Astro 10-Lecture 6: Formation and Structure of the Solar System

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Astro 10-Lecture 6Formation and Structure of
the Solar System

• What are the properties of the solar system?
• How are these properties explained by theories of
  the formation of the solar system?
• Why are the planets different from one another?

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Scale of the Solar SystemA reminder of Exercise
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Scale Model of the Solar System

• The Sun (basketball)
• Mercury (tiny ballbearing at 10 yds)

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Scale Model of the Solar System

• Venus (pinhead at 18 yds)

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Scale Model of the Solar System

• Earth (pinhead at 25 yds)
• Moon (tiny ball bearing 2.4 inches away).

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Scale Model of the Solar System

• Mars (tiny ballbearing at 39 yards)

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Scale Model of the Solar System

• Asteroids (A few thousand specks of dust between
  50 and 75 yards away from the volleyball)

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Scale Model of the Solar System

• Jupiter (1 inch diameter at 132 yards)

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Scale Model of the Solar System

• Saturn (0.7 inch diameter at 242 yards)

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Scale Model of the Solar System

• Uranus and Neptune (0.3 inch diameter at 487 and
  762 yards)

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Scale Model of the Solar System

• Pluto Charon (tiny ballbearings at 1000 yds)

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Characteristics of the Solar System(formation
theory must explain all)

• 1. All of the planets orbit in the same
  direction and in the same plane. (Within a few
  degrees) This plane corresponds with the equator
  of the sun.
• Exceptions Mercury (7 degrees), Pluto (17
  degrees)
• 2. Most of the planets rotate in the same
  direction and have their equators roughly aligned
  with the plane of the solar system.
• Exception Venus rotates in the opposite
  direction (retrograde).
• Exception Uranus and Pluto highly tilted (90
  degrees) with respect to the plane of solar
  system.

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Characteristics of the Solar System

• 3. Orbits of moons around planets are in the
  planets equatorial plane.
• Exception Earths moon rotates in plane of the
  solar system
• One of Neptunes moons orbits retrograde.
• 4. Two (three?) types of planets
• Small rocky planets (Mercury, Venus, Earth, Mars)
• Large gaseous planets (Jupiter, Saturn, Uranus,
  Neptune)
• Tiny icy planets? (Pluto?)

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Terrestrial Planets
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Terrestrial Planets

• Small and Rocky. Crust mainly composed of
  Silicon and Oxygen (Silicates).
• Atmosphere ranges from none to thick. Atmosphere

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Terrestrial Planets

• Near to the sun. (lt 5 astronomical units)
• Reminder of what an astronomical unit is.
• Small and Rocky. (From 0.1 to 1 Me)
• High density (volume/mass) (3-5x the density of
  water).
• How do we measure the density?
• Crust mainly composed of Silicon and Oxygen
  (Silicates) with small fraction of Sulfates.
• Exception Earth has higher percentage of
  carbonates in its rocks.
• Cores of Nickel and Iron
• Some solid, some liquid

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Terrestrial Planets

• Cratering is common on the surface of terrestrial
  planets.
• From the ages of the craters, we can tell that
  impacts were much more common early in the
  history of the solar system

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Terrestrial Planets

• Atmosphere ranges from none (Mercury) to thick
  (Venus).
• Typical atmospheric components are Carbon Dioxide
  (CO2) and Nitrogen (N2)
• Exception Earth has Oxygen (O2) and Water (H2O)
  in its atmosphere.
• Typically have no moons.
• Exception Earth has a very large moon.
• Exception Mars has two tiny moons.

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Terrestrial Planet Interiors
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Jovian (Gas Giant) Planets
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Jovian (Gas Giant) Planets

• Primarily composed of hydrogen (H2) and Helium
  (He)
• Massive (15 to 300 Me)
• Thick Atmosphere
• H2, He, Methane (CH4), Ammonia (NH3)
• Low density (0.7 to 1.8x the density of water)
• Saturn would float!
• Small rocky core surrounded by huge ocean of
  liquid hydrogen.
• Is the core a terrestrial planet?
• All are found more distant than about 5
  astronomical units from the sun.

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Jovian Planets

• Rings! All Jovian planets have them.

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Jovian Planets

• Moons! Jovian planets tend to have very many

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Jovian Planets

• Interior

Atmosphere
Liquid Metallic Hydrogen
Liquid Hydrogen
Core
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Jovian Planets

• Some (Jupiter and Saturn at least) radiate more
  energy than they receive from the sun.
• They generate energy from gravitational
  contraction.

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Tiny Icy Planets

• Pluto? Charon? Are they really planets
• The moons of the Jovian planets?
• These are like the terrestrial planets, but
  instead of SiO2 they have H2O
• Tiny rocky core underneath the ice.

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Characteristics of the Solar System

• 5. Three types of space debris
• Asteroids
• Chunks of rock between 10 meters and a thousand
  km in size.
• 20,000 of them
• Concentrated in the plane of the solar system
  between Mars and Jupiter

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Characteristics of the Solar System

• Comets
• Chunks of ice and rock between 10 meters and a
  thousand km in size.
• Billions? of them
• Most reside outside the orbit of Pluto in the
  Oort cloud. Not concentrated into the plane of
  the solar system
• Occasionally one will fall into the inner solar
  system (on a very elliptical orbit).

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Characteristics of the Solar System

• Meteoroids
• Tiny bits of rock and metal
• Most lt1 gram
• Heated by atmospheric friction until they glow.
• Most follow along the orbits of comets
• Debris left behind when a comet goes by.

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Characteristic of the Solar System

• 6. Age
• The objects in the solar system are all about 4.6
  billion years old.
• How do we know this?
• Radioactive dating
• A radioactive element decays into a daughter
  element.
• We dont know what time a specific atom will
  decay but we know how long it will take for half
  the atoms to decay.
• (Demo)

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Radioactive Dating

• U238gtPb206
• Halflife
• 4.5 billion years
• Oldest earth rocks
• 3.96 billion years
• Meteors and Moon rocks
• 4.6 billion years
• This is the time they solidified The solar
  system is older than this.

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Theory of the formation of the Solar System

• A story the fits the facts.
• Needs to explain, or at least be consistent with
  all the characteristics that we listed.
• Needs to also be consistent with what we know
  about the rest of the galaxy. Other stars and
  solar systems should form in the same way.

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The Solar Nebula Theory

• The sun and solar system formed from the collapse
  of a cloud of gas and dust.
• The cloud was slowly rotating, so centrifugal
  force made it into a disk (accretion disk)
  transferring matter to the center.
• Conservation of angular momentum made it rotate
  more quickly

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The Solar Nebula Theory

• Instabilities in the disk may have formed smaller
  sub-disks where giant planets formed

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The Solar Nebula Theory

• Dust, rock and ice condense and stick together to
  make small bodies called planetesimals.
• Heat from the forming sun only allowed certain
  elements to condense nearby. Ices could only
  condense far away.

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The Solar Nebula Theory

• Two ways of building planets
• Larger planetesimals attract smaller ones. They
  collide and merge to make a bigger planetesimals.
  These attract more and eventually form the
  planets
• Near the sun, the nebular hydrogen gas is too hot
  (moving to fast) to form an atmosphere around the
  planets. Distant planets begin to form hydrogen
  atmospheres once they get big enough.
• The Jovian planets captured their atmospheres.
• As time goes on the nebula cools, making the
  frost line move inward.

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The Solar Nebula Theory

• The young planets start out fairly warm (in a
  liquid or nearly liquid state). Heavy elements
  start to sink This concentrates the radioactive
  elements in the center (and explains why the
  earths core is hot).
• This process is still occurring in the giant
  planets. It releases gravitational energy.

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The Solar Nebula Theory

• On the terrestrial planets, gasses are released
  from the hot interior to form atmospheres.
• Volcanic processes release H2S, SO2, CO2, H2O,
  NH3, N2
• Solar UV radiation breaks apart NH3, hydrogen
  escapes, leaving N2
• Solar UV radiation also breaks apart H2O,
  hydrogen escapes leaving O, which reacts with
  rock to form solid oxides.
• H2O combines with H2S, and SO2 to make sulfuric
  and sulfurous acid. This eats away rock to form
  solid sulfates.
• Whats left? CO2 and N2 in the atmosphere.
  Oxygen and Sulfur in the rocks. (Why is Earths
  atmosphere different?)

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Where did the nebula go?

• Solar wind, heat, and light pressure drove the
  gas away.
• What about the left over planetesimals?
• Most of the rocky ones in the inner solar system
  eventually collided with planets. (Thats why
  the rate of impacts was high 4 Gya, but is low
  now.)
• Theres about 20,000 left over mostly between
  Mars and Jupiter (Asteroids!)
• Jupiters gravity prevented a planet from forming
  there.
• Encounters with the giant Jovian planets kicked
  most of the remaining icy ones into the outer
  solar system or interstellar space
• These are comets!
• The encounters would kick them in any direction.
  (This explains why comets arent concentrated in
  the plane of the solar system.)

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How does this theory fit the characteristics of
the Solar System?

• 1. 2. Collapse to a disk explains the
  concentration in the plane of the solar system,
  and why almost everything moves in the same
  direction.
• 3. The giant planets had disks of their own so
  their moons orbit in their equatorial plane
• 4a. Because the inner solar system was hot, only
  rock and metal could condense which resulted in
  terrestrial planets
• 4b. The outer solar system was cold enough for
  ices to condense and for hydrogen gas to be
  captured by a massive enough body. This resulted
  in Jovian planets.
• 4c. If an object in the outer solar system
  wasnt massive enough to capture hydrogen gas, it
  remained as a small icy body. (Pluto, the outer
  planet moons, comets)

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How does this theory fit the characteristics of
the Solar System?

• 4d. The terrestrial planets released their
  atmospheres from their interiors. The Jovian
  planets captured theirs. The icy planets werent
  massive enough to capture one, or hot enough to
  release one.
• 4e. The inner structure of the planets is
  explained by differentiation. Heavier elements
  sink to the core. Lighter ones float to the
  surface.
• 5. Asteroids and comets are left over
  planetesimals. Meteors are bits of dust that
  have fallen off of comets
• 6. Everything is the same age because it all
  formed at about the same time.
• What about the exceptions?

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For every exception there is a rule...

• Tilted orbits of Mercury and Pluto.
• Mercury probably suffered a large impact late in
  its formation
• Pluto might be a left-over planetesimal.
• Retrograde rotation of Venus
• Probably due to a large impact late in formation.
• Probability favors, but does not require,
  rotation in the same direction as the orbit.
• High axial tilt of Uranus and Pluto
• Also likely to be due to a large impact
• Also, in the outer solar system, computer models
  suggest the nebula was less concentrated in the
  plane, which could result in large tilt of
  sub-disks.

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For every exception there is a rule...

• Retrograde moon of Neptune.
• Probably a captured planetesimal.
• Oxygen in the atmosphere of earth.
• Earths atmosphere is highly modified by life.
• Earths moon orbits in the plane of the solar
  system.
• This is likely because the moon was formed from
  an impact with another body traveling in the
  plane of the solar system.