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

Lecture Content 1. Composition of the Atmospheres of Earth, Mars and Venus 2. Evolution of Earth’s Atmosphere 3. Structure of Planetary Atmospheres 4. Solar Radiation 5. Atmospheric Convection 6. Wind 7. Water 8. Storm Systems 9. Climate Change 10. Revision Reading: The Cosmic Perspective, chapter on planetary atmospheres by Bennett, Donahue, Schneider, Voit. Publisher: Addison Wesley Atmosphere, Weather and Climate by Barry, Chorley. Publisher: Routledge

Planets of the solar system Orbit the Sun Near Spherical Dominate its orbit IAU Prague (2006) Fit to be a planet ?

Atmospheric Composition: Earth, Mars & Venus EarthMarsVenus

EarthMarsVenus Atmospheric Composition: Earth, Mars & Venus

Comparison of physical quantities: EarthMarsVenus EarthMars Venus Mass * Distance from Sun * Radius * Gravitational Acc. * Surface Pressure (atm.) Surface Temp. (K) Planetary Albedo Rotation Period (days) Orbital period (years) * Relative to Earth

Comparison of Atmospheric Composition EarthMars Venus Nitrogen3.5%78%2.7% Oxygen <0.001%21%0.13% Carbon Dioxide 96.5% 0.035%95.3% Argon 0.007% 0.93%1.6% Water Vapour 0.004%.004%-4.0%0.03% Equilibrium state: production rate = loss rate Sources & Sinks (in balance) Sensitive dependant on: temperature, pressure, quantity, other gases, etc.

Partial Pressures Partial pressure of a gas = P total atmospheric × % concentration Examples: N 2 on Venus:3.5% × 90 = 3.15 atm. N 2 on Earth: CO 2 on Mars: CO 2 on Earth: 78% × 1 = 95.3% × 0.01 = 0.035% × 1 = 0.78 atm atm atm.

Some Important Gases CO 2 –Greenhouse gas: temperature –Needed for photosynthesis: life H 2 O –Sink for CO 2 –Liquid Water needed for life on earth Oxygen O 2 –needed for life on earth O 3 (Ozone) –Pollutants through human activities (Low atmosphere) –Formed by Sun’s radiation (High atmosphere) –UV screen

How does a planet GAIN atmospheric gases ? Born 4.5 million years ago without atmospheres Outgassing –Heating from the core –Volcanic activities –H 2 O, CO 2, N 2, H 2,S, SO 2 Evaporation/sublimation Water (Earth) /ice (Earth, Mars) and frozen CO 2 (Mars)

How does a planet LOSE atmospheric gases ? Five processes: 1)Condensation 2)Chemical Reactions 3) Thermal Escape 4) Impacts 5) Dissociation from solar rays These two can be reversed These three permanent loss

The presence of water affects CO 2 levels - CO 2 highly soluble in water - Acid rain which reacts with rock - CO 2 locked into rocks (over geological timescale) 1.Removal of CO 2 from atmosphere (see next lecture for Oxygen cycle) Chemical Reactions

Thermal Escape: Gravity and the atmosphere Escape velocity (V e ): G – Gravi. Const. (6.67 × m 3 s -2 kg -1 ) M – Mass of Planet r - Planet radius For Earth V e = 11km/s

Gravity and the atmosphere Motion of Gas molecules: - random - temperature dependant velocity No. of molecules Velocity R – gas constant (8.31 J mol -1 K -1 ) T – temperature m – molecular weight (=0.002kg/mol for H 2 ) At 280K, V max = 1.5km/s Ve

Temperature & escape velocity of planet are determining factors of atmospheric composition -Points show v escape and temperature of each planets -Curves show typical highest velocities for different gases Temperature (K) Thermal Escape V escape (km/s)

Earth’s atmosphere – other constituents Aerosols –Types: Dust, Organic matter, Smoke, Salt –Sources: Forest/Bush Fire, Volcano, Sea Spray, Dust Storm, Burning fossil Fuel Pollutants –Sulphur compounds (acid rain), Nitrogen compounds (NO x ), Ozone, CO, Hydrocarbons Water vapour –Cloud formation

Earth’s atmosphere – Cloud formation Planetary Albedo Water vapour saturation Temperature Pressure aerosols Highly reflective ( 70%-95% Visible range) Efficient scatterer Many different types of cloud

Example exam questions Q1. List the main differences between atmospheric composition of Mars and Earth? Q2. What are the main factors which affect surface temperature of planets? Q3. How does surface temperature affect the composition of planetary atmosphere? Next lecture – evolution of Earth’s atmosphere