Solar System III

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Solar System III Lecture 8
James J Marie, Astronomy 2005
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Origin of The Solar System

• In 1755, a German philosopher Immanuel Kant
  hypothesized that the solar
  
• system formed from the gravitational collapse
  of a great interstellar cloud
  
• of gas called a nebula (Nebular Hypothesis).
• Many scientist thought the Nebular Hypothesis
  was flawed and pursued
  
• other theories.
• In the early 1900s, the Close Encounter
  Hypothesis was proposed which
  
• suggested that the planets formed from blobs
  of gas pulled from the Sun
  
• during a near collision with another star.
• Over time, the scientific evidence for the
  Nebular Hypothesis mounted and
  
• the Close Encounter Hypothesis was discarded.

James J Marie, Astronomy 2005
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Solar Nebula

• The solar system is thought to have
• been born from a cloud of gas that
• collapsed under its own gravity.
• The initial solar nebula was roughly
• spherical with a diameter from 2 to 5
• light years across.
• The gas density was extremely low
• and its temperature was extremely
• cold.

James J Marie, Astronomy 2005
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Evidence For the Nebular Hypothesis

• Patterns of Motion
• a) All planets orbit the Sun in the same
  direction is nearly circular orbits
  
• within the same plane.
• b) The Sun and planets rotate in the same
  direction that they orbit.
  
• c) Most large moons also orbit in the
  same direction.
  
• Two Types of Planets
• a) Terrestrial
• b) Jovian
• Asteroids and Comets
• These objects are additional features that
  must be explained.
  
• Exceptions to the Patterns
• Examples include the odd tilt of the axis
  of Uranus and the Earths
  
• large Moon.

James J Marie, Astronomy 2005
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Evidence For the Nebular Hypothesis

• The Nebular Hypothesis make makes mathematical
  predictions for
  
• all 4 of the broad characteristics!
• In fact, the Nebular Hypothesis is more than a
  good theory. We actually
  
• see such phenomena occurring elsewhere in the
  cosmos!

• New stars, star systems
• and probably planets are
• forming out of this massive
• cloud of gas and dust
• called the Orion Nebula.

James J Marie, Astronomy 2005
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Eagle Nebula

• The Eagle Nebula lies 6500 ly from
• Earth and is 20 ly across.
• It is a spectacular stellar nursery
• bursting with creation.
• Our own solar system along with
• hundreds of other stars probably
• formed out of a similar cloud.
• Each forming solar system originates
• from Bok globules within the larger
• nebula.

James J Marie, Astronomy 2005
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Nature of a Collapsing Nebula
1. The collapse of the initially spherical
nebula needs a trigger.

• A shock wave from a nearby
• exploding star can trigger the
• collapse.
• Once the collapse is started,
• the strength of the gravitational
• force behind the collapse
• grows rapidly

• As the diameter decreases
• by ½, the force of gravity
• increases by 4.

James J Marie, Astronomy 2005
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Nature of a Collapsing Nebula
4. The temperature of the nebula increases as it
collapses, especially at the center. Gravit
ational potential energy gets converted to
kinetic energy of the atoms and molecules.
James J Marie, Astronomy 2005
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Nature of a Collapsing Nebula

• Mass becomes heavily
• concentrated near the
• center. Temperature and
• density become very high
• near the center.
• A proto-sun begins to form.
• The solar nebula rotates
• faster and faster as its
• radius shrinks due to the
• conservation of angular
• momentum.
• The rotation prevents the
• entire nebula from
• imploding, i.e. some of the
• mass is spread out radially.

James J Marie, Astronomy 2005
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Nature of a Collapsing Nebula

• The solar nebula
• flattens into a disk. The
• flattening is a natural
• consequence of
• collisions between
• particles in a cloud.

James J Marie, Astronomy 2005
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Layout of the Solar System

• The heating, spinning and flattening explain
  the layout of the solar system.
  
• By the time the solar nebula had a diameter of
  200 AU, the Sun (proto-sun)
  
• was forming at the center where the
  temperatures and densities were the
  
• highest.
• The planets were born in the spinning disk.

There are many examples of flat disks throughout
the Cosmos

• Spiral Galaxies
• Planetary Rings
• Accretion Disks Surrounding Black Holes and
  Neutron Stars

They are a consequence of natural processes.
James J Marie, Astronomy 2005
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Two Types of Planets

• The nebular theory can explain why there are 2
  types of planets

Terrestrial and Jovian.

• The solar nebula had a uniform and homogeneous
  composition

98 hydrogen and helium 2 everything else

• How is it possible that 2 different kinds of
  planets can come out of this?

James J Marie, Astronomy 2005
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Seeds

• The force of gravity is too weak for the
  gaseous material in the solar disk
  
• to clump up.
• There had to be seeds before chunks of matter
  could form.
  
• The seeds can be thought of as tiny snow
  flakes that freeze out of the
  
• gaseous nebula

• The snow flakes are simply
• ice particles and dust grains.

James J Marie, Astronomy 2005
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Frozen Seeds

• The temperature of the solar nebula varies It
  becomes cooler as the distance
  
• away from the center increases.
• If the gases in the nebula are cool enough,
  then molecular forces pull the
  
• molecules together to form the snow flakes.
  (Gravity is not involved at all.)
  
• The composition of the seeds depends on the
  condensation temperature (the
  
• temperature at which the gases freeze).
• Most of the solar nebula is made of hydrogen
  and helium.
  
• But the solar nebula was too warm for hydrogen
  and helium ice crystals to
  
• form.
• That leaves only 2 of the gases in the solar
  nebula to condense into seeds
  
• (dust particles).

James J Marie, Astronomy 2005
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Three Types of Seeds
1. Hydrogen Compounds - 1.4 of the solar
nebula mass
CH4 - methane NH3 - ammonia H2O - water

• These molecules form ices (seeds) if the
  temperature is below 150 K.

2. Rock 0.4 of the solar nebula mass
silicon based materials

• These molecules can freeze into dust grains
  (seeds) if the temperature
  
• is below the range 500 1,300 K.

?
James J Marie, Astronomy 2005
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Three Types of Seeds
3. Metals - 0.2 of the solar nebula mass
iron, nickel and aluminum

• These atoms freeze into dust grains (seeds) if
  the temperature
  
• is below the range 1,000 -1,600 K.

• Once the seeds form they start to stick
  together just like several snow flakes
  
• can stick together. (Gravity is still not
  involved yet.)
  
• Eventually, the dust grains form loose, porous
  aggregates that have very
  
• weak gravitational fields.

James J Marie, Astronomy 2005
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Temperature Differences in the Solar Nebula

• The inner regions of the solar nebula near the
  forming Sun had temperatures
  
• above 1600 K. These temperatures are too
  high for any condensation (no
  
• seeds).
• Farther out (but still within the orbit of
  Mercury) the temperature was low
  
• enough for particles of metal to condense. At
  distances comparable to
  
• Mercurys orbit it was cool enough for bits of
  rock to condense.
  
• At the distance of the asteroid belt, the solar
  nebula was cool enough to allow
  
• dark, carbon rich minerals to condense along
  with small amounts of water ice.

James J Marie, Astronomy 2005
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Frost Line

• A dividing line (actually a circle) that
  separates the solar nebula into 2 regions.
  
• Within the inner region, the temperature is too
  high for hydrogen compounds to
  
• condense. Only metal and dust grains condense
  in this region.
  
• In the outer region, temperatures were cool
  enough to allow hydrogen
  
• compounds to condense. Solid particles are
  made of ices, metals and rock.
  
• The number of seeds beyond the frost line were
  much higher.
  
• This division of the solar nebula allowed 2
  types of planets to be born

• Planets from metal and rock (inner planets).
• 2. Planets from seeds of ice (as well as metal
  and rock)
  
• which led to giant gaseous planets (outer
  planets).

James J Marie, Astronomy 2005
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Frost Line
James J Marie, Astronomy 2005
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Accretion

• The small aggregates collect into planetesimals
  which then accrete into
  
• the early planets.

James J Marie, Astronomy 2005
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Origin of the Asteroids and Comets

• The solar wind from the young
• Sun blew away most of the gas
• from the solar nebula.
• However many planetesimals
• remained scattered between the
• newly formed planets.
• These left-overs became the
• asteroids and comets.

James J Marie, Astronomy 2005
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Asteroid Belt

• Asteroids accumulated
• between the orbits of
• Mars and Jupiter.
• Asteroids in the region of
• the 4 inner planets were
• swept up by these
• planets.
• In the early solar system,
• there may have been
• enough asteroids to form
• another planet but Jupiters
• gravity sent many asteroids
• crashing into the inner
• planets or ejected them out
• of the solar system.

James J Marie, Astronomy 2005
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Comets

• Comets are icy left-over planetesimals of the
  outer solar system.
  
• Most of these planetesimals were either
  swallowed up by the gas giants
  
• or ejected into the outer fringes of the solar
  system.

James J Marie, Astronomy 2005
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Nucleus of a Comet
James J Marie, Astronomy 2005
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Oort Cloud

• The Oort cloud is made up of
• comets flung to the outer fringes
• of the solar system by the jovian
• planets.
• Roughly spherical in shape.
• The Oort cloud is mathematically
• predicted by the Nebular Theory.

James J Marie, Astronomy 2005
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Kuiper Belt

• The Kuiper Belt is made up of
• comets beyond the orbit of
• Neptune which orbit in the
• same direction as the planets
• and are concentrated near the
• plane of the ecliptic.
• Comets in the Kuiper belt were
• much less likely to be destroyed
• by collisions or cast off by
• gravitational encounters.
• The relative closeness of Kuiper
• belt objects allowed further
• accretion.
• The additional accretion led to
• the formation of a few larger
• objects (perhaps Pluto).

James J Marie, Astronomy 2005
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Solar Wind

• The solar wind was much stronger than it is
  today.
  
• Soon after the planets formed, most of the
  hydrogen and helium gas of the
  
• solar nebula was blown into interstellar
  space.
  
• The strong solar wind explains the slow
  rotation of the Sun.

• The spinning disk of the solar nebula was
  spinning much faster
  
• than the present rotational rate of the
  Sun.
  
• The solar wind caused the sun to lose angular
  momentum and its
  
• spin rate slowed down.

James J Marie, Astronomy 2005
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Solar Wind

• The solar wind ionized the
• gas in the solar nebula
• creating charged particles
• (ions).
• The rotating magnetic field of
• the Sun dragged the charged
• particles along as the Sun
• rotated.
• So as the charged particles
• gained angular momentum,
• the Sun lost angular
• momentum.
• Then the solar wind blew the
• charged particles into
• interstellar space.

?
Now the Sun is left spinning at the slow
rate that we observe today.
James J Marie, Astronomy 2005
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Heavy Bombardment Epoch

• The planets of the early solar system
  experienced a period of heavy
  
• bombardment during the first few hundred
  million years after formation.
  
• The bombardment resulted from collisions with
  left-over comets and asteroids.
  
• Every world in the solar system was pelted by
  impacts during this epoch.

James J Marie, Astronomy 2005
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Lunar Craters

• Evidence for the heavy bombardment
• epoch is especially apparent on the
• Moon. ?
• Photograph of the lunar surface taken by
• Apollo 16 astronauts.

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Martian Craters
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Meteors and the Early Earth
James J Marie, Astronomy 2005
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Impact Craters on Earth

• Geologic and weathering processes on Earth have
  erased almost all evidence
  
• for impact craters except for very recent
  impacts.

Barringer Meteorite Crater in Arizona Impact
occurred 50,000 years ago
James J Marie, Astronomy 2005
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Jupiters Influence

• Jupiters gravity played a
• major role in the Earths
• formation.
• Jupiter flung great numbers
• of asteroids and comets
• toward the inner solar
• system.
• As a result large amounts of
• water was deposited on the
• Earth and the oceans were
• formed.

James J Marie, Astronomy 2005
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Comet Impacts

• The liquid water, gaseous atmospheres
• and ices found on the terrestrial planets
• are thought to have been deposited by
• impacts from planetesimals that formed
• beyond the orbit of Mars.
• Many of these planetesimals were
• comets.
• The water that we drink was once inside
• of a comet!

James J Marie, Astronomy 2005
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Major Impacts

• The last major extinction on the Earth
• may have been due to an impact with
• a large asteroid or comet.
• Such impacts occur every 50 to 100
• million years.

James J Marie, Astronomy 2005