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Planetary Atmospheres, the Environment
and Life (ExCos2Y)
Topic 3: Structure of Planetary Atmospheres
Chris Parkes
Rm 455 Kelvin Building
Revision
2. Evolution of Earth’s Atmosphere
• Habitable zone – liquid water
• The Oxygen cycle
– photosynthesis & Oxygen
absorbtion in balance
• Oxygen in atmosphere over
time
– 3 reservoir model: oxygen
produced by life,
anaerobic/aerobic life
• Water Cycle
• Carbon dioxide Cycle
• Self-regulation and the “Gaia”
hypothesis
Reducing
Oxygenating
Atmosphere
volcanic
gases
photochemistry
Shallow Ocean
weathering
photosynthesis
volcanic
gases
Deep Ocean
Structure of planetary atmospheres
Mars
Earth
Venus
Earth’s atmosphere
Thermosphere
Earth’s radius = 6350km (4000 miles)
90% of mass in ~ 10 Km
No definitive upper boundary of atmosphere … (10000km)
Earth’s atmosphere
Distinct layers
Separated by
boundary layers
Temperature
profile different
in each layer
Pressure decreases
by factor of 10
every ~15km in
altitude
Layers of atmosphere
• Energy comes from
sunlight
• Temperature structure
due to interaction of
sun’s rays with gases
Mesosphere
• Lowest layer
–
–
–
–
–
–
–
Height (km)
Troposphere:
150
Thermosphere
100
Mesosphere
50
Stratosphere
Troposphere
8-16 km in height (latitude & season)
00
300 600
~90% mass of the atmosphere
Contains virtually all water vapour and aerosols
Capped by inversion layer – limits convection
region where convection occurs (i.e. weather, storms)
Most weather systems below tropopause
Little interaction with upper layers
• Temperature drops with altitude (climb a mountain)
– Visible light reaches surface and warms ground
– Infrared light radiated upwards and warms
• Warmer closer to surface
–
–
–
–
–
Height (km)
Stratosphere
• Second layer from Earth
150
Thermosphere
100
Mesosphere
50
Stratosphere
Troposphere
Aeroplanes just reach into this
00
300
Extends from tropopause to ~50km
99.7% of atmosphere below stratopause
Maximum ozone (O3) concentration at ~22km
“nacreous” clouds
600
• Temperature increases with height
– O3 absorbs UV from sun
– No convection, air cannot rise as higher air is hotter
• Air stratified, cooler to warmer layers, – hence ‘stratosphere’
• Stratsosphere occurs because O3 absorbs UV
– Lack of oxygen on Mars/Venus means no stratosphere
Ozone reduction / hole in Stratosphere
Stratosphere Ozone reduction of about 4% per decade since the late 1970s.
Decrease in stratospheric ozone over Earth's polar regions
• No convection in stratosphere means pollutants remain
• Chloroflourocarboms (CFCs) pollutants destroy Ozone
• Ozone protects us from harmful UV
– e.g. skin cancer in humans, plankton reduction
Montreal Protocol (1989) banned production
1992
2008
Largest Ozone hole recorded,
South pole, September 2006
NASA satellites
Purple – least Ozone
Nacreous clouds in Stratosphere
~15-25km
above
- mostly over
polar region
during winter
- wavy clouds
showing winds
& waves in the
stratosphere
•
3rd
Layer from ground
– ~50 - 80km altitude
– “Noctilucent” clouds
Height (km)
Mesosphere
150
Thermosphere
100
Mesosphere
50
00
Stratosphere
Troposphere
300
600
• Temperature decreases with height
– Decreasing heating as far above Ozone layer
– Top of mesosphere is coldest place in atmosphere
• Below water freezing throughout mesosphere
- From freezing of moisture
content in mesosphere
- (Ice) cloud formation possible,
structure due to convection
- When troposphere is clear of
cloud, cloud visible after sunset,
hence ‘noctilucent’
– ~80 - 400km
– International Space Station in this
• Temperature increases with height
– Temperature is “theoretical” due to low gas density
– Cosmic rays (X-rays) ionise gas molecules
Height (km)
• 4th layer from earth
Thermosphere
150
Thermosphere
100
Mesosphere
50
00
Stratosphere
Troposphere
300
600
• First gases reached
• Aurora
• Ionosphere - Band of ionised gas
– ionised atoms/molecules and electrons
– Reflects radio waves – long distance communication, not just line of sight
• Final Layer
Exosphere
– 400 km onwards
• Low density gas
– gradual boundary between atmosphere and space
• From here gases can sometimes reach escape velocity
– leave atmosphere
• At 600 km all atoms are ionised
Aurora: 100 – 120 km
Collision of high energy charged particle with gas in upper atmosphere
Excited gas atom fluoresces (colour depends on gases)
Motion of charged particles in Earth’s magnetic field
Magnetosphere
Region of Earth’s magnetic field
Protects Earth from ‘solar wind’ –
deflecting charged particle radiation
needed for life
Venus/Mars –
lost elements of atmosphere
due to solar wind
Van Allen Belts
•3000 Km, 16000 Km
•Charged particles
trapped in Earth’s
magnetic field
Inner – protons
Outer - electrons
Venus’ atmosphere
Surface Temperature > an oven
Dominated by CO2
•Venus closer to sum than Earth
does not explain extreme temp. difference
CO2 greenhouse gas – boosts temperatures
Venus’ atmosphere
Weather forecast: Hot, Cloudy, No wind
• Weak Coriolis – no strong winds
• High pressure
– efficient heat transport
– temperature same everywhere
• No axis tilt – no seasons
• Surface temperature drives strong
convection
• Covered with Sulphuric acid
clouds - highly reflective
• Lack of magnetic field
– solar wind stripped water
•Pressure: Gas density at surface 10% of water
Mars’ atmosphere
Thin atmosphere
Low pressure & temperature
Major components (by volume)
95.3% carbon dioxide (CO2)
2.7% nitrogen (N2)
1.6% argon (Ar)
0.15% oxygen (O2)
0.03% water vapor (H2O)
Global dust storm
CO2 polar ice caps
Water – is there life on Mars … ?
Present day surface temperature too cold, frozen as ice
Evidence of water on surface of Mars in the past
Mars’ atmosphere
Weather Forecast: Cold, strong winds, dust storms
•Pressure < 1% Earth
•Liquid water – evaporate or feeze
•CO2 – but weak greenhouse effect
as very little atmosphere
•Most lost to space
•Frozen in polar caps
•Mars has seasons
•Year twice as long
•See future lectures
•Temperature difference on planet
• strong winds
•dust storms
Temperature Profiles: Venus, Earth, Mars
Why are Earth/Mars/Venus atmospheres so different ?
• Why does Earth have ocean’s but not Venus, Mars ?
– Mars: lost due to solar wind, frozen in ice caps
– Venus: too hot, escape to space
– Earth: temperatures low enough to condense
• Why is there much less CO2 in Earth’s / Mars
atmosphere than Venus?
– Similar amounts of outgassing on Earth and Venus
– Mars: 1) No magnetsosphere lost to solar wind
2) Frozen in polar dry-ice caps
– Earth: CO2 dissolved in Oceans forming carbonate rocks
Why are Earth/Mars/Venus atmospheres so different ?
• Why is only Earth’s atmosphere mainly Nitrogen
and Oxygen ?
– Earth: water, CO2 gone – hence Nitrogen
– Earth: Oxygen produced by life
• Why does only Earth have a stratosphere ?
– Earth: has Oxygen, solar radiation forms Ozone
– Ozone absorbs UV light – the stratosphere
Pressure variation with height:
Hydrostatic
balance
(assume const. T)
Pressure at a point
due to weight of
air above
Using ideal gas law
ph = psea×e(-0.12h)
Temperature variation with height: Lapse rate
• In troposphere, temperature decreases with height at 6.5ºC/km
• Parcel of air at A moves up “straight” line in Temp vs Height graph
• At B it is unstable - warmer than
Typical real
surroundings, so continues to rise.
Height temp. profile
• At C it is stable - at same temperature
C
as surroundings, so stops.
B
Height Increases
Pressure decrease exponentially,
Temperature decrease linearly
For ideal gas law – pV=T
pressure decreases faster than Temperature
Volume must increase to compensate
A
Lapse rate
Temperature
Cloud formation & Lapse Rate
• Now consider air
containing water
vapour at A
• Air cools, reaches
dew point at B, and
water condenses
• Forms clouds
Height
Typical real
temp. profile
C
B
A
Lapse rate
• Dew point falls slowly with
height (2º C/km)
Temperature
Dew point
Example exam questions
Q1. Name the distinct layers of earth’s atmosphere.
How does the temperature vary with altitude
within these layers?
Q2.Give evidence of possible past existence of water
on Mars. Is there water on the surface of Mars
currently? Why?
Q3. How does the presence of CO2 affect the surface
temperature of Venus?
Next lecture – solar radiation, energy budget
Water – is there life on Mars … ?
Water – is there life on Mars … ?
How can water flow on the surface?
images from Mars Global Surveyor