Planetary Atmospheres, the Environment and Life (ExCos2Y)

Topic 1: Composition of Atmospheres

Chris Parkes c.parkes@physics.gla.ac.uk

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. RevisionReading: The Cosmic Perspective, chapter on planetary atmospheresby 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

Earth MarsVenus

Earth MarsVenus

Atmospheric Composition:

Earth, Mars & Venus

Comparison of physical quantities:

Earth MarsVenus Earth MarsVenus

Mass * 0.82 1.00 0.11

Distance from Sun * 0.72 1.00 1.52

Radius * 0.95 1.00 0.53

Gravitational Acc. * 0.90 1.00 0.38

Surface Pressure (atm.) 90.00 1.00 0.01

Surface Temp. (K) 750. 280. 240.

Planetary Albedo 0.76 0.39 0.16

Rotation Period (days) 243. 1.00 1.03

Orbital period (years) 0.62 1.00 1.88* Relative to Earth

Comparison of Atmospheric Composition

Earth MarsVenus

Nitrogen 3.5% 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:

N2 on Venus: 3.5% × 90 = 3.15 atm.

N2 on Earth:

CO2 on Mars:

CO2 on Earth:

78% ×1 =

95.3% × 0.01 =

0.035% × 1 =

0.78 atm.

0.00953 atm.

0.00035 atm.

Some Important Gases

• CO2 – Greenhouse gas: temperature– Needed for photosynthesis: life

• H2O– Sink for CO2

– Liquid Water needed for life on earth

• Oxygen• O2

– needed for life on earth

• O3 (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

– H2O, CO2, N2, H2,S, SO2

• Evaporation/sublimationWater (Earth) /ice (Earth, Mars) and frozen CO2 (Mars)

How does a planet LOSE atmospheric gases ?

Five processes:1) Condensation2) Chemical Reactions

3) Thermal Escape4) Impacts5) Dissociation from solar rays

These two can be reversed These three permanent loss

The presence of water

affects CO2 levels

- CO2 highly soluble in water- Acid rain which reacts with rock

- CO2 locked into rocks(over geological timescale)

1. Removal of CO2 from atmosphere(see next lecture for Oxygen cycle)

Chemical Reactions

Thermal Escape: Gravity and the atmosphere

Escape velocity (Ve):

G – Gravi. Const.

(6.67×10-11m3s-2kg-1)

M – Mass of Planet

r - Planet radius

For Earth Ve = 11km/s

r

GMve

2

Gravity and the atmosphere

Motion of Gas molecules:

- random

- temperature dependant velocity

No.

of

mol

ecu

les

Velocity

R – gas constant

(8.31 J mol-1 K-1)

T – temperature

m – molecular weight

(=0.002kg/mol for H2)

At 280K, Vmax = 1.5km/sVe

m

RTvrms

3

Temperature & escape velocity of planet are determining factors of atmospheric composition

- Points show vescape and temperature of each planets

- Curves show typical highest velocities for different gases

Temperature (K)

Thermal Escape

Vescape(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

(NOx), 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