Transcript Unit 03 Slides - Chapter 10

Chapter 10:
Planetary Atmospheres
Earth and the Other Terrestrial
Worlds
Comparing Terrestrial Atmospheres
What is an Atmosphere?
 A layer of gas which surrounds a world is called an
atmosphere.
• they are usually very thin compared to planet radius
 Pressure is created by atomic & molecular
collisions in an atmosphere.
• 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
 Pressure balances gravity in an atmosphere.
Earth's Atmosphere
 About 10 kilometers
thick
 Consists mostly of
molecular nitrogen (N2)
and oxygen (O2).
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Where does an atmosphere end?
 Small amounts of gas are present even above 300
kilometers.
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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
The Greenhouse Effect
 Visible Sunlight passes through a
planet’s atmosphere.
 Some of this light is absorbed by the
planet’s surface.
 Planet re-emits this energy (heat) as
infrared (IR) light.
• planet’s temperature lower than Sun
 IR light is “trapped” by the
atmosphere.
• its return to space is slowed
 This causes the overall surface
temperature to be higher than if there
were no atmosphere at all.
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.
Planetary Energy Balance
 Solar energy received by a planet must balance the
energy it returns to space
• planet can either reflect or emit the energy as radiation
• this is necessary for the planet to have a stable temperature
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) in lower atmosphere
• it works like a blanket
 In the absence of the Greenhouse Effect, what would
determine a planet’s surface temperature?
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the planet's distance from the Sun
the planet’s overall reflectivity
the higher the albedo, the less light absorbed, planet cooler
Earth’s average temperature would be –17º C (–1º F) without
the Greenhouse Effect
Greenhouse Effect on the Planets
 Greenhouse Effect warms Venus, Earth, & Mars
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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
Structure of Earth’s Atmosphere
 pressure & density of atmosphere decrease with altitude
 temperature varies “back and forth” with altitude
• these temperature variations define the major atmospheric layers
 exosphere
• low density; fades into
space
 thermosphere
• temp begins to rise at
the top
 stratosphere
• rise and fall of temp
 troposphere
• layer closest to surface
• temp drops with altitude
Atmospheres Interact with 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)
• passes right through gases
• some photons are scattered
 Infrared (IR)
• absorbed by greenhouse gases
Reasons for Atmospheric Structure
 Light interactions are responsible for the structure we see.
 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
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
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
Earth’s Magnetosphere
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
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 do not go directly from pole
to equator; air circulation is
diverted by…
 Coriolis effect
• 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 poleto-equator circulation cells distribute
heat efficiently
• surface temperature is uniform all
over the planet
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:
Four Major Factors which affect Long-term
Climate Change
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 highenergy photons blast atoms/molecules out of surface rock
 occurs only if the planet has no substantial atmosphere already
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!
Gain/Loss Processes of Atmospheric
Gas
Origin of the Terrestrial Atmospheres
 Venus, Earth, & Mars received their atmospheres through
outgassing.
• most common gases: H2O, CO2, N2, H2S, SO2
 Chemical reactions caused CO2 on Earth to dissolve in
oceans and go into carbonate rocks (like limestone.)
• this occurred because H2O could exist in liquid state
• N2 was left as the dominant gas; O2 was exhaled by plant life
• as the dominant gas on Venus, CO2 caused strong greenhouse
effect
 Mars lost much of its atmosphere through impacts
• less massive planet, lower escape velocity
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
Exospheres of the Moon and Mercury
 Sensitive measurements show that the Moon and Mercury have
extremely thin atmospheres.
 Gas comes from impacts that eject surface atoms.
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Martian Weather Today
• Seasons on Mars are more extreme than on Earth
• Mars’ orbit is more elliptical
• CO2 condenses & sublimes at opposite poles
• changes in atmospheric pressure drive pole-to-pole winds
• sometimes cause huge dust storms
Martian Weather: N Polar Ice Cap & Dust Storm
Changing Axis Tilt
 Calculations suggest
Mars's axis tilt ranges from
0° to 60°.
 Such extreme variations
can cause climate
changes.
 Alternating layers of ice
and dust in polar regions
reflect these climate
changes.
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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.
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?
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
The Uniqueness of Earth’s Atmosphere
Outgassing from volcanoes on Venus, Earth, & Mars released the same
gasses:
• primarily water (H2O), Carbon dioxide (CO2), and Nitrogen (N2)
So why did Earth’s atmosphere end up so different?
• why did Earth retain most of its H2O – enough to form oceans?
• why does Earth have so little CO2 in its atmosphere, when it should have
outgassed just as much CO2 as Venus?
• why does Earth have so much more Oxygen (O2) than Venus & Mars?
• why does Earth have an ultraviolet-absorbing stratosphere?
The Uniqueness of Earth’s Atmosphere
Earth’s H2O condensed because of the temperature.
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oceans formed, in which the CO2 gas dissolved
chemical reactions bound the C of CO2 into rocks like limestone
low level of atmospheric CO2 causes moderate greenhouse effect
temperatures on Earth remain where H2O can be a liquid CO2
There was once liquid H2O on Mars and maybe Venus.
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before CO2 could dissolve out, temperatures fell/rose so that
oceans boiled away on Venus and froze out on Mars
Earth’s O2 was not outgassed by volcanoes.
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O2 is a highly reactive chemical
it would disappear in a few million years if not replenished
no geologic process creates O2
The Uniqueness of Earth’s Atmosphere
Earth’s O2 was created through the evolution of life.
• plants & microorganism release O2 via photosynthesis
• they convert CO2 into O2
In the upper atmosphere, O2 in converted into ozone (O3).
• via chemical processed involving Solar ultraviolet light
• O3 absorbs Solar UV photons which heats the stratosphere
Venus & Mars lack plant & microbial life.
• so they have no O2 in their atmospheres and no stratospheres
The CO2 Cycle
The CO2 Cycle is a Feedback Mechanism which
Regulates Earth’s Climate
Stability of Earth’s Climate
Plate tectonics causes the relative stability of Earth’s climate.
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plate tectonics makes the CO2 cycle work
it takes about 40,000 years for the CO2 cycle to restore balance
There have been temporary episodes of extreme cooling and heating in Earth’s
history.
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these ice ages & hothouse period have their own feedback mechanisms
Studying Past Life
We know the history of life on Earth by studying fossil records.
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fossils are more difficult to find as we look back to earlier epochs
 more organisms which lacked skeletons leave fewer fossils
 erosion erases much old fossil evidence
 subduction destroys fossils carries deep beneath Earth’s surface
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we have found fossils of large & small animals, plants, microorganisms
the fossil record goes back 3.5 billion years
 deficit of 13C, a sign of life, in rocks as old as 3.85 billion years
Origin of Life
All known organisms:
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build proteins from same subset of amino
acids
use ATP to store energy in cells
use DNA molecules to transmit genes
All organisms share same genetic
code…sequence of chemical bases
Organisms have similar genes.
Indicates that all living organisms share a
common ancestor.
Life on Earth is:
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divided into three major groupings
plants & animals are just two tiny branches
Origin of Life
We have no direct evidence of when or how life began.
We have a plausible scenario of how chemistry begat biology:
• chemicals found on Earth, “sparked” by lighting, can form complex organic
molecules naturally
• RNA can form, and if some of it becomes self-replicating, it can lead to…
• DNA and full, self-replicating organisms.
Alternatively, this process could have begun on Mars or Venus.
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life could have been transported to Earth via meteorites
organic molecules have been found in meteorites
Evolution of Life
The oceans were full of single-celled life 3.5 billion years ago.
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conditions on land were too inhospitable due to lack of O3 in atmosphere
Organism’s DNA is reproduced, but can change due to copying errors or external
factors
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a change in the base sequence of an organism’s DNA is called a mutation
Some mutations are lethal, others make the cell better able to survive.
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those organisms which are better at adapting to their environment… thrive
this process is called natural selection, first proposed by Charles Darwin
Natural selection helps some species to dominate, and creates entirely new species
from older ones.
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life on Earth rapidly diversified
Some 2 billion years ago, life was still confined to the oceans.
The Rise of Oxygen
Single-celled organisms called cyanobacteria appear to have created O2 via
photosynthesis as early as 3.5 billion years ago.
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the first O2 produced was absorbed into rocks via chemical reactions
it was not until about 2 billion years ago than the rocks were saturated with O2 and so it
began to accumulate in the atmosphere
fossil record indicates current levels of O2 were reached 200 million yrs ago
The build-up of O2 in the atmosphere permitted:
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the evolution of oxygen-dependent animals
the formation of O3 in the stratosphere, making it safe for life to move out onto dry land
Plants first appeared on land some 475 million years ago.
Animals soon followed…
The Rise of Humans
Some 540 million years ago, most of the life on Earth was still one-celled and tiny.
For the next 40 million years, until 500 million years ago:
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there was a dramatic increase in the number of species, especially animals
animals diversified into all the basic body plans which we find today
this incredible diversification of species is called the Cambrian explosion
The first humans appeared a few million years ago.
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humans civilization is 10,000 years old, industrial society 200 years old
Although latecomers to the scene,
humans are the most successful
species to survive on Earth
Human population has grown
exponentially
How will this affect our host – the Earth?
Mass Extinctions
Evolution of species normally occurs gradually.
• one species goes extinct per century
Evolution can receive a jolt during a mass extinction.
• historically, this has been caused by an impact
• even species not directly killed by the impact will soon go extinct due to
lack of food and changes in the ecosystem
• species at the top of the food chain are most susceptible
Human activity in recent times have driven many more species to
extinction.
• could we be undergoing an episode of mass extinction now?
• what will the consequences be for humans?
Ozone Depletion
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O3 in the stratosphere shields
Earth’s surface from Solar UV
A depletion of O3 was observed
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1979
1998
The apparent cause of the ozone hole is a man-made chemical, CFC
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ozone hole over Antarctica
discovered in mid-1980’s
appears in Antarctic spring
used as a refrigerant, CFC is inert and rises to the stratosphere
Solar UV photons break down CFCs and create Cl – Chlorine gas
Cl serves as a catalyst in destroying O3
reaction goes faster at low temperatures … like over Antarctica
Mars is an example of no ozone, UV photons sterilize the surface.
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increased UV on Earth would increase cancer rates and genetic mutations
Long-Term Climate Change
 Changes in Earth's axis tilt might lead to ice ages.
 Widespread ice tends to lower global temperatures by increasing
Earth's reflectivity.
 CO2 from outgassing will build up if oceans are frozen, ultimately
raising global temperatures again.
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How is human activity changing our planet?
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Dangers of Human Activity
 Human-made CFCs in the atmosphere destroy
ozone, reducing protection from ultraviolet
radiation.
 Human activity is driving many species to
extinction.
 Human use of fossil fuels produces greenhouse
gases that can cause global warming.
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Global Warming
 Earth's average temperature has increased by 0.5°C in
past 50 years.
 The concentration of CO2 is rising rapidly.
 An unchecked rise in greenhouse gases will eventually
lead to global warming.
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CO2 Concentration
 Global temperatures
have tracked CO2
concentration for last
500,000 years.
 Current CO2
concentration is the
highest it's been in at
least 500,000 years.
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CO2 Concentration
 Most of the CO2 increase has happened in last 50 years!
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Modeling of Climate Change
 Complex models
of global warming
suggest that
recent
temperature
increase is
consistent with
human
production of
greenhouse
gases.
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Consequences of Global Warming
 Storms more numerous and intense
 Rising ocean levels; melting glaciers
 Uncertain effects on food production, availability of fresh
water
 Potential for social unrest
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