Transcript Planetary Atmospheres The layer of gas surrounding Earth and other Worlds QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Earth: N2 O2

Planetary Atmospheres
The layer of gas surrounding
Earth and other Worlds
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Earth: N2 O2 Ar CO2
• Oxygen we breathe
• 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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Homework due Friday (as always).
Telescope Opportunity:
View Saturn, Orion nebula,
Mars, Any Other Celestial Object
Sketch two of them.
TALC: every wed 7-9pm
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Earth’s Atmosphere:
Dynamic, Protective, Governing
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Troposphere:
Lower atmosphere
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Molecules Cycle: water, CO2
Aurora: Solar wind hits
Molecules in the atmosphere
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Atmosphere:
Protects against meteorites
Troposphere:
Lower atmosphere
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Planetary Atmospheres
The layer of gas surrounding
Earth and the Other Terrestrial Worlds
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Breathe: N2 O2 CO2
• Oxygen we breathe
• 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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Earth’s Atmosphere
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• 78% NITROGEN (N2)
• 21% OXYGEN (O2)
O
O
• Produced by plants during photosynthesis
• Necessary for breathing by animals.
• Arrived 3.5 billion years ago: algae & bacteria
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0.04%
CARBON DIOXIDE (CO2)
• ~1% ARGON (Ar)
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•
•
•
•
•
Water vapor (H2O)
Carbon monoxide (CO)
Neon (Ne)
Oxides of nitrogen
Methane (CH4)
Krypton (Kr)
Concentrations are a few
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Water
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Methane
Gases in the
Earth’s Atmosphere
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How Molecules Affect Visible Light
• Visible Light:
• Most passes through gas.
• Blue photons are scattered
more that red photons
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How Molecules Affect Visible Light
• Visible Light:
• Blue photons are scattered
more that red photons
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Clouds are “white” :
Reflect all wavelengths
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How Molecules Affect Visible Light
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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 1970 - 2005
CO2
Emission Amount
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Burning coal
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CO2 Causes Greenhouse Effect:
Stay tuned . . . Thursday .
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gasoline
Natural gas
Origin of Atmospheres
• Venus, Earth, & Mars received their atmospheres
through volcanic outgassing.
• H2O, CO2, N2, H2S, SO2 , NH3
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• On Earth:
• N2 was left as the dominant gas;
• CO2 dissolves in oceans and goes into carbonate rocks like limestone
(= calcium carbonate, Ca CO3.)
• Only possible because H2O could exist in liquid state
• O2 from photosynthesis by plants (cyanobacteria)
• 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
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What about Oxygen? Where did it come from?
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Origin of Oxygen on Earth:
Plants
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Shark’s Bay (Western Australia):
Colonies of microbes:
Stramatolite (blue-green algae)
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Octopus spring (Yellowstone)
Photosynthetic
Blue-green algae mats
Fossilized remains of blue-green algae
Produced
Oxygen
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cyanobacteria
Banded-iron Formation
Appears 3 billion
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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 atm.
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 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
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2kT
T is temp (K)
m is mass of molecule
k isEducation
Boltzmann’s
constant
= 1.38 x 10
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J-K
Temperature:
A Measure of the Speeds of Molecules
Quiz
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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 an Atmosphere ?
•
•
•
•
A layer of gas held to world by gravity.
Very thin compared to planet radius
Temperature: Measure of Avg. speed of molecules
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 radius were twice as
large, but its mass stays the same, and the
atmosphere had the same gas molecules.
Compared to our Earth, the pressure at the
surface would be:
a) 2x greater
b) 4x greater
c) The same
d) 4x smaller
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Energy Balance
• Planet receives Solar energy
• Surface Warms.
• Thermal emission in infrared light increases
until the energy emitted equals solar energy received:
===> stable temperature
What if Earth gets
too hot ? Can it
correct the temperature
Back to normal?
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Effects of an Atmosphere on a Planet
• Greenhouse effect
• makes the planetary surface warmer than it would be otherwise
• 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
• Creates auroras
• interaction with the Solar wind when magnetic fields are present
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The Greenhouse Effect:
Planetary Temperature
Questions:
• What is the greenhouse effect?
• Is it good, bad, or both . . .
• How would planets be different without the
greenhouse effect?
• Compare the greenhouse effect on
Venus, Earth, and Mars.
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The Greenhouse Effect
• Visible Sunlight passes through a
planet’s atmosphere.
• Some of this light is absorbed by
the planet’s surface.
• Planet warms. Emits its own
light: “thermal radiation”, as
infrared (IR) light - back out to
space.
• IR light is absorbed by the
molecules and sent back to
Earth !
• Result: the temperature is
higher than if there were no
atmosphere at all.
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Greenhouse Gases
• Key to Greenhouse Effect…gases
which absorb IR light effectively:
• water [H2O]
• carbon dioxide [CO2]
• methane [CH4]
• These are molecules which rotate
and vibrate easily.
• they re-emit IR light in a random
direction
• The more greenhouse gases which
are present, the greater the amount
of surface warming.
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What Determines a Planet’s Surface Temperature?
• Greenhouse Effect cannot change incoming Sunlight,
so it cannot change the total energy returned to space.
• it increases the energy (heat) trapped in lower atmosphere
• it works like a blanket
• In the absence of the Greenhouse Effect, what would
determine a planet’s surface temperature?
• the planet's distance from the Sun
• the planet’s overall reflectivity, “albedo” (fraction reflected)
• the higher the albedo, the less light absorbed, planet cooler
• Earth’s average temperature would be
–17º C (–1º F) without the Greenhouse Effect !
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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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Greenhouse Effect on the Planets
albedo
• Greenhouse Effect warms Venus, Earth, & Mars
•
•
•
•
on Venus: it is very strong
on Earth: it is moderate
on Mars: it is weak
avg. temp. on Venus & Earth would be freezing without it
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Atmosphere: Layered Structure
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•
•
•
Basic structure of Earth’s atmosphere.
Heating from causes atmospheric structure
Contrast Venus, Earth, and Mars.
Magnetosphere?
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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
• exosphere
• low density; fades into space
• thermosphere
• temp begins to rise at the top
(mesosphere)
Stratosphere
Ozone Layer (absorbs UV)
Troposphere
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•
•
stratosphere
• rise and fall of temp
troposphere
• layer closest to surface
• temp drops with altitude
CFCs Attack Ozone
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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How Molecules Affect Light
• X rays
• ionize atoms & molecules
• dissociate molecules
• absorbed by almost all gases
• Ultraviolet (UV)
• dissociate some molecules
• absorbed well by O3 & H2O
• Visible (V)
• Most passes through
• Blue photons are scattered
more that red photons
• Infrared (IR)
• absorbed by greenhouse gases
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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 and high gas density causes storms (convection)
• Stratosphere
• lies above the greenhouse gases (no IR absorption)
• absorbs heat via Solar UV photons which dissociate 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 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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Magnetospheres
• The Sun ejects a stream of charged particles, called the
solar wind.
• it is mostly electrons, protons, and Helium nuclei
• Earth’s magnetic field attracts and diverts these charged
particles to its 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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What are 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
mid-latitudes 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; heads
for poles
• cold air moves towards 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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Clouds, Rain and Snow
• Clouds strongly affect the surface conditions of a planet
• they increase its albedo, thus reflecting away more sunlight
• they provide rain and snow, which causes erosion
• Formation of rain and snow:
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Four Major Factors that affect
Long-term Climate Change
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Gain/Loss Processes of Atmospheric Gas
• Unlike the Jovian planets, the terrestrials were too small
to capture significant gas from the Solar nebula.
• what gas they did capture was H & He, and it escaped
• present-day atmospheres must have formed at a later time
• Sources of atmospheric gas:
• outgassing – release of gas trapped in interior rock by
volcanism
• evaporation/sublimation – surface liquids or ices turn to gas
when heated
• bombardment – micrometeorites, Solar wind particles, or
high-energy photons blast atoms/molecules out of surface rock
• occurs only if the planet has no substantial atmosphere already
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Gain/Loss Processes of Atmospheric Gas
• Ways to lose atmospheric gas:
• condensation – gas turns into liquids or ices on the surface
when cooled
• chemical reactions – gas is bound into surface rocks or
liquids
• stripping – gas is knocked out of the upper atmosphere by
Solar wind particles
• impacts – a comet/asteroid collision with a planet can blast
atmospheric gas into space
• thermal escape – lightweight gas molecules are lost to space
when they achieve escape velocity
gas is lost forever!
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Origin of the Terrestrial Atmospheres
• Lack of magnetospheres on Venus & Mars made
stripping by the Solar wind significant.
• further loss of atmosphere on Mars
• dissociation of H2O, H2 thermally escapes on Venus
• Gas and liquid/ice exchange occurs through
condensation and evaporation/sublimation:
• on Earth with H2O
• on Mars with CO2
• Since Mercury & the Moon have no substantial
atmosphere, fast particles and high-energy photons
reach their surfaces
• bombardment creates a rarified exosphere
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11.6 The Climate Histories of Venus, Earth,
and Mars
Our goals for learning:
• Describe major, seasonal features of Martian
weather today.
• Why did Mars’s early warm and wet period
come to an end?
• Why is Venus so hot?
• Could Venus ever have had oceans?
• After studying Mars and Venus, why does
Earth’s atmosphere seem surprising?
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Martian Weather Today
• Seasons on Mars are more extreme than on Earth
• Mars’ orbit is more elliptical
• CO2 condenses & vaporizes at opposite poles
• changes in atmospheric pressure drive pole-to-pole winds
• sometimes cause huge dust storms
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Mars’ Thin Atmosphere
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• Martian sunset
illustrates just how
thin the Martian
atmosphere is.
Martian Weather: N Polar Ice Cap &
Dust Storm
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Climate History of Mars
• More than 3 billion years ago, Mars must have had a
thick CO2 atmosphere and a strong greenhouse effect.
• the so-called “warm and wet period”
• Eventually CO2 was lost to space.
• some gas was lost to impacts
• cooling interior meant loss of magnetic field
• Solar wind stripping removed gas
• Greenhouse effect weakened until Mars froze.
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Venusian Weather Today
• Venus has no seasons to speak of.
• rotation axis is nearly 90º to the ecliptic plane
• Venus has little wind at its surface
• rotates very slowly, so there is no Coriolis effect
• The surface temperature stays constant all over Venus.
• thick atmosphere distributes heat via two large circulation cells
• There is no rain on the surface.
• it is too hot and Venus has almost no H2O
• Venusian clouds contain sulfuric acid!
• implies recent volcanic outgassing?
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Climate History of Venus
• Venus should have outgassed as much H2O as Earth.
• Early on, when the Sun was dimmer, Venus may have had
oceans of water
• Venus’ proximity to the Sun caused all H2O to evaporate.
• H2O caused runaway greenhouse effect
• surface heated to extreme temperature
• UV photons from Sun dissociate H2O; H2 escapes, O is stripped
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