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

Structure of planetary atmospheres Earth Mars Venus

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 surface Height (km) Troposphere Stratosphere 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 (O 3 ) concentration at ~22km –“nacreous” clouds Temperature increases with height –O 3 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 Height (km) Troposphere Stratosphere 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 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

3 rd Layer from ground –~ km 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’ Height (km) Troposphere Stratosphere Mesosphere Thermosphere

4 th layer from earth –~ km –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 Height (km) Troposphere Stratosphere 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

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, Km Charged particles trapped in Earth’s magnetic field Inner – protons Outer - electrons

Venus’ atmosphere Dominated by CO 2 Venus closer to sum than Earth does not explain extreme temp. difference CO2 greenhouse gas – boosts temperatures Surface Temperature > an oven

Venus’ atmosphere Weather 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 atmosphere Low pressure & temperature Major components (by volume) 95.3% carbon dioxide (CO 2 ) 2.7% nitrogen (N 2 ) 1.6% argon (Ar) 0.15% oxygen (O 2 ) 0.03% water vapor (H 2 O) Global dust storm CO 2 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’ atmosphere Weather Forecast: Cold, strong winds, dust storms Pressure < 1% Earth Liquid water – evaporate or feeze CO 2 – 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 CO 2 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: CO 2 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, CO 2 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 p h = p sea × 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 Increases Pressure 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 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 CO 2 affect the surface temperature of Venus? Next lecture – solar radiation, energy budget

Water – is there life on Mars … ?

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