Chris Parkes Rm 455 Kelvin Building

1. Planetary Atmospheres, the Environment and Life (ExCos2Y)Topic 3: Structure of Planetary Atmospheres Chris Parkes Rm 455Kelvin Building

2. 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

3. Structure of planetary atmospheres Mars Earth Venus

4. Earth’s atmosphere Thermosphere Earth’s radius = 6350km (4000 miles) 90% of mass in ~ 10 Km No definitive upper boundary of atmosphere … (10000km)

5. 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

6. Layers of atmosphere • Energy comes from sunlight • Temperature structure due to interaction of sun’s rays with gases Mesosphere

7. 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 surface 150 Thermosphere 100 Height (km) Mesosphere 50 Stratosphere Troposphere 0 0 300 600

8. 150 Thermosphere 100 Height (km) Mesosphere 50 Stratosphere Troposphere 0 0 300 600 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

9. 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 Largest Ozone hole recorded, South pole, September 2006 NASA satellites Purple – least Ozone 1992 2008

10. Nacreous clouds in Stratosphere ~15-25km above - mostly over polar region during winter - wavy clouds showing winds & waves in the stratosphere

11. 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 150 Thermosphere 100 Height (km) Mesosphere 50 Stratosphere Troposphere 0 0 300 600 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’

12. 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 150 Thermosphere 100 Height (km) Mesosphere 50 Stratosphere Troposphere 0 0 300 600 Thermosphere Exosphere

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. Temperature Profiles: Venus, Earth, Mars

21. 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

22. 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

23. Hydrostatic balance (assume const. T) Pressure at a point due to weight of air above Using ideal gas law ph = psea×e(-0.12h) Pressure variation with height:

24. 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 Increases Pressure decrease exponentially, Temperature decrease linearly For ideal gas law – pV=T pressure decreases faster than Temperature Volumemust increase to compensate Typical real temp. profile Height C B A Lapserate Temperature

25. 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) Typical real temp. profile Height C B A Lapserate Temperature Dew point

26. 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

28. Water – is there life on Mars … ?

29. Water – is there life on Mars … ? How can water flow on the surface? images from Mars Global Surveyor