Introduction to the Introduction to the Solar SystemSolar System

Chapter 6Chapter 6

The Solar System

Ingredients?

The Solar System

Ingredients?

● 1 Star: the Sun

● 8 Planets + a few “minor planets”

● 126 moons around these planets

● Asteroids, meteoroids, comets

● A lot of nearly empty space

QuestionsQuestions What percentage of the total mass of the What percentage of the total mass of the

solar system does the Sun contribute?solar system does the Sun contribute?

How is the solar system laid out in space? How is the solar system laid out in space? Spacing between planets? Orbital Spacing between planets? Orbital directions?directions?

Mass in Solar SystemMass in Solar System

Sun 99.8%

Jupiter 0.1%

Comets 0.05%

All Other Planets 0.04%

Earth 0.0003%

Sun, Planets and Moon to scale

Sun accounts for 99.9% of solar system mass!

Solar System Solar System TemperaturesTemperatures

Planet Distance Temperature(top of atmosphere)

Mercury 0.38 AU 450 K

Venus 0.72 AU 330 K

Earth 1.00 AU 280 K

Mars 1.52 AU 230 K

Jupiter 5.20 AU 120 K

Saturn 9.54 AU 90 K

Uranus 19.22 AU 60 K

Neptune 30.06 AU 50 K

Pluto 39.5 AU 40 K

45 F

-390 F

350 F

Comparative Comparative PlanetologyPlanetology

Categorize planets by propertiesCategorize planets by properties

Compare similarities and differencesCompare similarities and differences

Ask: What physical processes can explain Ask: What physical processes can explain these properties?these properties?

6.2 Planetary Properties

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6.3 The Overall Layout of the Solar SystemAll orbits paths are close to the ecliptic plane

Pluto’s orbit does not (17° tilt)

Planet OrbitsPlanet Orbits

Planet OrbitsPlanet Orbits

Orbits aligned in same plane (the ecliptic)Orbits aligned in same plane (the ecliptic) Explains why planets always found in ZodiacExplains why planets always found in Zodiac Pluto’s orbit tipped the most (17 degrees)Pluto’s orbit tipped the most (17 degrees)

All planets orbit Sun counter-clockwiseAll planets orbit Sun counter-clockwise

Planets rotate counter-clockwisePlanets rotate counter-clockwise except Venusexcept Venus

Rotation axis roughly perpendicular to orbitRotation axis roughly perpendicular to orbit except Uranus and Plutoexcept Uranus and Pluto

The Terrestrial PlanetsThe Terrestrial Planets

Terrestrial PlanetsTerrestrial Planets

Terrestrial = Terrestrial = Earth-likeEarth-like MercuryMercury VenusVenus Earth (and Moon)Earth (and Moon) MarsMars

Small, low massSmall, low mass

No large moons (except Earth) No large moons (except Earth) Mars has two small ones…Mars has two small ones…

Close to SunClose to Sun

Terrestrial PlanetsTerrestrial Planets Rocky SurfaceRocky Surface

High density (3-5 gm/cmHigh density (3-5 gm/cm33) ) (water = 1 gm/cm(water = 1 gm/cm33))

Geologic Activity (volcanoes, continental Geologic Activity (volcanoes, continental drift)drift) Present on larger planets (Earth and Venus)Present on larger planets (Earth and Venus) Absent on smaller planets (Moon, Mercury, and Absent on smaller planets (Moon, Mercury, and

Mars)Mars)

AtmosphereAtmosphere Little hydrogen and heliumLittle hydrogen and helium Mostly carbon dioxide (Venus and Mars)Mostly carbon dioxide (Venus and Mars)

or nitrogen (Earth) or nitrogen (Earth) Smaller planets have no atmosphere (Mercury, Smaller planets have no atmosphere (Mercury,

Moon) Moon)

Origin of Pluto

Large member of a class of objects in the outer reaches of the Solar System:

The Kuiper Belt Objects

100's found since 1992.

Orbits tend to be more tilted, like Pluto's.

Leftover planetesimals from Solar System formation?

AsteroidsAsteroids

Mars

The Asteroid Belt

Asteroid Belt

Perhaps a planet was going to form there. But Jupiter's strong gravity disrupted the planetesimals' orbits, ejecting them out of Solar System. The Belt is the few left behind.

And Finally . . .

Remaining gas swept out by intense period of solar wind activity.

The Jovian PlanetsThe Jovian Planets

Jovian PlanetsJovian Planets

Jovian = Jovian = Jupiter-likeJupiter-like JupiterJupiter SaturnSaturn UranusUranus NeptuneNeptune

Large, massiveLarge, massive

Many moonsMany moons

Far from SunFar from Sun

Jovian PlanetsJovian Planets

Low density (1 gm/cmLow density (1 gm/cm33))

No obvious surfaceNo obvious surface

AtmosphereAtmosphere Mostly hydrogen and heliumMostly hydrogen and helium Other gases (methane, ammonia)Other gases (methane, ammonia)

may form icesmay form ices

The Outer Solar The Outer Solar SystemSystem

Comets

Kuiper Belt and Oort Cloud

Let’s consider a scale model of the Solar System!

6.4 Terrestrial and Jovian PlanetsRelative sizes of the Sun & Planets

It would take 109Earths to span the Sun!

6.4 Terrestrial and Jovian Planets

Terrestrial planets:

Mercury, Venus, Earth, Mars

Jovian planets:

Jupiter, Saturn, Uranus, Neptune

Pluto is neither but a new class called the

Dwarf planets

6.4 Terrestrial and Jovian PlanetsDifferences (Comparative Planetology) between the terrestrial planets:

• Atmospheres and surface conditions are very dissimilar

• Only Earth has oxygen in atmosphere and liquid water on surface

• Earth and Mars rotate at about the same rate; Venus and Mercury are much slower, and Venus rotates in the opposite direction

• Earth and Mars have moons; Mercury and Venus don’t

• Earth and Mercury have magnetic fields; Venus and Mars don’t

The image at right shows apicture of the Sun. The darkspots located on this imageare sunspots. How does thesize of Earth compare to thesize of the sunspot that isidentified on the right side ofthe image of Sun?

A) Earth and the sunspotare about the same size.B) The sunspot is muchlarger than Earth.C) The sunspot is muchsmaller than Earth.

Sunspot

If you were constructing a scale model of the solar system that used a Sun that was the size of a basketball (approximately 12 inches in diameter), which of the following lengths would most closely approximate the scaled distance between Earth and the Sun?

A) 3 feet (length of an outstretched arm)B) 10 feet (height of a basketball goal)C) 100 feet (height of an 10 story building)D) 300 feet (length of a football field)

QuestionsQuestions What are some of the smaller objects (or What are some of the smaller objects (or

debris) found in the solar system?debris) found in the solar system?

What information do they contain that the What information do they contain that the planets and moons do not?planets and moons do not? (Hint: What effects do erosion, geological (Hint: What effects do erosion, geological

activity, vulcanism, etc. have on a planet?)activity, vulcanism, etc. have on a planet?)

QuestionsQuestions What are some of the smaller objects (or What are some of the smaller objects (or

debris) found in the solar system?debris) found in the solar system?Comets, asteroids, meteoroidsComets, asteroids, meteoroids

What information do they contain that the What information do they contain that the planets and moons do not?planets and moons do not?Solar system debris is unevolved => gives Solar system debris is unevolved => gives

direct evidence of conditions during solar direct evidence of conditions during solar system formation!system formation!

Solar System DebrisComets

Comet Halley (1986) Comet Hale-Bopp (1997)

Short Period Comets Long Period Comets

50-200 year orbits

Orbits prograde, close to plane of Solar System

Originate in Kuiper Belt

Few times 105 or 106 year orbits

Orbits have random orientations and large ellipticities

Originate in Oort Cloud

Oort Cloud is a huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects?

A passing star may redirect Oort cloud objects, creating long period comets.

Kuiper Belt object can be redirected by Neptune, creating a short-period comet.

QuestionQuestion What causes the tail of a comet?What causes the tail of a comet?

(Hint: The tail always points directly away (Hint: The tail always points directly away from the sun.)from the sun.)

Nucleus: ~10 km ball of ice, dust

Coma: cloud of gas and dust around nucleus (~106 km across)

Tail: Always points away from Sun.

Comet Structure

Coma and tail due to gas and dust removed from nucleus by the Solar Wind.

Far from Sun, comet is a nucleus only.

Comet TrajectoryComet Trajectory

Meteor Showers

Comets break up when near Sun - solar wind, evaporation, tidal force.

e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years.

Debris spreads out along comet orbit.

Intersection of orbits => meteor shower

How did the Solar System Form?

What must be explained?

● Solar system is very flat.

● Planetary orbits are nearly circular.

● Almost all moons and planets (and Sun) rotate and revolve in the same direction.

● Planets are isolated in space.

● Terrestrial - Jovian planet distinction.

● Leftover junk (comets and asteroids).

Solar Nebula Start with rotating cloud of gas Start with rotating cloud of gas

and dustand dust

Collapses because of gravityCollapses because of gravity spins fasterspins faster flattens into disk-shapeflattens into disk-shape gets hottergets hotter

Sun forms in centerSun forms in center

Temperature decreases Temperature decreases outwardoutward

As nebula cools, gas condensesAs nebula cools, gas condenses Forms solid particles (dust Forms solid particles (dust

grains)grains)

Nebular TheoryNebular Theory• Nebula: Cloud of interstellar dust and gas Nebula: Cloud of interstellar dust and gas

about a light-year acrossabout a light-year across

• Condensing cloud heats up - star forms at Condensing cloud heats up - star forms at centercenter

• But why is solar system flat?But why is solar system flat?Conservation of Angular Momentum!

Ang. Mom. = mass x rotation speed x “size”Ang. Mom. = mass x rotation speed x “size”

Conservation of Conservation of angular momentumangular momentum

(Demo)(Demo)

So, as nebula contracted it rotated faster.

It became a flattened disk, like a pizza crust. (Centrifugal hoops demo)

But, clumps in rotating gas tend to disperse. Need modified theory.

Solar Nebula: 98% of mass is gas 2% in dust grains

Condensation theory:

1) Dust grains act as "condensation nuclei. Also radiate heat => help to cool gas => faster gravitational collapse.

2) Accretion: Clumps collide and stick

3) Gravity-enhanced accretion: objects now have significant gravity => faster growth

Forming Planets Dust grains stick Dust grains stick

together together form rocksform rocks

Grow into planetesimalsGrow into planetesimals some still survive todaysome still survive today

asteroidsasteroids cometscomets

Larger planetesimals Larger planetesimals attract smaller ones attract smaller ones (gravity)(gravity)

Planetesimals accretePlanetesimals accrete form protoplanets / planet form protoplanets / planet

corescores initially coldinitially cold

Collisions become violentCollisions become violent heating melts protoplanetheating melts protoplanet differentiation occursdifferentiation occurs

Forming Jovian PlanetsForming Jovian Planets

Snow lineSnow line Location beyond which ices formLocation beyond which ices form

Building blocks (solids)Building blocks (solids) both silicates both silicates and icesand ices

Protoplanets / planet core Protoplanets / planet core grew larger grew larger gravity captured hydrogen & heliumgravity captured hydrogen & helium

composition similar to Suncomposition similar to Sun gaseous accretion disk forms around planetgaseous accretion disk forms around planet

Moons form in disk around planetMoons form in disk around planet

Evolution of the Solar Evolution of the Solar SystemSystem

Collisions dominate early-onCollisions dominate early-on produces early heavy bombardmentproduces early heavy bombardment comets collide with terrestrial planetscomets collide with terrestrial planets

Deposit volatiles that form atmosphereDeposit volatiles that form atmosphere(water, carbon dioxide, etc.)(water, carbon dioxide, etc.)

Planets sweep up / throw out remaining Planets sweep up / throw out remaining planetesimalsplanetesimals Ones thrown out:Ones thrown out:

Oort cloudOort cloud Ones that remain:Ones that remain:

Comets (Kuiper belt)Comets (Kuiper belt) Asteroids (asteroid belt)Asteroids (asteroid belt)

Planetary EjectionPlanetary Ejection

Planetary Evolution - Planetary Evolution - GeologicalGeological

Internal heating leads to geologic activityInternal heating leads to geologic activity volcanism, tectonicsvolcanism, tectonics active worldsactive worlds

As core cools & solidifies, activity slows, As core cools & solidifies, activity slows, eventually stopseventually stops e.g. Moone.g. Moon

Earth, Venus large enough to still be activeEarth, Venus large enough to still be active

Planetary Evolution - Planetary Evolution - AtmosphereAtmosphere

Atmosphere formed by Atmosphere formed by gases escaping from interior gases escaping from interior impacts of comets (volatile-rich debris)impacts of comets (volatile-rich debris)

Fate of water depended on temperature Fate of water depended on temperature (distance from Sun)(distance from Sun)

Atmospheres changed chemically over timeAtmospheres changed chemically over time

Life on Earth substantially changed the Life on Earth substantially changed the atmosphereatmosphere