Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 2: Evolution of Earth’s Atmosphere Chris Parkes Rm 455 Kelvin Building

Physical characteristics of planets Atmospheric composition of Planets –Earth: Nitrogen, Oxygen Mars/Venus: CO2 –Mars: low pressure, Venus: high pressure Gain & Loss mechanisms Thermal Escape: –Temperature –Gravity, Mass & Radius of planet – Escape Velocity 1. Composition of the Atmospheres of Earth, Mars and Venus Revision

Lecture 2: Evolution of the Earth’s atmosphere “..the thickness of the Earth's atmosphere, compared with the size of the Earth, is in about the same ratio as the thickness of a coat of varnish on a schoolroom globe is to the diameter of the globe.” Carl Sagan Skeptical Enquirer, Volume 19, Issue 1, January-February 1995

The presence of water – the life zone Too near the sun (hot) water boils off Too far away (cold) water freezes Orbit of Venus Orbit of Mars Orbit of Earth Sun BUT – significant effect of greenhouse gases on temperature, see later

A home away from home? (liquid water)

Far from equilibrium Atmospheres on Mars and Venus –CO2 rich and in chemical equilibrium Earth’s atmosphere –not in chemical equilibrium –held in a precarious state by the biosphere O 2 not naturally free e.g. rusting Oxygen production CO 2 removal Oxygen removal

The modern O 2 cycle 1. Plants release O 2 - photosynthesis. 2. Animals/plants respiration use O 2 to break down sugars. 3. CO 2 is released by respiration & used in photosynthesis. 4. O 2 cycles between oceans & atmosphere, maintaining equilibrium.

Modern O 2 cycle Photochemistry: 2H 2 O  O 2 + 2H 2 (H 2 thermal escape) 2O 2  O + O 3 (solar ray formation of ozone) rate: kg Weathering (chemical reactions): rate: kg/year Volcanism:emits CO, Sulphur, react; rate: - ~10 10 kg/year water vapour (as above) gives a source of oxygen Photosynthesis:CO 2  O 2 ; rate: kg/year Respiration & decay: O 2  CO 2 ; rate: - ~10 14 kg/year (balancing) Burial of Carbon: (no longer reacts with O 2 ) rate: kg/year Recycling of sediments: rate: < kg/year Fossil fuel combustion (O 2  CO 2 ): rate: kg/year Note: rough mass of O 2 in atmosphere kg

Origin of Oxygen and Ozone Oxygen and aerobic life linked

Uncertainty Oxygen produced by life first organisms anaerobic later aerobic, plants & animals Oxygen in oceans forms banded iron Free Oxygen, redbeds occur UO 2, FeS 2 (pyrite) Oxygen Content of atmosphere over time

The three-reservoir model of Earth Evolution of Atmosphere - Treat earth as covered with ocean - 3 reservoir of O 2  atmosphere shallow ocean deep ocean -Each has a combination of processes which are grouped into -O 2 reducing (R) & Oxygenating (O). 3 different states: A) reducing: very little O 2 present B) oxidising: enough O 2 to oxidise mineral but not enough for respiration C) aerobic: enough O 2 to support aerobic respiration Atmosphere Shallow Ocean Deep Ocean Reducing Oxygenating volcanic gases weathering volcanic gases photochemistry photosynthesis

4 stages in the history of O 2 on Earth Stage I: After water is established Photochemistry  O 2 Reach balance with weathering & volcanism Very little O 2 in atmosphere ~between to PAL (present atmospheric level) Atmosphere (reducing) Shallow Ocean (reducing) Deep Ocean (reducing) Reducing Oxygenating volcanic gases weathering volcanic gases photochemistry

Shallow Ocean (oxidising) Reducing Oxygenating volcanic gases weathering volcanic gases photochemistry photosynthesis Stage II: photosynthesising organism spread new source of O 2 possible increase burial rate due to tectonic activities O 2 level at ~2 billion years ago Atmosphere (oxidising) Deep Ocean (reducing) 4 stages in the history of O 2 on Earth

Stage III: Abundance of photosynthesising organism O 2 level limited because primitive anaerobes can’t tolerate high O 2 level Organisms had to evolve to cope with high O 2 level Atmosphere (aerobic) Shallow Ocean (aerobic) Deep Ocean (oxidising) Reducing Oxygenating weathering photosynthesis respiration/decay volcanic gases 4 stages in the history of O 2 on Earth

Stage IV: Deep ocean becomes aerobic New organisms Balance between respiration and photosynthesis Reducing Oxygenating respiration photosynthesis Atmosphere (aerobic) Shallow Ocean (aerobic) Deep Ocean (aerobic) respiration As O 2 increases, photochemistry creates ozone in upper atmosphere. When ozone layer is thick enough to shield from solar rays then living organisms can live out of water 4 stages in the history of O 2 on Earth

Dependency of life on Oxygen Oxygen (%)Health Effects in Humans 17Accelerated heartbeat 16Increased reaction time 15Poor judgment 10 – 12Loss of consciousness 8 – 10Coma < 8Brain damage < 6Death

Water Cycle Clouds form by convection in high, cold regions of troposphere (see next lecture) Stronger convection more clouds –Thunderstorms on summer afternoons –Lush jungle regions at equator –Desert at o, depleted of moisture (see lecture on wind)

CO 2 cycle Critical for greenhouse effect (see later lecture) Cycle driven by water Without water CO2 stays in atmosphere as on Venus

The “Gaia” feedback mechanism Self regulating Earth An hypothesis “a complex entity involving the Earth's biosphere, atmosphere, oceans, and soil; the totality constituting a feedback or cybernetic system which seeks an optimal physical and chemical environment for life on this planet.” James Lovelockbiosphereatmosphere oceanssoilcybernetic Arguments: Earth’s surface temperature remained roughly constant, despite change of 30% in solar energy input Even though out of equilibrium, atmospheric composition remains constant Ocean salinity is constant Criticism: What mechanism drives self-regulation ? “there was no way for evolution by natural selection to lead to altruism on a Global scale”altruism Richard Dawkins, Extended Phenotype

The “Gaia” feedback mechanism Daisy world - A computer model planet orbiting a sun & slowing getting more heat from it planet inhabited by two types of daisy – black & white reproduction rate of both have same dependence on T However, white – reflect light – cooling planet black – absorb light – heating up planet Black hotter, reproduce more Leave to run – reach equilibrium Planet goes from black daisy dominating to white daisy dominating as it keep surface temperature constant Stable and Self-regulating – within temperature limits

Example exam questions Q1. Name three processes which add oxygen to the Earth’s atmosphere? Q2. Describe the main features of Daisy world? What is its significance? Q3. What will happen to oxygen in the earth’s atmosphere if living organisms were to die off? Next lecture – structure of planetary atmosphere

O2 & CO2 cycles

Height (km) Temperature (K) Troposphere Stratosphere Mesosphere Thermosphere