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Formation of the Solar
System
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Department
1
Midterm!
Part I (Take home exam, including 10 points
from Mastering Astronomy, 50 pts) is
available, due October 26th, noon
Next week, Part II (in class exam, 50 pts.)
– Taken in 3rd hour (week of 10/22 to 10/25)
– Bring SCANTRON (882 form) and #2 pencil
– Based on “Review Questions” handout, available
now!
Also: 10 of the 25 extra credit points are due
by October 26th, noon.
Lecture 8a: The Formation of the Solar system
The Formation of the Solar System
What properties must a planetary formation
theory explain?
It must explain the patterns of motion of the
present solar system (last week).
2. It must explain why planets form into 2 groups.
3. It must explain the huge existence of asteroids
and comets.
4. It must allow for possible exceptions to the rules.
The theory may be able to be used on other solar
systems in the Galaxy
1.
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Department
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Lecture 8a: The Formation of the Solar system
The Formation of the Solar System
Evolutionary Theories
All evolutionary theories have their start with
Descartes’s whirlpool or vortex theory
proposed in 1644.
Using Newtonian mechanics, Kant (in 1755)
and then Laplace (around 1795) modified
Descartes’s vortex to a rotating cloud of gas
contracting under gravity into a disk.
The Solar Nebular Hypothesis is an
example of an evolutionary theory.
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Department
Solar
Nebula
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Lecture 8a: The Formation of the Solar system
The Formation of the Solar System
Catastrophic Theories
Catastrophic theory is a theory of the
formation of the solar system that involves an
unusual incident such as the collision of the Sun
with another star.
The first catastrophic theory - that a comet
pulled material from the Sun to form the planets
- was proposed by Buffon in 1745.
Other close encounter hypotheses have been
proposed too.
Catastrophic origins for solar systems would be
quite rare (relative to evolutionary origins) due to
the unusual nature of the catastrophic incident.
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
Origin of the Solar Nebula
Galactic
recycling
Galactic recycling
– Most of the universe started as Hydrogen and Helium. All
other heavy elements (loosely called “metals” by
astronomers) were formed in stars
– When stars die they release much of the content into
space
– While this has been going on for 4.6 billion years, only
2% of all the have been converted to “metals”
Evidence from other gas clouds
– All new systems that we can observed formed within
interstellar clouds, such as the Orion Nebula
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Department
Orion
Nebula
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
Towards a Solar Nebula Hypothesis
A supernovae shock wave likely triggered the events
which led to the birth of our solar system
The nebular cloud collapsed due the force of gravityCloud
on the cloud. But the cloud does not end up
collapse2
spherical (like the sun) because there are other
processes going on:
– Heating – The cloud increases in temperature, converting
gravitational potential energy to kinetic energy. The sun would
form in the center where temperatures and densities were the
greatest
– Spinning – as the cloud shrunk in size, the rotation of the disk
increases (from the conservation of angular momentum).
– Flattening – as cloud starting to spin, collisions flattened the
shape of the disk in the plane perpendicular to the spin axis
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Lecture 8a: The Formation of the Solar system
Testing the Model
If the theory is correct, then we should see
disks around young stars
Dust disks, such as discovered around
beta-Pictoris or AU Microscopii, provide
evidence that conditions for planet formation
exist around many Sun-like stars.
AU Mircoscopii
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Department
HD 141569A
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Disks around other stars
Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
categories
The Formation of Planets
As the solar nebula cooled and flattened into a disk some 200 AU
in diameter, materials began to “freeze” out in a process called
condensation (changing from a gas to a solid or liquid).
The ingredients of the solar system consist of 4 categories (with %
abundance):
–
–
–
–
Hydrogen and Helium gas (98%)
Hydrogen compounds, such as water, ammonia, and methane (1.4%)
Rock (0.4%)
Metals (0.2%)
Since it is too cool for H and He to condense, a vast majority of the
solar nebula did not condense
Hydrogen compounds could only condense into ices beyond the
frost line, which lay between the present-day orbits of Mars and
Jupiter
Frost line
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
Building the Terrestrial Planets
In the 1940s, Weizsächer showed that eddies would form in a
rotating gas cloud and that the eddies nearer the center would be
smaller.
Eddies condense to form particles that grow over time in a
process called accretion. Materials such and rock and metalplanetesimals
(categories #3 and #4).
These accreted materials became planetesimals which in turn
sweep up smaller particles through collision and gravitational
attraction.
Mixed rock
These planetesimals suffered gravitational encounters which meteorite
altered their orbits caused them to both coalesce and fragment.
Only the largest planetesimals grew to be full-fledged planets.
Verification of this models is difficult and comes in the form of
theoretical evidence and computer simulations.
© Sierra College Astronomy
Department
Jovian
planetesimals
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
Building the Jovian Planets
Planetesimals should have also grown in the
outer solar system, but would have been
made of ice as well as metal and rock.
But Jovian planets are made mostly of H and
He gas…
The gas presumably was captured by these
ice/rock/metal planetesimals and grew into
the Jovian planets of today.
© Sierra College Astronomy
Department
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
A Stellar wind is the flow of nuclear
particles from a star.
Some young stars exhibit strong stellar
winds. If the early Sun went through such a
period, the resulting intense solar wind
would have swept the inner solar system
clear of volatile (low density) elements,
molecules and compounds.
The giant planets of the outer solar system
would then have collected these outflowing
gases.
© Sierra College Astronomy
Department
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
An object shrinking under the force of gravity
heats up. High temperatures near the newly
formed Sun (protosun) will prevent the
condensation of more volatile (low density)
elements. Planets forming there will thus be
made of nonvolatile, dense material.
Planet
Building
Farther out, the eddies are larger and the
temperatures cooler so large planets can form
that are composed of volatile elements (light
gases).
© Sierra College Astronomy
Department
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
Problem: The total angular momentum of
the planets is known to be greater than that
of the Sun, which should not occur according
to conservation laws (i.e. the present Sun is
spinning too slowly).
Solution: As the young Sun heated up, it
ionized the gas of the inner solar system.
– The Sun’s magnetic field then swept through the
ions in the inner solar system, causing ions to
speed up.
– As per Newton’s third law, this transfer of energy
to the ions caused the Sun to slow its rate of
rotation.
© Sierra College Astronomy
Department
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
Explaining Other Clues
Over millions of years the remaining
planetesimals fell onto the moons and
planets causing the cratering we see today.
This was the period of heavy bombardment.
Comets are thought to be material that
coalesced in the outer solar system from the
remnants of small eddies.
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Department
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Lecture 8a: The Formation of the Solar system
Solar Nebula Hypothesis
The formation of Jovian planets and its
moons must have resembled the
formation of the solar system. Jupiter
specifically:
– Moons close to Jupiter are denser and
contain fewer light elements;
– Moons farther out decrease in density and
increase in heavier elements.
© Sierra College Astronomy
Department
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Lecture 8a: The Formation of the Solar system
The Exceptions to the Rule
Phobos Deimos
Captured Moons – satellites which go the
opposite way were likely captured. Most of
these moons are small are lie far away from
the planet.
Giant impact
Moon
Giant impacts – may have helped form the
Moon and explain the high density of Mercury
and the Pluto-Charon system. Furthermore,
the unusual tilts of Uranus and Venus can
also be explained by giant impacts.
Solar Nebula Theory
Summary
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Department
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Lecture 8a: The Formation of the Solar system
Solar System Destiny
The nebular hypothesis accounts for all major
features in the solar system
It does not account for everything, however
It probably took about a few tens of million of years,
about 1% of the current age of the solar system
The solar system was probably not completely
predestined from the collapse of the solar nebula,
though the initial were orderly and inevitable
The final stage of accretion and giant impacts were
fairly random in nature and made our solar system
unique
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Department
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Lecture 8a: The Formation of the Solar system
Radioactivity
Radioactivity
Periodic
Table
Certain isotopes (elements which contain
differing number of neutrons) are not stable and
will decay into two or more lighter elements
The time it takes for half of a given isotope to Half-life
K-40
decay is called the half-life
By noting what percentage a rock (or human
body) has left of a radioactive element can
enable us to estimate the age of that object. This
process is called radioactive dating. See
Mathematical Insight 8.1
Half-life 2
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Department
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Lecture 8a: The Formation of the Solar system
Radioactivity
Radioactivity - examples
Potassium-40 decays into Argon-40 with a half-life of
1.25 billion years
Half-life
K-40
– Since Argon-40 is an inert gas, it is very unlikely to have formed
inside a rock as the solar nebula condensed, so it must have
formed via decay
Uranium-238, after a series of decays, turns into Lead206 with half-life of 4.5 billion years
Periodic
– Lead and Uranium have very different chemical behavoirs
Table
– Some minerals have nearly no lead to begin with, so when
uranium is mixed with lead, we can assume that the lead formed
via decay
© Sierra College Astronomy
Department
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Lecture 8a: The Formation of the Solar system
Radioactivity
Periodic
Radioactivity
Table
The general formula for the age of a radioactive
material is (see Mathematical Insight 8.1):
 current amount 
log10 

original amount 

t  thalf 
1
log10  
2
Half-life
K-40
Half-life 2
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Department
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Lecture 8a: The Formation of the Solar system
Radioactivity
Half-life 2
Earth rocks, Moon rocks, and meteorites
The oldest Earth rock date back to 4 billion years
and some small grains go back to 4.4 billion
years. Moon rock brought back from the Apollo
mission date as far back as 4.4 billion years.
– These tell us when the rock solidified, not when the
planet formed
The oldest meteorites, which likely come form
asteroids, are dated at 4.55 billion years,
marking the time of the accretion of the solar
system
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Department
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Lecture 8b: Terrestrial Geology Basics
Earth’s Structure & Composition
EarthQ
The Interior of the Earth (overall density = 5.5 g/cm3)
Earth’s interior is determined by analyzing travel times
of two types of waves generated by earthquakes.
Earth’s interior is made up of three layers:
Terr.
insides
– Crust is the thin (<100 km) outermost layer of the Earth and
has a density of 2.5–3 g/cm3.
– Mantle is the thick (2,900 km), solid layer between the crust
and the Earth’s core. Density of the mantle is 3–9 g/cm3. The
crust “floats” on top of the mantle.
– Core is the central part of the Earth, composed of a solid inner
core and a liquid outer core. Density of the core ranges from
9–13 g/cm3 and is probably composed of iron and nickel.
Increasing density trend is called differentiation sinking of denser materials toward the center of planets
or other objects.
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Department
EarthQ2
interior
Diff
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Lecture 8b: Terrestrial Geology Basics
Earth’s Structure & Composition
Layering by Strength
Most of the Earth is not molten and most of the
lava from volcanoes rises upward from a
narrow region of the mantle which is partially
molten.
The shape of a planet is determined by the
strength and fluidity of the inside as well as the
strength of gravity
Shapes
– Large worlds (> 500 km diameter) are round
– Small worlds are irregular in shape
The crust and the top part of the mantle is
relatively cool region of rock called the
lithosphere that floats on the rest of the
mantle.
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Terr.
insides
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Lecture 8b: Terrestrial Geology Basics
Causes of Geological Activity
Geological Activity describes how much
ongoing change occurs on the surface of
a solar system body
Interior heat is the primary driver for
geological activity
But how do interior heat up and cool off?
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Lecture 8b: Terrestrial Geology Basics
Causes of Geological Activity
How planets heat up
Heat of accretion
– Energy brought from afar from colliding planetesimals – potential energy
converted into kinetic energy
Heat of differentiation
– As the planet redistributes its mass and denser material sinks towards
the core gravitational potential energy is converted to thermal energy via
friction
Heat from radioactive decay
– Decay from radioactive materials heats up the interior as some of the Heat
nuclear decay energy ( E = mc2 ) gets transferred to thermal energy
sources
Note: the first of these two tend to happen early in a planet’s history
while the last (radioactive decay) happens throughout the history of
the planet, but is strongest at the beginning of the formation of the
planet. Radioactive decay likely contributes several times more
energy over the life of the planet than does accretion and
differentiation.
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Lecture 8b: Terrestrial Geology Basics
Transfer of Energy
How Interiors Cool Off:
Conduction
– Transfers occurs between atoms
– Examples: metal rod in fire, Earth’s core and lithosphere
Three
Types
Convection
– Warmer (less dense) air rises and carries energy into cooler (denser)
regions
Demo
– Requires large temperature gradient
Lava lamp
– Examples: Lava lamp, Earth atmosphere and mantle, Sun’s outer layers
Radiation
–
–
–
–
Photons directly transfer energy
Less efficient in high density situations
Photons take ~ 200,000 years to get of Sun.
Examples: Heat lamp, Earth’s surface, Sun’s interior
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Earth
cooling
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Lecture 8b: Terrestrial Geology Basics
Transfer of Energy
How the Earth moves energy from the core to
the surface:
Convection is the most important process in
the Earth’s deep interior
– The ongoing process of transferring heat upward
creates convection cells
– Ongoing mantle convection goes at the rate of 1
cm/year: It would take about 100 million years to
move the mantle from the base to the top
At the lithosphere, conduction is probably the
most important process
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Department
Earth
cooling
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Lecture 8b: Terrestrial Geology Basics
Planetary Size
A small object cools more quickly than a large
object
So size is the most important factor in
planetary cooling
This can be seen in the terrestrial worlds:
– Earth and Venus: still very active.
– Mars: Activity in the past, but mostly dead now.
– Moon and Mercury have been dead for 3 billion
years or so.
© Sierra College Astronomy
Department
Earth
cooling
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Magnetic
field
Lecture 8b: Terrestrial Geology Basics
Earth’s Magnetosphere
Basic
Earth Mag
field
Earth’s Magnetic Field
A magnetic field is a region of space where
magnetic forces can be detected. The region
around a planet is called a magnetosphere
Earth’s magnetic poles are not located at its
poles of rotation. The location of the
magnetic poles changes with time.
Demo
Dynamo effect is the model that explains
the Earth’s and other planets’ magnetic
fields as due to currents within a liquid iron
core and a rapidly spinning planet.
dynamo
Earth
dynamo
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Department
magnetosphere
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Lecture 8b: Terrestrial Geology Basics
Earth’s Magnetosphere
The Van Allen belts are doughnut-shaped
regions composed of charged particles
(protons & electrons) emitted by the Sun &
captured by the magnetic field of the Earth.
Auroras result from disturbances in the
Earth’s magnetic field that cause some of
the particles to follow the magnetic field lines
down to the atmosphere, where their
collisions with atoms of the air cause it to
glow.
Aurora
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Aurora from the Ground
Aurora From Space
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Lecture 8b: Terrestrial Geology Basics
Shaping the Earth
There are 4 processes which shape the
virtually all features on Earth
1. Impact Cratering
 Bowl shaped from asteroids or meteors
2. Volcanism
 Eruption of lava from planet’s interior
3. Tectonics
 Disruption of planet’s surface by internal forces
4. Erosion
 Wearing down or building of geological
features by wind, water, ice etc…
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Department
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Lecture 8b: Terrestrial Geology Basics
Impact Cratering
impact
As a general rule the craters made by meteors
are 10 times bigger than the impactor and 1020% as deep as the crater is wide.
Most impacts happened very early in the history
of the solar system
The most prominent impact crater on Earth is
Meteor Crater near Winslow, Arizona (only
Meteor
crater
50,000 years ago).
Many of the craters on the Earth have been
wiped out by erosion processes
– Not true for Moon and Mercury
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Department
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Lecture 8b: Terrestrial Geology Basics
Volcanism
Volcanism occurs when underground molten
rock finds it way through the lithosphere. This is
due for 3 reasons:
– Molten rock is generally less dense than solid rock
– Most of the Earth’s interior is not molten and it
requires a chamber of molten rock to be squeezed
up the surface
– Molten rock often has gas inside of it, leading to
dramatic eruption and to outgassing
The most common gasses released are water
vapor, carbon dioxide, nitrogen, and sulfur
gasses (H2S or SO2)
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Department
Drift
Plates
Rift
Subduc
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Lecture 8b: Terrestrial Geology Basics
Plate Tectonics
Plate Tectonics
Alfred Wegener is credited with first
developing the idea of continental drift the gradual motion of the continents relative
to one another.
Rift zone is a place where tectonic plates
are being pushed apart, normally by molten
material being forced up out of the mantle.
Subduction Zone is where two plates are
Drift
Plates
Rift
Subduc
forced together.
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Department
tectonics
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Lecture 8b: Terrestrial Geology Basics
Erosion
The surface of the Earth is changed by
erosion, the processes that break down or
transport rock through the action of ice, liquid,
or gas
Erosion
– Valleys shaped by glaciers
– Canyons carved by rivers
– Shifting of sand dunes by the air
Erosion can pile up sediments into layers called
sedimentary rocks (Ex. Grand Canyon)
The Earth has the most erosion of any
terrestrial planet
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Department
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Lecture 8b: Terrestrial Geology Basics
Age of surfaces
The number of craters in a given region
can tell one the age of the planet/moon
since the last major change on surface
– Does not necessarily indicate formation age
Erosion from wind, water, and lava will
wipe out craters in a given region
– This led to determining the development of
different parts of the planet/moon
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Department
Craters
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The End
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Department
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