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Chapter 2: Solar System
Learning objectives: You will be able to…
• Explain the solar nebula hypothesis (origin of the
solar system)
• Describe the Sun and how it works
• Explain how Mercury, Venus and Mars differ form
Earth
• Characterize the gas giants
• Describe dwarf planets, comets, and asteroids
• Identify key events in the formation of Earth
The Universe
Universe is finite
Universe is expanding (Doppler Effect)
Expansion is accelerating
Our solar System is not the center of the Universe
Universe has no center nor an edge
The Solar System consists of:
The Sun
Eight classical planets
Mercury, Venus, Earth, Mars,
Jupiter, Saturn, Uranus, &
Neptune
Five dwarf planets
Pluto, Ceres, Haumea,
Makemake, and Eris
240 known satellites (moons),
including 162 orbiting the
classical planets
Millions of comments and asteroids
Kuiper Belt and Oort Cloud
Countless particles and
interplanetary space
Origin of Solar System
Pick a theory, any theory, but it must be consistent
with these facts:
1) Planets all revolve around the Sun in the same
direction in nearly circular orbits.
2) The angle between the axis of rotation and the
plane of orbit is small (except Uranus).
3) All planets (except Venus and Uranus) rotate in
the same direction as their revolution; their
moons do, too.
Origin of solar system
4) Each planet is roughly twice as far as the next inner planet is
from the Sun (the Titus-Bode rule).
5) 99.9 % of mass is in the Sun; 99 % of angular momentum is
in the planets.
6) Planets in two groups:

terrestrial (inner): Mercury, Venus, Earth, Mars & Mercury is
mostly Fe ( = 5.4)
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Jovian (outer): Jupiter, Saturn, Uranus, & Neptune mostly
gas and ice (  = 0.7)
7) Terrestrial planets are have a lot of Fe and O, Si & Mg. The
Sun is almost entirely H & He (also important in Jovian
planets).
Collision hypothesis
Portions of the Sun were torn off
by a passing star: planetesimals
then collided to form planets.
Problems: gases coming from
Sun would be too hot to
condense; stellar collision
exceedingly rare
Protoplanet hypothesis

Large gas cloud begins to condense.

Most mass in the center, turbulence in outer
parts.

Turbulent eddies collect matter meters
across; small chunks grow and collide,
eventually becoming large aggregates of gas
and solid chunks.

Protoplanets, much bigger than present
planets, eventually contracted due to their
own gravity.
Nebular hypothesis


Most popular theory is called the nebular
hypothesis
Fell out of favor for a number of years, but
now it is considered the definitive model
Nebular hypothesis
Primeval slowly rotating gas cloud
(nebula) condensed into several
discrete blobs.
fits
rotation
mass
doesn't fit
angular momentum
Solar Nebula
Hypothesis
The beginning of the
Solar System 6.9 billion
years ago –
nebula formation
An ancestral star –
ended its life: Red Giant
Explosion - Nebula
Early Nebula
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Solar System began when part of a molecular
cloud of interstellar gas filled with particles of ice,
dust, rock, and other particles, collapsed
Clouds collapsed caused it to heat up and
eventually turn into a star
Most of the cloud formed the Sun
Other material from the cloud flattened around
the Sun forming a planetary disc

The material from the planetary disc went to form the
planets and other objects in our Solar System
A nebula is a
cloud of gas and
dust made by an
exploding star.
Planetary nebula:
remaining mineral
particles and gas
after a star
explodes

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Stars release energy
and build elements
through nuclear
fusion.
Nuclear fusion creates
new elements.
Stars “burn” hydrogen,
becoming brighter.
Eventually, stars
become Red Giants
and explode.
Nebula Collapse and Condensation
Rotation started by shockwaves
from a nearby explosion (?)
Because the solar nebula
was rotating, it contracted
into a disc, and the planets
formed with orbits lying in
nearly the same plane.
Planetesimal
accretion - ~5 to
4.6 billion yrs ago
Planet Formation

Some material formed solid objects
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Eventually, each object got large enough to
attract more dust and ice with its gravitational
influence
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Got larger as other particles collided with it and stuck
together
These balls then form the cores of the planets
Astronomers believe that it took millions of years
for the planets to form
Asteroids and other planetesimals are “failed
planets” – objects formed from the solar nebula
that never got large enough to turn into planets
Inner and Outer Planets

Nebula was composed mostly the two simplest and
lightest atoms hydrogen and helium, in gaseous form

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Also contained heavier atoms (mostly oxygen, carbon, silicon,
iron), some of them in the form of dust particles
As the Sun took shape in the center, matter coalesced in
the outer regions of the disk

Critical phase in development of the Solar System
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Earth and other planets would formed in an area of the disk that
was enriched with heavier elements because it was much warmer
The outer region of the disk cooled rapidly, with the result that some
rock and frozen volatile elements condensed as tiny particles
Particles grew by colliding with one another, forming
larger masses that eventually developed gravitational
fields
Terrestrial Planets
Gas Giants
•Rocky particles and metallic compounds formed solids in the inner portion of the
condensing solar nebula
•It was too hot for hydrogen compounds to solidify
•In the cooler outer region, hydrogen compounds, metals, and rocks condensed to a
solid state
•The transition zone between the two regions is known informally as the frost line.
When the Solar wind “turned
on”, volatiles were expelled
from inner Solar System
Earth’s Geomagnetic Field
Blown into a streamlined shape by the Solar Wind.
•Early in the history of the Solar System, the solar wind stripped the inner planets of
their primitive atmospheres
•In this image, the modern solar wind “ blows” the geomagnetic field into a streamlined
shape with the blunt end facing into the solar wind and the tail extending downwind
Our Sun: A Massive Hydrogen Bomb held together by gravity
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Solar core is site of
nuclear fusion.
H is converted to He,
which has less mass

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4 H atoms fuse together
to form one He atom
Mass differential is
expelled as energy (light
and heat).
The Sun is getting “lighter”
through time.
Enough fuel to last another
4 to 5 billion years.
•Everything in Solar System orbits around
the Sun
•Balances between Sun’s gravity with the
centrifugal force
•The Sun contains over 99% of the Solar
System’s mass
Mercury
•Vertical axis (no
seasons)
•Probable molten
(Fe) core
•Silicate (SiO2) shell
•Atmosphere
created by solar
wind
•227oC to -137oC
Venus
•Axis spin opposite to
other planets (upside
down?)
•Is core liquid or
solid? - Unknown
•Active volcanism? Probably
•Atmosphere 96.5%
(CO2)
•460oC “runaway”
Greenhouse Effect
Greenhouse Worlds

Why is Venus so much hotter than Earth?

Although solar radiation 2x Earth, most is reflected
but 96% of back radiation absorbed
Energy Budget
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Earth’s temperature constant ~15C
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Energy loss must = incoming energy
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Heat loss called back radiation
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Earth is constantly receiving heat from Sun,
therefore must lose equal amount of heat back
to space
Wavelengths in the infrared (long-wave
radiation)
Earth is a radiator of heat

If T > 1K, radiator of heat
Energy Budget
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Average Earth’s surface temperature ~15C
Reasonable assumption
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Surface of Earth radiates heat with an average
temperature of 15C
However, satellite data indicate Earth
radiating heat average temperature ~-16C
Why the discrepancy?
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What accounts for the 31C heating?
Energy Budget

Greenhouse gases absorb 95% of the long-wave,
back radiation emitted from Earth’s surface
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Trapped radiation reradiated down to Earth’s
surface
Accounts for the 31C heating
Satellites don’t detect radiation
Muffling effect from greenhouse gases
Heat radiated back to space from elevation of
about 5 km (top of clouds) average 240 W m-2

Keeps Earth’s temperature in balance
Greenhouse Worlds

Why is Venus so much hotter than Earth?
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Although solar radiation 2x Earth, most is reflected
but 96% of back radiation absorbed
What originally controlled C?
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During formation of solar system most carbon
was CH4
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Lost from Earth and Venus
Earth captured 1 in 3000 carbon atoms
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60 out of every million C atoms
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CaCO3 (limestone and dolostone) and organic residues (kerogen)
Bulk of carbon in sediments on Earth
Venus probably had similar early planetary history
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Tiny carbon fraction in the atmosphere as CO2
Most carbon is in atmosphere as CO2
Venus has conditions that would prevail on Earth
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All CO2 locked up in sediments were released to the
atmosphere
Earth and Venus
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Water balance different on Earth and Venus
If Venus and Earth started with same components
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Venus should have either
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Sizable oceans
Atmosphere dominated by steam
H present initially as H2O escaped to space
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H2O transported "top" of the Venusian atmosphere
Disassociated forming H and O atoms
H escaped the atmosphere
Oxygen stirred back to surface
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Reacted with iron forming iron oxide
Planetary Evolution Similar
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Although Earth and Venus started with
same components
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Earth evolved such that carbon buried
safely in early sediments
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Avoiding runaway greenhouse effect
Venus built up CO2 in the atmosphere
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Build-up led to high temperature
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High enough to kill all life
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If life ever did get a foothold
Once hot, could not cool
Why Runaway Greenhouse?
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Don't know why Venus
climate went haywire
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Extra sunlight Venus
receives?
Life perhaps never got
started?
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No sink for carbon in
organic matter
Was the initial
component of water
smaller than that on
Earth?
Mars
•Most Earth-like of
planets in Solar
System
•Iron core, partially
liquid
•Silicate (SiO2) mantle
and crust
•Active volcanism? Probably
•Atmosphere 95.3%
(CO2)
•Past “flooding” and
fluvial erosion of
surface
Water has flowed in the past. But is now
locked up as ice in the ground and as
polar ice caps. Drainage features due to
short-lived melting events
Mars Exploration
Broad Goals of Mars Exploration
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Determine whether life ever arose on
Mars
Characterize the climate of Mars
Characterize the geology of Mars
Prepare for human exploration
Mars Rover Curiosity
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Curiosity landed on Mars August 5, 2012
Mission – robotic exploration of Mars
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Rather than look for life directly, Curiosity was
designed to see if Martian environments could ever
have hosted life
Curiosity carries the most advanced suite of
instruments ever sent to the Martian surface
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Curiosity analyzes samples scooped from the soil and
drilled from rocks
The record of the planet's climate and geology is
"written in the rocks and soil“
Curiosity's onboard laboratory will study rocks, soils,
and the local geologic setting in order to detect the
chemical building blocks of life (e.g., forms of carbon)
on Mars and will assess what the Martian environment
was like in the past
Curiosity discoveries
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Mars is a suitable home for life
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Organic carbon was found on Mars
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Information necessary for a human mission
Thick atmosphere with water
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[CH4] changed 10-fold!
Radiation could be health risks for humans
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Raw ingredients necessary for life to get started
Methane concentration is variable on Mars
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Environment suitable for living microbes
Found S, N, O, P and C
“Heavy” isotopes of H, C and Ar; “light” lost to space
Evidence that there was once more water on Mars
Ancient streambed found
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Water about knee-deep on flowed on Mars
MERCURY
VENUS
EARTH
Terrestrial planets are small and rocky, with thin
atmospheres, silicate and metallic shells.
O, Fe, Si, Mg, Ca, K, Na, Al
MARS
•Not all of the planetesimals ended up becoming planets
•Some were made up primarily of rocky and metallic substances, and they
became asteroids
•Most asteroids reside in a belt of rocky debris between Earth and Jupiter that
may be left over from the early solar system
•The total mass of all the asteroids is less than that of our Moon.
•Jupiter is the largest planet in Solar
System – more than twice as massive as all
other planets combined
•Enormous size is due to its huge gaseous
atmosphere
•Holdover from the early days of the
Solar System
•While the terrestrial planets lost their
early atmospheres to the heat of the
evolving Sun, Jupiter did not
Jupiter's Great Red
Spot - A hurricane
the size of Earth
lasting several
centuries
Does Jupiter have
a hard surface?
Lack of hard
surface may allow
for different winds
at different speeds
– hence, banding
90% Hydrogen,
10% Helium
•Saturn: 9 rings of rock
and ice particles, 10,000
km wide and 200 km
thick
•Outer layer of frozen
ammonia (NH3)
96% Hydrogen, 3.35%
Helium
•62 moons
•Uranus: axis tilted
completely on its side
•Uranus north pole points
toward the Sun for half of a
Uranian year; its south pole
points toward the Sun for
the other half of the year
•Results in extreme
seasonal effects
• The polar regions
get the greatest
amount of sunlight
• Rest of the planet
fails to experience
the daily heating and
cooling experienced
by other planets
•82.5% Hydrogen, 15.2%
Helium, 2.3% Methane
(CH4)
•Neptune is the outermost
classical planet in our Solar
System
•The inner two-thirds of the
planet are thought to be
composed of a mixture of
molten rock, water, liquid
ammonia, and methane
•Neptune: highest winds in
Solar system, 2000 km/hr
•80% Hydrogen, 18.5%
Helium, 1.5% Methane
(CH4)
Gas Giants are massive planets with thick
atmospheres.
He, H, CO2, H2O, N2, CO, NH3, CH4
Neptune
Jupiter
Uranus
Saturn
Dwarf Planet
“an object in the Solar System that orbits the Sun and is not a
satellite of a planet or other celestial body. It must be
spherical (or nearly so) in shape.”
•Dwarf planets do not resemble the inner, terrestrial planets or the outer, gaseous
planets in their makeup
•Closely resemble the ice moons of the outer planets
•Researchers suspect that they are large icy chunks of debris left over from the
formation of the Solar System.
Formation of Earth
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Accretion from the nebular cloud as particles smashed into
each other, forming planetesimals
Earth’s mass and gravitational field grew causing it to
compress into a smaller, denser spherical body
Compression generated heat in Earth’s interior
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Heat also produced by
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Earth’s interior began to melt
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Decay of radioactive elements in Earth’s interior
Impacts of thousands of extraterrestrial objects
Iron is abundant and heaviest common element - droplets of liquid iron
sank toward the planet’s center and condensed to form the core
Melting iron moving through the planet generated friction that
contributed more heat and raised Earth’s temperature
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Deep magma ocean formed on the planet surface
Earth began to differentiate
In time EARTH’S interior
accumulated heat
New atmosphere created by volcanic
outgassing and delivery of gases and
water by ice-covered comets.
“Hadean Era”
Formation of our Moon
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Capture hypothesis
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Earth’s gravity captured a passing planetesimal, which became
the Moon
Hypothesis predicts that Earth and Moon were formed at
separate locations
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Chemistry of Moon rocks is similar to that of Earth rocks, suggesting
that both developed either from a common source or at least at the
same location in the planetary nebula
Double planet hypothesis
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Earth and the Moon were formed concurrently from a local cloud
of gas and dust
Hypothesis fails to account for the unusual tilt of the Moon’s axis,
melting of its surface rocks, and the fact that it is less than half
as dense as Earth
Formation of our Moon
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Fission hypothesis
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Centrifugal force associated with Earth’s spin caused
a bulge of material to separate from Earth in the area
of the equator
Hypothesis requires that Earth rotate once every 2.5
hours in order to develop the necessary force.
Impact hypothesis
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Most widely accepted explanation for the Moon’s
formation
During planetesimal accretion, Earth suffered a
massive collision with a huge object the size of Mars,
and this collision led to the formation of our Moon
•The impact hypothesis
suggests that Earth
suffered a massive
collision that led to
formation of our Moon
•This image shows Earth
and a Mars-size object,
each peppered by
hundreds of smaller
impacts, colliding with
one another in the early
Solar System
Why worry about the beginning?
 The
evolutionary course is
significantly influenced by the
initial state
 We
know the state of the Earth
today relatively well; knowing
the beginning will help constrain
the in between