Lecture Slides
CHAPTER 7: Atmospheres of Venus, Earth, and Mars
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
Atmospheres of Venus, Earth, and Mars
For these three terrestrial
worlds, understand:
 Primary versus
secondary atmospheres.
 Compositions.
 Greenhouse effect.
Origin of Atmospheres
 New planets formed with primary
atmospheres (mostly hydrogen)
 All terrestrial bodies lost their primary
atmospheres.
 Planets could not hold onto the gas.
• Combination of high gas speeds and low
planetary gravity.
 Secondary atmospheres were acquired
later by:
• Accretion (but locked in the interior).
• Volcanism (releasing the gas from the
interior).
• Comet impacts.
Origin of Atmospheres
Origin of Atmospheres: Atmospheres of Planets
 Mercury and the Moon have basically no atmosphere.
 Venus, Earth, and Mars were able to obtain significant
secondary atmospheres.
• Similar geologic histories.
• Should have had similar secondary atmospheres
Class Question
The Moon and Earth have approximately the same
average temperature. Why does the Moon lack an
atmosphere?
A. Comets, which deliver water and air, collide only
with Earth.
B. The Moon has no life. Life produces the
atmosphere.
C. The Moon never had any volcanism.
D. The Moon has a low gravity.
Compositions
 Earth: Primarily nitrogen and oxygen.
 Venus and Mars: Mostly carbon dioxide.
 Venus and Earth have much more atmosphere
than Mars.
• Mars has a much lower surface gravity than either
Earth or Venus.
Greenhouse Effect
 Incoming sunlight heats the planet.
 Outgoing infrared radiation cools the planet.
Greenhouse Effect: Greenhouse Gases
 Some gases, especially carbon dioxide and water
vapor, block some infrared radiation, preventing the
planet from cooling.
 These are examples of greenhouse gases.
Greenhouse Effect: Equilibrium
 The heated atmosphere also warms the ground.
 This greenhouse effect means the planet’s surface
reaches a higher temperature. Equilibrium is attained
between absorbed sunlight and thermal energy
radiated away by the planet.
Greenhouse Effect: Temperature of Planets
 The average temperature of the Earth would be
below freezing without the greenhouse effect! (some
global warming is a good thing!)
 Due to the greenhouse effect, Venus is hotter than
Mercury, although about twice as far from the Sun.
Greenhouse Effect
 Venus: Hot early in its history, so no water could
remain on surface.
 Earth: Mild temperature early in its history allowed
water to remove carbon dioxide from atmosphere (by
being absorbed into the oceans) and life further
removed carbon dioxide.
 Two effective greenhouse gases, water and carbon
dioxide, were therefore constrained on Earth; on
Venus, they were not, which led to a runaway
greenhouse effect.
Compositions
 Earth’s atmosphere is mostly nitrogen and oxygen.
• Life is responsible for the oxygen’s presence.
 Other planets do not have substantial amounts of
oxygen in their atmospheres.
Class Question
Why does Earth have a substantial amount of
oxygen in its atmosphere while the other planets
do not?
A. The Earth is the only planet with life.
B. The oceans emit oxygen.
C. The oxygen in the primary atmosphere
escaped.
Layers: Troposphere
 Troposphere (surface to
10–15 km altitude):
• Temperature and pressure
decline with altitude.
• Water vapor mainly is
located here.
• Weather happens here!
Layers: Stratosphere
 Tropopause: Temperature
stops declining with altitude.
 Stratosphere (15–50 km):
• Temperature rises with
altitude.
• Ozone absorbs ultraviolet
light, heats stratosphere.
Layers: Mesosphere
 Mesosphere (50–90 km):
• Temperature declines with
altitude.
• Above ozone layer
• Upper mesosphere is
coldest part of atmosphere.
Layers: Thermosphere and Ionosphere
 Thermosphere (> 90 km):
• Ultraviolet radiation and
solar wind can ionize atoms.
• Solar wind = flow of
particles from the Sun.
 The ionosphere overlaps
the thermosphere but is
mostly in the upper portions
of it, extending up to
1000km.
Layers
 Earth’s magnetic
field makes a region
surrounding the
Earth called the
magnetosphere,
which extends out
into space.
 It blocks much of the
solar wind.
Layers: Auroras
 The magnetosphere can trap charged solar wind
particles.
 The particles collide with the atmosphere, heating it
up and causing it to glow.
 This creates the aurora.
Layers: Auroras (Cont.)
Layers: Auroras (Cont.)
Layers: Hadley Circulation
 Parts of the Earth
are heated
differently.
 Vertical circulation
of air (convection)
distributes surface
heating.
 Global winds carry
heat from hot to
cool regions.
Layers: Coriolis Effect
 The circulation
depends on
heating pattern
and rotation
period.
 The Coriolis
effect, due to
rotation, breaks
the circulation into
zonal winds.
Layers: Coriolis Effect (Cont.)
Layers: Coriolis Effect (Cont.)
Venus
 Venus: Hot, dense
atmosphere,
completely cloudcovered.
 96% carbon dioxide
 Runaway greenhouse
effect.
 Surface temperature
about 737 K, hotter
than Mercury, and hot
enough to melt lead!
 Clouds are nasty:
made of sulfuric acid!
Venus: Atmosphere
 Thick atmosphere
• Uniform temperature over
entire planet
• Opaque in visible light
• Atmospheric pressure at
surface is almost 100x
times the pressure at
Earth’s surface!
• Strong winds in upper
atmosphere
 Surface imaged by radar.
 Rotates on its axis very
slowly, and in the opposite
direction as rest of solar
system.
Mars
 Mars: Cold, thin atmosphere, composed mostly of
carbon dioxide
 Thin atmosphere => extreme temperature variations.
Mars: Atmosphere
 Daily, seasonal, and latitudinal temperature variations
create large winds.
=> Planet-covering dust storms!
Class Question
Why is Mars so cold compared to Venus if both of
their atmospheres are made mostly of carbon
dioxide, a greenhouse gas?
A.
B.
C.
D.
Mars is farther from the Sun.
Mars has lower gravity.
The atmosphere on Mars is much thinner
All of the above.
Greenhouse Effect
 Why care about the
greenhouse effect on Earth?
=> We do not want a runaway
greenhouse effect, like Venus!
 Earth
temperature
measurements
show a steady
increase over
the past 130
years.
 More
greenhouse
gases
increase the
greenhouse
effect.
 Earth’s climate
is a complex
system.
Class Question
Earth started with about as much carbon dioxide as
Venus has. In Venus, the carbon dioxide is mainly in
the atmosphere. Where is it on Earth?
A. In rocks, the ocean, and in life.
B. It was lost when the primary atmosphere
escaped.
C. It evaporated from Earth’s atmosphere while
other gases were retained.
Chapter Summary
 The atmospheres of Venus, Earth and Mars have
evolved since they first formed.
 The atmospheres of the three planets differ in
temperature, pressure and composition.
 All three planets have greenhouse gases in their
atmospheres.
 The conditions of the atmospheres of Venus, Earth
and Mars can be explained by the greenhouse effect.
Astronomy in Action
Changing Equilibrium
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Astronomy in Action
Charged Particles and Magnetic Forces
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AstroTour
Atmospheres Formation and Escape
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(Requires an active Internet connection)
AstroTour
Greenhouse Effect
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Nebraska Applet
Gas Retention Plot
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(Requires an active Internet connection)
Nebraska Applet
Gas Retention Simulator
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(Requires an active Internet connection)
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture slides for
CHAPTER 7: Atmospheres
of Venus, Earth, and Mars
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company