Chapter 7
Earth and the Terrestrial Worlds
Understanding the similarities and differences between the
planets of the solar system, in particular, the four terrestrial
planets, can tell us how Earth becomes the way it is today.
• The similarities and differences of the terrestrial worlds.
• The small terrestrial worlds: Mercury and our Moon.
• The large terrestrial worlds: Mars, Venus, and Earth.
What makes the environment on Earth suitable for life?
• Mars?
What’s the future of Earth?
Similarities and Differences of the
Terrestrial Worlds
From a distance, they appear very
similar…
• rocky and small (we really
can’t see the surface of Venus
directly) !
• No or few moons
• No rings
Examined close-up, They are very different…
• Mercury and Earth’s Moon are airless and barren
• Mars has a very thin atmosphere
• Earth has oxygen, water, and life!
• Venus has a thick atmosphere and very hot!
•
•
•
•
Internal Structure
Surface Features
Atmosphere
What makes the Earth
hospitable to life?
– Venus
– Mars
• Global Warming?
Internal Structure of the Terrestrial Planets
The internal structure of the terrestrial planets are similar. They all have
• Core – High density metal
• Mantle – Medium density rocky materials, such as silica (SiO2), hot, semi-solid
• Crust – lowest density rocks, such as granite and basalt (black lava rock…)
The layering of different density materials occurs due to differentiation – heavy materials
sink to the bottom while lighter material rise to the top…
Lithosphere: The coolest and most rigid layer of rock near a planet’s surface.
Molten lava of Earth exists at a very narrow region beneath the lithosphere
Inside the Earth: More Details
Lava comes from a thin layer under the lithosphere…
Illustration by J. C. Butler
http://www.uh.edu/~jbutler/physical/chapter19.html
Illustration from USGS
http://pubs.usgs.gov/gip/dynamic/inside.html
Heating of the Terrestrial Planets
The interiors of the terrestrial planets are
heated by
• Gravitational potential energy of the
accreting planetesimals are converted
into thermal energy.
• Radioactive heating
Radioactive Heating
Radioactive materials (e.g., uranium,
potassium, thorium) decay by emitting
subatomic particles (alpha particle—nuclei
of helium, beta particle– electrons or
positron, neutron, proton, etc.) and often
gamma-ray, which collide with surrounding
atoms, heating them up.
− Potassium-40 → Argon-40
− Uranium-234 → ……. → Lead-206
Internal heating causes Mantle
Convection---hot rock rises to the top
and cools off, cool rock sinks to the
bottom, resulting in the cooling of the
planet…
Earth’s Magnetic Field
Another important characteristics of the Earth is its magnetic fields, which shield
us from the bombardment of the high-energy charged particles, mostly from the
Sun.
• The rapid rotating liquid outer metal core of Earth generate magnetic field.
• The charged particles from the Sun must move along the magnetic field lines,
and are directed to the north and south polar regions. The interactions between
the charged particles and the molecules of the atmosphere cause the glow of
atmosphere near the north and south poles  aurora borealis and aurora
australis
• Without magnetic field, solar wind can strip much of the Earth’s atmosphere…
Building a Magnet
•
•
We can generate magnetic field by circulating electric charges (running a
electric current) in a spiral path.
We do not have a complete theory of how the magnetic field of the Earth is
formed yet…but generally, it is believed to form by the rotation of the Earth
carries the electrically conducting molten metals in the core around,
generating Earth’s magnetic field.
Lines indicate
points with equal
magnetic field
strength
Reaching Inside the Earth
We can study the interior structure of the Earth by studying how
seismic waves travel through Earth…
Seismic waves propagate through Earth in two modes:
• P wave: Primary (Pressure, or Pushing) wave
P wave can travel through any material.
• S wave: Secondary (Shear, or side-to-side) wave.
S wave cannot travel through liquid.
•
•
•
•
Internal Structure
Surface Features
Atmosphere
What makes the Earth
hospitable to life?
• Global Warming?
Surface Features
Processes shaping the surface of the planets
• Impact cratering: the blasting of bowl-shaped impact
craters by asteroids or comets striking a planet’s surface.
• Volcanism: the eruption of molten rock, or lava, from a
planet’s interior onto its surface.
• Tectonics: the disruption of a planet’s surface by internal
stresses.
• Erosion: the wearing down or building up of geological
features by wind, water, ice, and other phenomena of
planetary weather.
Impact Crater in Arizona
Volcanism
• Volcanism occurs when
underground molten rock
finds a path through the
lithosphere to the surface
• In addition to shaping the
surface of the planet, it
explains the existence of
our atmosphere and
ocean…water and gases
trapped in the interior of
Earth are released into
the atmosphere through
volcanic activities…
• The best results…the
formation of our island
paradise…Hawaii!
Plate Tectonic
• Tectonics is particularly important on Earth, because the
underlying mantle convection fractured Earth’s lithosphere
into more than a dozen pieces, or plates. These plates
move over, under, and around each other, leading to a
special brand of tectonics that we call plate tectonics.
Erosion
Erosion is a blanket term for a variety of
processes that break down or transport rock
through the action of ice, liquid, or gas.
– The shaping of valleys by glaciers (ice),
– the carving of canyons by rivers (liquid),
– and the shifting of sand dunes by wind (gas)
are all examples of erosion.
Mauna Kea and Mauna Loa are very tall, but as we
move toward the west, the volcanoes gets lower and
lower…
•
•
•
•
Internal Structure
Surface Features
Atmosphere
What makes the Earth
hospitable to life?
• Global Warming?
The Atmosphere of the Terrestrial Worlds
According to the Nebular Theory, the terrestrial planets were formed by
metallic and rocky planetesimals. So,
Where did the gas come from?
•
•
The gases came from comets and asteroids impact during the period of
heavy bombardment.
The gases are trapped in the interior of the planets, later released through
volcanic out-gassing.
But, why are their
atmosphere so different?
• How come Earth has
so much H2O?
• How come Earth
don’t have much
CO2?
• How come Earth has
so much O2?
Why Mercury and the Moon don’t
have an Atmosphere?
Mercury and the Moon don’t have an
atmosphere because they are too
small. Their weak gravitation field is
not enough to keep the gas.
1 2
kT  mv
2
2kT
v
m
Given the same temperature…
– Light gases escape easily,
– Heavier gases are trapped by
gravity…
Given the same temperature, the thermal velocity of lighter gases
are higher compared with velocity with heavier gases. Therefore,
the lighter gases have better chance of acquiring a speed greater
than the escape velocity of the Earth and escape…(remember that
the escape velocity of the Earth is about 14 km/sec, and
independent of the mass of the escaping object).
Mercury and Earth’s Moon
Similarities between Mercury and the Moon:
•
Size
•
No Atmosphere
•
Dense impact craters on the surface  No geological activities to
alter the surface features after the period of heavy bombardment –
they are geologically ‘dead’ long time ago…
•
Large day/night temperature difference
The similarities between these two worlds
can be explained by their small sizes:
• Small size  low surface gravity 
low escape velocity  gas cannot be
trapped by gravity on the surface.
• No atmosphere  large day/night
temperature difference
• Small size  small initial heat content
 they cool off fast  low level of
geological activities
Surface of Mercury looks very similar to the Moon
Venus, Mars and Earth
Venus and Earth are very similar in size, and Mars is a little smaller. Their
surface features are somewhat similar also – they both have few impact
craters, and they all have volcanoes and evidence of tectonic activities.
Although there is a very large difference in the amount of gases on Venus
and Mars, their chemical compositions are very similar – high percentage of
CO2
Among the three large terrestrial worlds, the surface environment of
Earth
is very
unique.
The surface
of the
Earth is characterized by
•
•
•
•
Abundant surface liquid water
Abundant atmospheric oxygen
Plate tectonics
Climate stability
These are features that are very important to
support life on Earth.
Surface Temperature of the Terrestrial
Worlds
The “No Greenhouse Temperature” of a planet depends on
its distance from the Sun, and its albedo (or the reflectivity
of a surface or a body)…
However, the presence of an atmospheres can drastically
change the surface temperature of a planet.
The “No Greenhouse Temperature”
• Without an atmosphere, the surface
materials of the planets absorb some of
the visible light from the Sun.
• The temperature of the surface material
increases, depending on the amount of
energy it absorbs (the albedo).
• The planet surfaces re-radiates the
absorbed energy in the form of thermal
radiation.
Objects with temperature • The re-radiated energy is equal to the
absorbed energy.
of a few hundred degree
• The amount of energy the planet
Kelvin emit thermal
surface radiates depends on its
radiation in the IR
temperature. The equilibrium
temperature is around a few hundred
degree Kelvin…
How does the Atmosphere Affects
the Environment on the Surface?
• X-rays are absorbed by the
atoms and molecules of the
atmosphere at high
altitude…
• UV is absorbed by the ozone
(O3) in the stratosphere…
• Visible light reaches the
ground and warms the
surface.
• IR radiation is absorbed by
the water vapor…
• Radio waves are not affected
by the atmosphere…
The Effects of the Atmosphere on
Planet Surface Temperature
Depending on the composition of the atmosphere, the
effect can be very different
• Venus: 96% CO2, 3.5% N2
 T = 740 K.
• Earth: 77% N2, 21% O2, 1% Argon (‘dry air’),
– Variable H2O (~ 10%...order-of-magnitude)
– Small amount of CO2 (~0.03%)
 288 K.
Why?
Greenhouse Effect!