Download 11. Planetary Atmospheres Earth and the Other Terrestrial Worlds

Planetary Atmospheres
The layer of gas surrounding
the Earth and other Worlds
Lecture 13
The Planets
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Planetary Atmospheres
The layer of gas surrounding
the Earth and other Worlds
Homework
• Read Chapter 10: Atmospheres of Planets
• MasteringAstronomy: Assignment Chapter 10
Due Friday, Oct 18
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Observing Project
due today!
Planetary Atmospheres
The layer of gas surrounding
the Earth and other Worlds
Subsection 1:
Molecules and Light
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http://i.dailymail.co.uk/i/pix/2009/11/26/article
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Troposphere:
Protection against solar wind and
Earth’s
Atmosphere:
Lower atmosphere
Ultraviolet light
Dynamic, Protective, Governing
Molecules Cycle: Water, CO2
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Protection against meteorites
Roles of Atmospheres
Breathe: N2 O2 CO2
• Oxygen: respiration for animal life
• Shields UV photons
• Protects us from meteorites
• Traps heat in: Greenhouse Effect
• Earth’s Atmosphere Unique in Solar System:
Only one with oxygen ! Luck . . . ?
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Structure of the Atmosphere
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Molecules in the Earth’s Atmosphere
• 78% NITROGEN (N2)
O
• 21% OXYGEN (O2)
• Produced by plants during photosynthesis
• Necessary for breathing by animals.
• Arrived 3.5 billion years ago: algae & bacteria
O
• ~1% ARGON (Ar)
O
C
O
• 0.04% CARBON DIOXIDE (CO2)
•
•
•
•
•
•
Water vapor (H2O)
Carbon monoxide (CO)
Neon (Ne)
Oxides of nitrogen
Methane (CH4)
Krypton (Kr)
Concentrations are a few
parts per million (ppm)
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Water
Methane
N2
78%
O2
20.9%
Ar
0.93%
Gases in the
Earth’s Atmosphere
CO2
0.035%
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How Molecules Affect Visible Light
• Visible Light from Sun:
• Most light passes through
atmosphere.
• Blue photons are scattered
more than red photons
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How Molecules Affect Visible Light
At Sunset: Blue scattered away
Red photons survive.
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Quiz
A certain city has street lights that are white
light bulbs. The night sky appears:
a) faintly blue
b) faintly red
c) faintly white with no color
d) white, but missing the blue and red
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Carbon Dioxide
in Our Atmosphere is
Increasing Rapidly
Worldwide CO2 1960 - 2012
CO2 (ppm)
Burning coal
Added CO2 causes
The Greenhouse Effect:
Next Lecture . . .
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gasoline
Natural gas
Section 2
The Origin of Atmospheres
What holds them up against gravity?
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Origin of Atmospheres
• Venus, Earth, & Mars received their atmospheres
through volcanic outgassing.
• H2O, CO2, N2, H2S, SO2 , NH3
• On Earth:
• N2 was left as the dominant gas;
• CO2 dissolves in oceans and goes into carbonate rocks like limestone (=
calcium carbonate, Ca CO3.) Most CO2 is in the oceans as carbonates
• O2 from photosynthesis by plants (cyanobacteria and blue-green algae)
• Mars and Venus: CO2 is dominant gas
• Mars: lost much of its atmosphere through impacts
• less massive planet, lower escape velocity
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Origin of Earth’s Atmosphere
Volcanic Outgassing:
H2O, CO2, N2, H2S, . . .
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Origin of Atmospheric Gas:
Volcanic Outgassing
What about Oxygen? Where did it come from?
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Origin of Oxygen on Earth:
Plants, Algae: Photosynthesis
Octopus spring (Yellowstone)
Photosynthetic
Blue-green algae mats
cyanobacteria
Fossilized remains of blue-green algae
Shark’s Bay (Western Australia):
Colonies of microbes:
Stramatolite (blue-green algae)
Banded-iron Formation
Produced
Oxygen
Appears 3 billion
years ago.
(radioactive age dating)
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Gain/Loss Processes of Atmospheric Gas
Ways to
Gain Gases
Ways to
Lose Gases
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Comparing Terrestrial Atmospheres
Mercury: none
Venus: CO2 massive atmosphere
90x Earth’s Mass: Mostly CO2
Earth: modest
Mars: CO2 1% of Earth’s pressure
Moon: None
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What is an Atmosphere ?
• A layer of gas held to a world by gravity.
• Very thin compared to planet radius
• Temperature:
A measure of the
average speed of molecules . . .
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Temperature:
A Measure of the Speeds of Molecules
2kT
T is temp (K)
m is mass of molecule
k isEducation
Boltzmann’s
constant
= 1.38 x 10
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-23
J-K
Temperature:
A Measure of the Speeds of Molecules
Quiz
2 k T/m
T is temp (K)
m is mass of molecule
k is Boltzmann’s constant.
In a refrigerator, food is preserved
longer because:
a) Chemical reactions are slower
b) Chemical reactions are faster
c) Reaction rates stay the same
d) Outside air doesn’t get in.
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What is Pressure?
• Pressure: Force per area caused by
atoms & molecules
colliding with walls or each other.
• heating a gas in a confined space increases pressure
• number of collisions increase
• unit of measure: 1 bar = 14.7 lbs/inch2
Earth’s atmospheric pressure at sea level
• Upward Pressure balances
Downward gravity.
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Pressure
pushes balloon
walls outward.
Why doesn’t the
atmosphere fall down
due to gravity?
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Atmospheric Pressure:
Balances Gravity
Atmosphere
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• Upward pressure
supports air against
weight of air above.
Quiz
Suppose the Earth’s atmosphere contained twice
the number of molecules. Compared to our
Earth, the pressure at the surface would be:
a)
b)
c)
d)
2x as great
4x as great
1/2 as great
1/4 as great
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Section 3
Layers in the
Atmosphere
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Planetary Atmospheres
• Layers of the Atmosphere
• Global Wind Patterns
• Energy Balance
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Structure of
Earth’s Atmosphere
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Structure of Earth’s Atmosphere
• Pressure & density of atmosphere decrease with altitude
• Temperature increases and decreases with altitude
• Temperature domains define the major atmospheric layers
•
•
(mesosphere)
•
Stratosphere
Ozone Layer (absorbs UV)
Troposphere
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•
Exosphere
• Low density; fades into space
• Molecules can escape
Thermosphere
• Xrays heat gas
• Temp begins to rise at the top
Stratosphere
• UV from Sun absorbed by Ozone
• Rise (and fall) of temperature
Troposphere
• Layer closest to surface
• Heating: Convection, IR trapping
• Temp drops with altitude
CFCs Attack Ozone (O3)
The stratospheric ozone is an
environmental success story.
Scientists detected the declining
ozone in the atmosphere, collecting
the evidence that convinced
governments around the world to
take regulatory action.
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Energy Balance: Heating = Cooling
Heating:
• A planet absorbs light energy from the Sunlight
• Surface warms: Temperature increases.
Cooling:
• Planet emits light energy by thermal emission.
• Surface cools: Temperature decreases.
• Absorption of energy =
Thermal emission of energy
===> stable temperature
What if Earth gets
too hot ? Can it
correct its temperature
back to normal?
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What Determines a
Planet’s Surface Temperature?
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Quiz
Consider two moons around Jupiter (5.2 AU
from the Sun). Moon #2 has twice the
radius of Moon #1 (no atmospheres,
volcanoes or tidal heating). The ratio of
their temperatures (T2/T1) is:
a) 1
b) 2
c) 4
d) 8
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Effects of an Atmosphere on a Planet
• Scattering and absorption of light
• absorb high-energy radiation from the Sun
• scattering of optical light brightens the daytime sky
• Creates pressure
• can allow water to exist as a liquid (at the right temperature)
• Creates wind and weather
• promotes erosion of the planetary surface
• Protects surface from UV and solar wind
• Magnetic fields Auroras: Trap Solar wind particles
• Greenhouse effect
• makes the planetary surface warmer
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Planetary Atmospheres
The layer of gas surrounding
the Earth and other Worlds
End of Lecture 13
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Lecture 13: Atmospheres
The Greenhouse Effect
&
Global Warming
Section 4:
Comparison of Atmospheres:
Earth to Venus and Mars
4.6 Billion Years Ago ...
Venus
Earth
Mars
SUN
0.7 AU
1 AU
(150 million km
from Sun)
1.5 AU
Temperature: Top of Atmosphere
Temperature (Celsiu)
Temperature (C)
decreases with distance from Sun
500
500
50
0
400
30
300
0
200
Earth
-18oC (0oF)
400
300
200
10
100
00
Venus
100
0
0
0-100
-100
-100
0.2
0.4
0.6
0.8
1
1.2
Distance From Sun
1.4
Mars
1.6
Climate History of Venus
Venus outgassed as much H2O as Earth (similar planets).
•Early on, when the Sun was dimmer, Venus probably had oceans.
Venus’ proximity to the Sun caused H2O to vaporize.
• H2O vapor traps IR light from surface of planet
• H2O caused runaway greenhouse effect
• Surface heated to extreme temperature
• CO2 released from rocks: Adds to greenhouse effect
• UV photons from Sun dissociate H2O; H2 escapes. Water lost forever.
Suppose the
Earth moved
to
Venus’Orbit
Temperature (Celsius)
EARTH:
Surface
15oC (60oF)
Top of Atm:
-18oC (0oF)
500
500
400
300
300
Surface
200
 All three phases
of water
100
10
0
0
0
0.5
1
No Greenhouse
1.5
2
-100
-100
Surface warmer than top of atm  Greenhouse Effect
Clue: atm composit
The Earth is Changing Rapidly
Who oversees the
health of the Earth?
Earth’s Magnetic Field:
Magnetospheres
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Magnetosphere: Diverts Solar Wind
• The Sun ejects a stream of charged particles, called the solar wind.
• It is mostly electrons, protons, and Helium nuclei
• Earth’s magnetic field diverts these particles to the magnetic poles.
• the particles spiral along magnetic field lines and emit light
• this causes the aurora (aka northern & southern lights)
• this protective “bubble” is called the magnetosphere
• Other terrestrial worlds have no strong magnetic fields
• solar wind particles impact the exospheres of Venus & Mars
• solar wind particles impact the surfaces of Mercury & Moon
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Earth’s Magnetosphere
Solar
Wind:
Electrons,
protons,
helium
nuclei
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Weather and Climate
Weather – short-term changes in wind, clouds, temperature, and
pressure in an atmosphere at a given location
Climate – long-term average of the weather at a given location
• These are Earth’s global wind
patterns or circulation
• local weather systems move along
with them
• weather moves from W to E at midlatitudes in N hemisphere
• Two factors cause these patterns
• atmospheric heating
• planetary rotation
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Global Wind Patterns
• Air heated more at equator
• Warm air rises at equator;
Pressure pushes air to poles
• Cold air moves from poles to
equator along the surface
• Two circulation cells are
created in each hemisphere
•
Cells of air do not go directly from pole to
equator; air circulation is diverted by…
• Coriolis effect
•
•
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moving objects veer right on a surface
rotating counterclockwise
moving objects veer left on a surface
rotating clockwise
Global Wind Patterns
• On Earth, the Coriolis effect breaks each circulation
cell into three separate cells
• winds move either W to E or E to W
• Coriolis effect not strong on
Mars & Venus
• Mars is too small
• Venus rotates too slowly
• In thick Venusian atmosphere,
the pole-to-equator circulation
cells distribute heat efficiently
• surface temperature is
uniform all over the planet
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Reasons for Atmospheric Structure
• Absorption of sunlight energy causes layering structure.
• Troposphere
• absorbs IR photons from the surface
• temperature drops with altitude
• hot air rises to warm upper region (convection)
• Stratosphere
• absorbs Solar UV photons by dissociating ozone (O3)
• UV penetrates only top layer; hotter air is above colder air
• no convection or weather; the atmosphere is stratified
• Thermosphere
• absorbs heat via Solar X-rays which ionizes all gases
• contains ionosphere, which reflects back human-made radio signals
• Exosphere
• hottest layer; gas extremely rarified; provides noticeable drag on satellites
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Structure of Terrestrial Planet Atmospheres
• Mars, Venus, Earth all
• have warm tropospheres (and
greenhouse gases)
• have warm thermospheres
which absorb Solar X rays
• Only Earth has
• a warm stratosphere
• an UV-absorbing gas (O3)
• All three planets have warmer
surface temps due to
greenhouse effect
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