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The Terrestrial Worlds
© Sierra College Astronomy Department
The Terrestrial Worlds
Overview
We will be comparing the Earth to the other
terrestrial planets and the Moon
While there are obvious differences as one looks
at the surface features:
– Moon and Mercury: barren, crater filled worlds
– Mars: Large canyons and volcanoes, some craters
– Venus: Thick atmosphere, some volcanoes, and
features indicating activity, some craters
– Earth: Modest atmosphere, volcanoes, water, life,
great geological activity
So why is the Earth geologically active?
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The Terrestrial Worlds
Earth’s Structure & Composition
Some Basic Facts:
– Earth bulges at equator about 21 km more than at
poles.
– The earth velocity at the equator is ~1600 km/hr
(1000 miles/hr), whereas at mid-latitudes the velocity
is 1100 km/hr (700 miles/hr).
– Density - ratio of mass to volume - of the Earth is 5.52
g/cm3. (Water’s density is 1 g/cm3, aluminum is 2.7
g/cm3, and iron is 7.8 g/cm3.)
Overall Composition
– The crust of the earth is mostly (73%) composed of
silicates - (silicon and oxygen).
– Other elements are aluminum, iron, calcium,
magnesium, sodium, potassium, titanium and others
(1%).
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Earth’s Structure & Composition
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:
– Crust is the thin (<100 km) outermost layer of the Earth and
has a density of 2.5–3 g/cm3. The top part of the crust is
relatively cool region of rock called the lithosphere.
– 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. © Sierra College Astronomy Department
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The Terrestrial Worlds
Earth’s Structure & Composition
Interior temperature increases ~ 2o K/100
meters of depth. The Earth’s core is believed to
be ~ 6500 K.
The Earth and other planets gained heat form
the process of formation
Core is heated by radioactive decay which
releases heat and is contained in the core by
the outer layers.
Recall that through radioactive dating, the Earth
is about 4.5 billion years old.
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Internal Heat and Geologic Activity
Interior heat drives the geologic activity on the
terrestrial planets
Convection is the process by which hot
material expands and rises while cooler
material contracts and falls
– Air does this very quickly
– The Earth’s mantle moves a few cm/year
Internal heat depends on the size of the planet
(actually the surface-area-to-volume ratio, see
Cosmic Calculations 7.1), so Earth and Venus
have a great left over, while Mercury and the
Moon have little and Mars has some
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Earth’s Magnetosphere
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.
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.
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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.
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The Terrestrial Worlds
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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The Terrestrial Worlds
Impact Cratering
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
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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The Terrestrial Worlds
Impact Cratering
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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The Terrestrial Worlds
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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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
forced together.
© Sierra College Astronomy Department
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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
– 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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The Terrestrial Worlds
Earth’s Atmosphere
The Earth’s atmosphere formed in two ways.
– Heating the solid material by volcanic action or
by violent asteroid impacts. This released the
gases from the rocks.
– Gases were brought here by comets.
Both hypotheses are difficult to validate, but a
combination of both is most probable.
Biological activity then altered the original
atmosphere.
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The Terrestrial Worlds
Earth’s Atmosphere
Earth’s atmosphere consists of about 77%
nitrogen (N2), 21% oxygen (O2), with minor
amounts of water vapor (H2O), carbon
dioxide (CO2), argon (Ar), and trace amounts
of ozone (O3).
The Earth’s atmosphere is “layered”
according to temperature and composition.
– 2/3 of the atmospheric is only 10 km (6 mi) from
the surface of the Earth
The Earth’s atmosphere absorbs certain
amounts of light and allow other to pass
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Earth’s Atmosphere
About 25-50 km above the surface of the
Earth (in the stratosphere) is the ozone
layer, which is an efficient absorber of
UV radiation from the Sun. This
absorption causes the temperature of
the atmosphere to peak at the ozone
layer.
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The Terrestrial Worlds
Earth’s Atmosphere
The greenhouse effect is the atmosphere’s
process of “trapping” heat emanating from the
ground (originally captured from the sun).
– The atmosphere becomes another source of heat (in
addition to the sun)
The greenhouse effect is enhanced by so-called
greenhouse gasses : mainly water vapor, carbon
dioxide, and methane
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The Moon and Mercury
The Moon’s geology
The Moon’s surface can be divided into
two main landforms: lunar maria and
highlands (mountainous and cratered)
regions.
Maria (plural of mare) are any of the
lowlands of the Moon (some circled by
mountains) that resemble a sea when
viewed from Earth.
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The Moon’s Surface
The Maria were caused (3 to 4 billion years ago, just after the
Moon was formed) by large impacts cracking through the crust
and the consequent magma flow from the Moon’s mantle.
Asymmetry of maria between the two sides of Moon is caused
by differences in crust thickness (which ranges in depth from
60-100 km and is thinner on Earth-facing side).
This asymmetry also lead to the “locking” of one face of the
Moon always towards the Earth (since the maria are made of
denser materials).
The interior of the Moon has cooled too much for this to occur
again
Micrometeorites, sand sized particles from space, remain as
the only major erosion process
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The Moon and Mercury
Mercury’s geology – extreme conditions
Radar observations show that Mercury rotates once
very 58.65 Earth days, which is precisely 2/3 of its
orbital period.
Mercury’s solar day is quite different from its sidereal
day. The solar day is 176 Earth days long (two
Mercurian years.)
Only 2 longitudes on Mercury experience noon while
the planet is at perihelion
High temperatures on Mercury can reach 425°C
(790°F), well above the melting point of lead (330°C
or 626°F).
On the night-side of Mercury, temperatures can fall to
-150°C (-250°F).
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Mercury and the Moon
Mercury’s geology - Moon Comparison
Mariner 10 flew by Mercury in 1974 (and
subsequently twice more), returning a total of 4,000
photographs for the three fly-bys.
Mercury appears similar to our Moon; both are
covered with many impact craters.
Mercury’s craters are less prominent; the planet’s
surface gravity is twice that of the Moon so loose
material will not stack as steeply.
Ray patterns are also less extensive on Mercury
because of the higher gravity.
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The Terrestrial Worlds
Mercury’s Surface
Mercury’s surface history is thought be:
– Mercury was hot and melted due to radioactive
decay and expanded in size
– This fractured the crust and allowed lava to reach
the surface to form the intercrater plains
– Lava eruptions in impact basins formed the
smooth plains
– Then the interior cooled and the planet shrunk
cracking the surface forming the scarps
– This probably happened in the first 700 million
years after Mercury formed
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Mercury and the Moon
A large “bulls-eye” impact crater called
Caloris Basin is visible.
The Moon has a similar impact region
This impact was so intense that there is
broken terrain in the region opposite of
the Caloris basin
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Mars
Mars’s Basics
Mars orbits the Sun at an average of 1.524 AU
(about 228 million km).
Mars’ orbit is more eccentric than Earth’s, so
Mars’ distance from the Sun varies from 210
million km to 250 million km.
Mars takes 1.88 Earth years to complete its
orbit around the Sun.
Polar caps of water-ice and carbon dioxide can
be seen
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Mars
Mars’ sidereal period is 24h37m; its solar day is
24h40m long, very similar to that of Earth.
Mars’ equator is tilted 25.2° with respect to its
orbital plane, close to Earth’s 23.4°.
We see seasons on Mars as we do on Earth.
– The polar caps grow and shrink accordingly
Because of Mars’ eccentric orbit, the southern
hemisphere exhibits greater seasonal shifts in
temperature than does the northern
hemisphere.
– These can have significant effect on the winds of
Mars
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Mars
Geology of Mars
Besides the polar caps, Mars has other
remarkable features
The southern hemisphere has most of the
higher elevation and the great impact region
called Hellas Basin and most of the impact
craters
The northern hemisphere has the lower
elevation, few impact craters and most of
the volcanoes
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Mars
The largest volcano is Olympus Mons, who
height of 24 km (15 mi) is twice that of Earth’s
largest mountain.
– Several other large volcanoes can be found in the
surrounding Tharsis Region
One reason Mars can “grow” larger volcanoes
than Earth is because they lack Earth-like
tectonic plates. Formed over a hot spot of lava
that wells up from within a planet, a volcano
can grow to enormous size if it does not move
off the hot spot.
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Mars
There were some tectonic activities in Mars’ past:
Valles Marineris is an enormous canyon on Mars
that stretches nearly 4,800 km (3,000 mi).
– However, it was not carved out by a river nor a result of
Earth-like plate tectonics
– Instead it is a split in the crust which caused the Tharsis
Region to bulge outward
– There do appear to be runoff channels on the edges of the
canyon which may have been formed by the outpouring of
subsurface water
There may be current geologic actively, though Mars
will “die” in the next few billion years
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Mars
Ancient Water on Mars
Could Mars have been water filled in its past?
– Outflow channel seem to imply that water flowed 2-3
billion years ago
Rovers Spirit and Opportunity (Mars Exploration
Missions: MER-A and MER-B; Rovers) landed on
Mars in 2004 looking for evidence of ancient water
– Opportunity found rocks that must have been soaking
in water at some time: Jarosite and the “blueberries”
containing hematite
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Mars
Present Water on Mars
Under the current conditions, free flowing water
is unlikely to exist on Mars since the pressure
and temperature are too low.
– Water will only exist as a gas or solid on Mars
– However, there is evidence of “gullies” which seemed
to have running water in the recent past
However, water or water-ice may exist just
underneath the surface of the planet.
– Odyssey and Mars Express orbiter both saw evidence
for subsurface water
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Mars
Why did Mars Change?
Mars’ likely was wetter and possibly warmer some 3
billion years ago
– Probably had a thicker atmosphere to allow rainfall
– Outgassing of water and CO2 from volcanoes would have
created a sufficient greenhouse effect
– A denser atmosphere would allow liquid water to flow on
surface
– CO2 in the atmosphere dissolved in water and into rocks,
causing the atmospheric temperature to drop, freezing all
the water, though much of the water and CO2 was lost into
space
– Oxygen formed from water which remained, CO2 rusted
the surface giving Mars the ruddy appearance of today
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Current and Upcoming Mars Missions
Currently there:
– Mars Global Surveyor and Odyssey (Orbiters;Relays)
– Spirit and Opportunity (Mars Exploration Missions: MER-A
and MER-B; Rovers)
– Mars Express
Beagle 2 rover crashed on surface, but orbiter is working fine
and it is taking some of the highest resolution pictures of the
Martian surface ever from orbit
– Mars Reconnaissance Orbiter
Even higher resolution of surface, subsurface, atmosphere
– Phoenix lander (2008) – mission ended
Digger arms, oven and portable laboratory
Landed in polar regions, discovered ice just under the surface
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Current and Upcoming Mars Missions
Mars Science Laboratory (Landed Aug 2012)
Called “Curiosity”
Bigger and better rover with many cameras and instruments
Up Next:
Maven (Mars Atmospheric and Volatile EvolutioN; launched
2013)
Exploring the Martian atmosphere and how it is changing today
and in the past
ExoMars/Trace Gas Orbiter (2016 by ESA)
Includes “demonstration” lander
Orbiting spacecraft that will help with telecommunications
Orbiter looking for trace gases
ExoMars/Trace Gas Rover- (2018 by ESA)
Looking for organic materials on Mars.
Prelude to return sample mission.
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The Terrestrial Worlds
Venus
Is Venus Geologically Active?
Since Venus is only 5% smaller than the Earth, we
expect it to be geologically active
Orbiting probes Pioneer Venus 1 (1978), Venera 15
and 16 (1983-84), and Magellan (1990-93) have
produced detailed radar maps of Venus’s surface.
About two-thirds of Venus’s surface is covered with
rolling hills. Highlands occupy <10% of the surface,
with lower-lying areas making up the rest.
Venus has about 1,000 craters that are larger than a
few kilometers in diameter
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Venus
Is Venus Geologically Active?
While it has volcanoes and a lithosphere contorted
by tectonics, Venus has some unique features, such
as coronae, probably made of hot rising plumes of
mantle rock.
Volcanoes are still active (erupting in the last 100
million years) since the atmosphere contains sulfuric
acid
There is the lack of erosion on Venus: the winds are
very weak.
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The Terrestrial Worlds
Venus
Is Venus Geologically Active?
Venus has a lack of Earth-like plate tectonics: no super high
mountain ranges
Crater counts are uniform across the planet, suggesting an
uniform age for the planet’s surface which is estimated to be
750 million years old. The uniformity of this age suggest that
the entire planet “repaved” itself at that time.
Since Venus should be a warm underneath the lithosphere
as the Earth, the lithosphere of Venus must be thicker than
that of the Earth and resists fracturing into pieces
– No direct proof of this
– May have come about from higher temperature surface
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Venus
Why Is Venus So Hot?
Venus surface temperature is about 880°F, hotter than Mercury.
About twice as much sunlight hits Venus as it does Earth
However, due to the larger abundance of highly reflective clouds of
Venus today, the Earth actually gets more sunlight to the surface
than Venus.
– Then why is it hotter on Venus than Earth and Mercury?
Venus’s closer distance from the Sun created conditions for a runaway greenhouse effect.
Water did not condense out of its atmosphere into oceans that could
absorb CO2 (as happened on Earth). Water vapor also contributed
to the greenhouse effect before it was broken apart the UV radiation
from the Sun and never reformed.
Life on Earth may have also help remove CO2 from the atmosphere
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The Terrestrial Worlds
Earth as a Living Planet
The Earth has several unique features
for life to exist
– Surface liquid water
– Atmospheric oxygen
– Plate tectonics
– Climate Stability
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Earth as a Living Planet
Our Unique Oceans and Atmosphere
– Water came from outgassing from volcanoes
(though the water may have come from
comet impacts)
– Oxygen did not come from outgassing, but
instead came from life itself
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Earth as a Living Planet
Plate Tectonics
– The lithosphere is broken down into more than a
dozen plates
– These plates get renewed in a process called
subduction and so the seafloor crust is never
more the 100 million years old.
– As a result, the continents have been spreading
away from each other for 200 million years.
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The Terrestrial Worlds
Earth as a Living Planet
Climate Stability
– The Earth’s climate has remained stable
enough to keep water liquid on the surface
– The Earth’s temperature has remained in
much the same range despite the fact the
Sun has brightened substantially in the last
4 billion years (by about 30%).
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The Terrestrial Worlds
Earth as a Living Planet
Climate Stability
– The self-regulation of the Earth temperature is done
by the CO2 cycle:
Atmospheric CO2 dissolves in rainwater, creating a mild
acid
This mild acid rain breaks down minerals in the rocks and
send the material to the oceans
These minerals combine with dissolved CO2 making
carbonate rocks such as limestone
Plate tectonics carries the carbonate rocks to subduction
zones to the mantle
As these rocks are pushed deeper into the mantle some of
the subducted rock melts and releases CO2 which outgases
back into the atmosphere
– So CO2 acts like a thermostat
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Earth as a Living Planet
Changes made by Human Activity: Global Warming
– Global Warming seems to be a reality because of
three facts:
By burning fossil fuels we are increasing the amount of
greenhouse gases
We understand the greenhouse effect enough to know that
this increase make our planet warm up at some time
Sophisticated models have matched the climate data quite
well and may be able to accurately predict to future increase
in global temperatures
– Consequences include the raising a water levels due
to slight warmer water temperature and the melting of
inland ice.
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The Terrestrial Worlds
Summary
Major Characteristics for Planets
– Orbit Parameters (Semi-major Axis, Eccentricity,
and Orientation)
– Planet Parameters (Mass, Radius, Avg. Density,
Sidereal & Solar Spin Rates, and Tilt)
– Interiors (Liquid vs Solid)
– Surfaces (Structures and Erosion)
– Atmospheres (Composition and Altitude Variation)
– Ionospheres and Magnetospheres
Depends on what?
– History in the Solar System Evolution
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The Terrestrial Worlds
Epilogue: Comparative Planetology
Surface features vs. planet size
– A competition between internally driven forces
vs. external bombardment
– The smaller the planet the more quickly it
cooled
– Major bombardment ended 4 billion years ago
Planetary atmospheres
– The key players: planet Mass, distance from
sun, planet radius
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The Terrestrial Worlds
Epilogue: Comparative Planetology
General principles:
– Larger planets are more likely to have
internal geological activity
– The larger the planet is, the younger its
surface features are likely to be
– The larger and cooler a planet is, the more
likely it is to have an atmosphere, and the
more likely this atmosphere is to have
retained its original gases
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The End
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