Source Material for This Week

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Source Material for This Week

• Wednesday Friday (Lecs 23 24)
• What are the phases of matter? section of CP
  5.3
• pp. 159-162
• CP 8.1-8.4 Formation of the Solar System

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Source Material for next week

• Monday (Lec 25)
• Introduction to Light Matter
• CP 5.1-5.2 Introduction to Light
• CP 5.3 Properties of Matter
• Youve already read What are the phases of
  matter?. Read the rest now (structure of matter,
  energy stored in atoms)
• Wednesday Friday (Lecs 26 27)
• CP 5.4 Learning from Light
• CP 5.5 Using the Doppler Effect

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Formation of the Solar SystemThe Big Picture

• Application of Physics How did our Solar System
  Form?
• Temperature vs. Heat
• Phases of Matter
• Solar Nebula Theory to explain general features /
  characteristics of SS

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9. Formation of the Solar System
The evolution of the world may be compared to a
display of fireworks that has just ended some
few red wisps, ashes, and smoke. Standing on a
cool cinder, we see the slow fading of the suns,
and we try to recall the vanished brilliance of
the origin of the worlds.
George Lemaître (1894 1966) Astronomer and
Catholic Priest
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Characteristics of the Solar SystemTypes of
Planets How do we know?

• Locations of planets
• Motions / positions in night sky (Keplers 3rd
  Law!)
• Presence of moons/rings
• Telescopic views / images from visiting
  spacecraft
• Detailed surface / atmosphere composition
• Find out what surface / atmosphere are made of
  using spectra and/or visits (more later)

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Characteristics of the Solar SystemTypes of
Planets How do we know?

• Mass
• See how planets gravity affects the motion of
  something small (moon, satellite)
• Size angular size distance
• Angular size
• Measure angle on sky
• Distance
• Use orbit period Keplers 3rd Law
• Radar ranging (return travel time of radio
  signals which travel at speed of light)

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Characteristics of the Solar SystemTypes of
Planets How do we know?

• Density/makeup of planets
• Density mass / volume
• Size ? volume
• Infer what is inside planets from density
• What we see on the surface may not match what
  lies deeper!

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Clicker Question

• We measure the density of a planet by observing
  to see whether its surface is rocky or icy, then
  using what we know about the density of rock
  ice.
• True
• False

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Characteristics of the Solar SystemTypes of
Planets Summary

• Patterns in characteristics ? 2 categories

• Jovian
• Mostly Gaseous
• Large
• Far from Sun
• Massive
• LowDensity
• Lots of Moons
• Rings

• Terrestrial
• Mostly Rocky
• Small
• Close to Sun
• Lower mass
• Higher density
• Few large moons
• No rings

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Characteristics of the Solar SystemShapes and
Motions How do we know?

• All planets orbit in same plane/direction
• Always see planets near ecliptic
• Motions / positions of planets in night sky
• Rotational (spin) motions mostly same direction
• Telescope views of the surfaces
• Radar measurements (more later)
• Monitor magnetic field (more later)
• Orbits of moons are in the planets equatorial
  plane
• Telescope views

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Theory of Solar System Formation

• A story that fits the facts needs to.
• Unify details
• Explain general characteristics
• Be consistent with what we know about the rest of
  the galaxy
• Other stars and solar systems should form in the
  same way
• How do we explain the general characteristics of
  the solar system?
• Like a detective story
• Solar Nebula Theory
• The sun and solar system formed from a cold,
  dense cloud of gas and dust which collapsed

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Solar Nebula Theory Preview

• Collapsing cloud heated, flattened, spun more
  rapidly
• Dense center formed protoSun
• Solids condensed
• planetesimals combined ? terrestrial planets
• In outer SS, some grew large enough to
  gravitationally attract gas ? gas giant planets
• Solar wind cleared away small particles
• Asteroids and comets are leftover planetesimals

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Reminder Heat / Thermal Energy / Temperature

• Temperature average KE per particle.

lower T
higher T
less heat
more heat

• Heat (thermal energy) total KE of all particles

same T
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Phases of Matter

• the phases
• solid
• liquid
• gas
• plasma
• depend on bonds between atoms and/or
  molecules
• As temperature increases, bonds loosen

Click Here
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Clicker Question

• According to our theory of solar system
  formation, which law best explains why the solar
  nebula spun faster as it shrank in size?
• A) Law of conservation of angular momentum
• B) Law of conservation of energy
• C) Law of universal gravitation
• D) Einsteins law that E mc2

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Theory Origin of the Solar System
Nebular Theory Solar System formed from giant,
swirling cloud of gas dust

• As nebula collapses, it heats up, spins faster,
  and flattens
• Why and how?

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Gravitational Collapse

• Solar nebula initially spherical, few light
  years across
• very cold
• rotating slightly
• Mostly H, He
• enriched (2) in heavy elements
• Nebula pushed by some event gt began to
  collapse
• As nebula shrank, gravity increased gt material
  fell in faster

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Gravitational Collapse

• As nebula falls inward, GPE converts to heat.
• Conservation of Energy
• GPE converted to KE
• Friction converts bulk motion KE into thermal E
• Hottest near center
• As nebulas radius decreases, it rotates faster
• Conservation of Angular Momentum

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During Collapse Flattening of the Solar Nebula

• Clumps of gas collide merge
• Random velocities average into rotation
• Spinning nebula becomes disk-shaped

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Forming a disk

• Parallel to the axis of rotation
• gas collapse unrestricted
• Perpendicular to this axis
• acceleration of rotation stops further collapse,
  once it balances gravity

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STOPPING the Collapse

• Acceleration of rotation slows material infall
• Material falls around the center
• Increased pressure
• Density and pressure of compressed and heated
  material both increase
• pushes outward, resists gravity

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Explaining Orderly Motions

• Center very hot and dense a protostar
• Planets form in rest of disk
• This explains
• all planets in a plane (disk)
• all planets orbit in one direction (spin
  direction of the disk)
• Sun rotates in the same direction (Conserve Ang.
  Mom.)
• planets tend to rotate in same direction
• most moons orbit in same direction
• most planetary orbits nearly circular (collisions
  in the disk)

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CLICKER QUESTIONS

• Suppose we found a solar system with the
  following property. Surprising (A) or
  Unsurprising (B)?
• A solar system has 10 planets that all orbit the
  star in approximately the same plane. However, 5
  planets orbit in one direction, while the other 5
  orbit in the opposite direction.
• A solar system whose major planets have highly
  elliptical orbits

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Building the Planets Getting Solids

• Nebula
• gas has fallen ? hot
• GPE ? KE ? Thermal E
• hottest near protosun (fallen farthest)
• all elements gaseous
• Gas Cools
• Solids condense (i.e. solidify)
• Different materials condense at different T
• Metals condense 1000-1600 K
• Rocks condense 500-1300 K
• Ices condense lt 150 K
• H / He dont condense in nebula

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Condensation Sequence Planet Formation

• Gas Cools
• Solids condense (i.e. solidify)
• Different materials condense at different T
• Type of material available to make planets at
• each location depends on temperature at that
  
• location before gas clears out
• Solid particles clump together eventually
• form planets

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Where did the nebula go?

• Q Why isnt there still gas (H/He) between the
  planets?
• Q If the gas keeps cooling, why isnt everything
  covered with ice, from when the gas got cold?
• A The Sun becomes a star, and the Solar Wind
  turns on
• At the center, temperature and pressure became
  high enough for nuclear fusion to begin (H into
  He, converting mass into energy, Emc2)

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Where did the nebula go?
solar wind --- charged particles streaming out
from the newly formed Sun
Solar wind and light pressure cleared the
leftover gas, but not the leftover planetesimals
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Raw Materials for Planets Whats Solid Where?
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Raw Material for Planets Frost Line
Frost line 3.5 AU - ONLY rocks metals
condensed inside - BOTH rocks / metals AND ices
condensed outside
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Building the Planets Growth of larger bodies
accretion -- small grains stick togethergt
planetesimals

• planetesimals
• combine near SS center ? rocky planets
• combine far from SS center ? icy planetesimals
• larger because
• both ice and rock/metal
• ices more common
• Animation of planet growth
• comets asteroids are leftover planetesimals

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Building the Planets Gas Giants

• Largest icy bodies capture H/He
• ? gas giants
• Formed own mini-nebula

• Gravity ? shrinking
• Conservation of Energy ? heating
• Conservation of Angular Momentum ? disk
• Condensation ? grains
• Accretion into solid chunks, growth into moons

Click image for animation
Gas Giant formation mini solar system
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Clicker Question

• Terrestrial (Earth-like) planets
• are found close to the Sun because
• A) solar wind of the young Sun blew gassy
  elements out of the inner SS towards the gas
  giant planets
• B) icy planetesimals melted close to the Sun
• C) only rocky/metallic elements (not ices) could
  condense into solids near the Sun, so dense
  planets are near the Sun
• D) the gravity of the Sun dominated in the inner
  solar system, leaving no gas for the terrestrial
  planets
• E) dense materials sank to the center of the
  nebula

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Explaining Types of Planets and Moons

• Solids condensed according to temperature
• Explains
• density differences (Jovian vs. Terrestrial)
• size differences (Jovian vs. Terrestrial)
• Why asteroids are rocky comets are icy
• Large outer planetesimals attract gas from the
  nebula via gravity, forming mini-disks
  Explains
• size, mass, density of outer planets
• existence of gas giant moons and their properties

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Explaining Debris Where did the leftover
planetesimals go?

• Most leftover rocky bodies in inner solar system
  collided with newly-formed planets moons early
  on
• heavy bombardment period ? Cratering!
• Leftovers Asteroids!
• Most icy bodies in outer solar system sent out
  even farther by gravitational interactions with
  giant planets
• Comets in all directions! (Oort Cloud)

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Exceptions to the Rules
Explaining the things that didnt fit the
patterns
IMPACTS and ENCOUNTERS

• Impacts
• Some orbits tilted (esp. Pluto)
• Spin axes of some planets are tilted (e.g. Earth,
  Uranus)
• Some planets spin faster than others
• Earth is only terrestrial planet with large Moon
• Encounters
• Some moons orbit opposite their planets rotation

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Formation of the Moon(Giant Impact Theory)

• Part of Earths outer layers ejected

• Earth struck by a Mars-sized planetesimal

• Coalesced into the Moon

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Concept Question

• True (A) or False (B) According to our model /
  theory of solar system formation, small bodies in
  the outer solar system are composed entirely of
  ice, with no rocky / metallic elements.

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Concept Question

• True (A) or False (B) According to our model of
  SS formation, the early phase of planet formation
  from the nebula for both the terrestrial and
  jovian planets was the accretion of solids that
  condensed out of the nebula.

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Review Explaining the SS

• Nebula Collapses
• Conservation of energy heats the collapsing
  nebula
• Hot dense center eventually forms Sun
• Motions average out (collisions) and flatten it
  into a disk
• Planets lie in a plane, orbits nearly circular
• Conservation of angular momentum caused disk to
  spin faster
• Single direction of motion

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Review Explaining the SS (2)

• Condensation of solids depends on temperature
• Closer to protosun, denser materials condense
• Inner planets denser, outer planets/moons less
  dense
• Particles accreted into planetesimals
• merged / grew into small planets
• Terrestrial planet formation
• Asteroids/comets leftover planetesimals
• Far from Sun some planetesimals grew large enough
  to attract gas from the nebula
• Because ice also condenses, more material ? more
  massive protoplanets
• Giant planet systems formed like mini-solar
  systems

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