Planetary Atmospheres, the Environment and Life (ExCos2Y)
Topic 3: Structure of Planetary Atmospheres
Chris Parkes
Rm 455 Kelvin Building
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
Atmosphere
Shallow Ocean
Deep Ocean
Reducing Oxygenating
volcanic gases
weathering
volcanic gases
photochemistry
photosynthesis
Revision
Earth’s atmosphere
Thermosphere
Earth’s radius = 6350km (4000 miles)
90% of mass in ~ 10 Km
No definitive upper boundary of atmosphere … (10000km)
Distinct layers
Separated by boundary layers
Temperature profile different in each layer
Pressure decreases by factor of 10 every ~15km in altitude
Earth’s atmosphere
Layers of atmosphere• Energy comes from
sunlight
• Temperature structure due to interaction of sun’s rays with gases
Mesosphere
Troposphere:
• Lowest layer– 8-16 km in height (latitude & season)– ~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 surfaceH
eigh
t (k
m)
00 300 600
50
100
150
TroposphereStratosphere
Mesosphere
Thermosphere
Stratosphere
• Second layer from Earth– Aeroplanes just reach into this– Extends from tropopause to ~50km – 99.7% of atmosphere below stratopause– Maximum ozone (O3) concentration at ~22km– “nacreous” clouds
• 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
Hei
ght
(km
)
00 300 600
50
100
150
TroposphereStratosphere
Mesosphere
Thermosphere
Ozone reduction / hole in Stratosphere
• 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
Stratosphere Ozone reduction of about 4% per decade since the late 1970s.Decrease in stratospheric ozone over Earth's polar regions
Largest Ozone hole recorded, South pole, September 2006NASA satellitesPurple – least Ozone
1992 2008
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
• Temperature decreases with height– Decreasing heating as far above Ozone layer– Top of mesosphere is coldest place in atmosphere
• Below water freezing throughout mesosphere
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’H
eigh
t (k
m)
00 300 600
50
100
150
TroposphereStratosphere
Mesosphere
Thermosphere
Thermosphere• 4th layer from earth
– ~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
• 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 – 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
Exosphere
Hei
ght
(km
)
00 300 600
50
100
150
TroposphereStratosphere
Mesosphere
Thermosphere
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
MagnetosphereRegion of Earth’s magnetic field
Protects Earth from ‘solar wind’ – deflecting charged particle radiation
needed for lifeVenus/Mars – lost elements of atmosphere due to solar wind
Van Allen Belts•3000 Km, 16000 Km•Charged particles trapped in Earth’smagnetic fieldInner – protonsOuter - electrons
Venus’ atmosphere
Dominated by CO2
•Venus closer to sum than Earth does not explain extreme temp. difference
CO2 greenhouse gas – boosts temperatures
Surface Temperature > an oven
Venus’ atmosphereWeather forecast: Hot, Cloudy, No wind
•Pressure: Gas density at surface 10% of water
• 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
Thin atmosphereLow 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
Mars’ atmosphere
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’ atmosphereWeather Forecast: Cold, strong winds, dust storms
•Pressure < 1% Earth•Liquid water – evaporate or feeze•CO2 – but weak greenhouse effectas 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
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
surroundings, so continues to rise.• At C it is stable - at same temperature
as surroundings, so stops.
Height IncreasesPressure decrease exponentially, Temperature decrease linearly
For ideal gas law – pV=T pressure decreases faster than Temperature Volume must increase to compensate
Temperature
Height
A
B
C
Typical real
temp. profile
Lapse rate
Cloud formation & Lapse Rate
• Now consider air containing water vapour at A
• Air cools, reaches dew point at B, and water condenses
• Forms clouds
• Dew point falls slowly with height (2º C/km)
Height
A
B
C
Typical real
temp. profile
Lapse rate
TemperatureDew 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