet Types

• Terrestrial planets are rocky, relatively small,

d l t th Sand close to the Sun

• Jovian planets are gaseous, larger, and farther from Sun

Swarms of Smaller Bodies

• Many rocky asteroids, rocky-ice Kuiper Belt Objects (KBOs) and i licy comets populate the solar system

Rosetta Stones

• Entire Uranian system (planet, rings & (p gmoons) are all knocked on their sides

• Likely from a megaimpact before the planetesimal period

• Venus flipped almost 180°

7.3 Robotic Exploration

• Census• Description• Explanation• Understanding• Hardware:

FlybysOrbitersLanders/ProbesSample Returns

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Stepping out

Pre-1970’s: Planets were astronomical objectsPost-1970’s: Planets have become geologic objects

The Golden Age of Planetary Exploration is right nowCensus, Description, Explanation & Understanding

Flybys• Technically

simplest way to get close up. Flys past planet just once

• Cheaper than other mission types but have less time to gather data

• First lunar flyby in 1959

Orbiters• Go into orbit around

another world (Moon, 1966)

• Full planetary coverage• More time to gather dataMore time to gather data

but limited high resolution data about world’s surface.– Still hundreds of miles

away• Spectroscopy techniques

perfected

Probes or Landers

• Initially “impact” landers (1962)• Initially impact landers (1962)– Suicide missions but got real close early on

• Land on surface of another world (Venus, 1965)

• Goal to explore surface in detail – Mars, 1997-present– Titan, 2006– Asteroids, 2004-present

Sample Return Missions• Gather samples from another body,

return them to Earth

• Apollo/Luna missions (Moon), and Stardust (Comet P/Wild 2) are only

l i i dsample return missions to date

• Mars sample return likely next…

Biologically dangerous?Perhaps redundant

Current StatusCurrent Status

•• SpiritSpirit and and OpportunityOpportunity rovers active on rovers active on surface of Mars. surface of Mars. Global SurveyorGlobal Surveyor in orbitin orbit

•• CassiniCassini orbiter at Saturnorbiter at Saturn

•• SpiritSpirit and and OpportunityOpportunity rovers active on rovers active on surface of Mars. surface of Mars. Global SurveyorGlobal Surveyor in orbitin orbit

•• CassiniCassini orbiter at Saturnorbiter at Saturn

•• Messenger at MercuryMessenger at Mercury

•• New HorizonsNew Horizons in route to Kuiper Beltin route to Kuiper Belt

•• UlyssesUlysses solar polar mission is out of the solar polar mission is out of the plane of the solar systemplane of the solar system

•• Both Both VoyagersVoyagers headed for interstellar spaceheaded for interstellar space

•• Messenger at MercuryMessenger at Mercury

•• New HorizonsNew Horizons in route to Kuiper Beltin route to Kuiper Belt

•• UlyssesUlysses solar polar mission is out of the solar polar mission is out of the plane of the solar systemplane of the solar system

•• Both Both VoyagersVoyagers headed for interstellar spaceheaded for interstellar space

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Where are the Voyagers?Where are the Voyagers?

Overlay of Oort Cloud of comets @ 1 to 10 BillionOverlay of Oort Cloud of comets @ 1 to 10 BillionOverlay of Oort Cloud of comets @ 1 to 10 BillionOverlay of Oort Cloud of comets @ 1 to 10 Billion

Launched in 1977,may cross heliopause

in 2008

Chapter 8: How did it form?

1. Any credible theory of solar system formation must explain…

2. Patterns of motion of the large bodies • Orbit in same direction and planeOrbit in same direction and plane

3. Existence of two types of planets• Terrestrial and jovian

4. Existence of smaller bodies• Asteroids, comets, Kuiper Belt objects

5. Notable exceptions to usual patterns• Rotation of Uranus, Earth’s moon, etc.

What theory best explains the features of our solar system?

• The nebular theory states that our solar system formed from the gravitational collapse of a giant interstellar gas cloud—the solar nebula

(Nebula is the Latin word for cloud)(Nebula is the Latin word for cloud)

• Kant and Laplace proposed the nebular hypothesis over two centuries ago

• A large amount of evidence now supports this idea…especially some Hubble images

• Older references will still discuss the planetesimal or close encounter theory.

8.2 The Birth of the Solar System

• Our goals for learning– A review of basic physical principles

– Where did the solar system come from?

– What caused the orderly patterns of motion in our solar system?

Review 1• Gravity

– Mass will be attracted to other masses proportionally to the masses involvedM ill t t i f t t– Mass will concentrate in areas of greatest gravitational attraction (usually the center of mass)

– Sun’s gravity actsthroughout the solar system

Review 2

• Density–Mass per unit volume

For a given composition density–For a given composition, density increases directly with mass

–Phases Changes arereflections of densityGases ↔ Liquids ↔ Solids

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Review 3• Temperature

– A measure of molecular motion Absolute Zero = temperature at which all molecular motion stops

• Phases are directly related to temperature: Solids = frozen liquids Liquids = condensed gas Gases = free clouds of

atoms or molecules

Review 4• Pressure

– A measure of force per unit areaHeated molecules move faster more force

•Pressure changes most readily in a gas, least so in a solid.

Review 5• Temperature & Pressure

– Changing temperature changes pressure

• Increasing pressure in a gas (compressing) causes it to heat up.

• Releasing pressure in a gas causes it to cool down.

Review 6• OK, maybe this isn’t a review, but…

• Phase Diagrams

Ph Di• Phase Diagrams show where and how phase changes occur

• Every natural material has a phase diagram

• Triple Point• Critical point

Review 7•Chaos Theory

– Self-organization– Phase changes are an

example of self-organizationp g– The stability of an

equilibrium distribution is a consequence of the fact that individual events are random and independent of other events. Individual chaos therefore implies collective determinism. -Heinz Pagels

Where did the solar system Where did the solar system come from?come from?

Cosmos animationof solar system

formation

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Galactic Recycling

• Elements that form planets were made in 1st

generation stars and then recycled through interstellar space to form 2nd

generation stars & their planets

Evidence from Hubble

• We can see stars forming in otherforming in other interstellar gas clouds, lending support to the nebular theory

Examples of proplyds(Protoplanetary disks)in Orion

Conservation of Angular Momentum

• Rotation speed of the cloud from which our solar

f dsystem formed must have increased as the cloud contracted

• Starting with a hemispherical cloud: Random collisions between particles will eventually form a disk with a random orientation.

Flattening

Slightly dominantdirection vector will

take up angularmomentum which isconserved into the

present day.

• Form because they survive.

• Collisions between d d

“Circular” Orbits

gas and dust particles in cloud reduce random motions.

• Particles NOT on collision courses survive.

Disks around other Stars

• Observations of disks around other stars support the nebular hypothesis

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What have we applied?• Solar System Formation?

– Galactic recycling built the elements from which planets formed.

– We can observe stars forming in other gas clouds.

• Orderly patterns of motion?– Solar nebula spun faster as it contracted because of

conservation of angular momentum– Collisions between gas & dust particles caused the

nebula to flatten into a disk– We have observed such disks around newly forming

stars

8.3 Formation of Planets

• Our goals for learning– Why are there different types of planets?

H did t t i l l t f ?– How did terrestrial planets form?

– How did jovian planets form?

– What ended the era of planet formation?

Why are there different types of planet?

Numerous small particles build big ones

Coalescence of particles

In-falling volatiles sublimated….ergo, The Frost Line

Inner parts of disk are hotter than outer parts.

Rock can be solid at muchsolid at much higher temperatures than ice.

Fig 9 5

The Frost Line

Inside the frost line: Too hot for hydrogen compounds to form ices.

Outside the frost line: Cold enough for ices (CH4 or NH3) to form.

Fig 9.5

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How did terrestrial planets form?

• Small particles of rock and metal were present inside the frost linePl t i l f k d t l b ilt• Planetesimals of rock and metal built up as these particles collided

• Gravity eventually assembled these planetesimals into terrestrial planets

•Small particles of rock and metal present inside the frost line

•Planetesimals of rock and metal built up as these particles collided

•T-tauri stage blows volatiles out of inner solar system.

•Gravity accretes planetesimals into terrestrial planets

Accretion of Planetesimals

• Many smaller objects collected into just a few large ones = accretion

• Individual planets differentiated into layers with densest materials at core

How did jovian planets form?

• Ice crystals forms small particles outside the frost line.

• Larger planetesimals and planets formLarger planetesimals and planets form by sweeping out larger area of orbit.

• Metal particles form cores of gas giants

• Gravity of these larger planets was able to draw in surrounding H and He gases.

Gravity of rock

GoingJovian

Gravity of rock and ice in jovian region draws in H and He gases

Moons of jovian planets form in miniature disks

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O tflo ing

Onset ofSolarWind

Outflowing matter from the Sun -- the solar wind --blew away the leftover gases

Solar Rotation

• In nebular theory, young Sun was spinning much faster p gthan now

• Friction between solar magnetic field and solar nebular slowed rotation over time

Summary• Why multiple types of planets?

– Ices sublimate inside the frost line– Gases blown out by solar wind– Rock, metals, and ices/gases condensed outside the

frost lineH did h i l l f ?• How did the terrestrial planets form?– Rock and metals collected into planetesimals– Planetesimals then accreted into planets– Planets differentiated into core, mantle, crust

• How did the jovian planets form?– Additional supply of ice particles and gases outside

frost line made planets there more massive– Gravity of these massive planets drew in H, He gases