Evolution of Earth’s atmosphere (Greek - vapor ball)

For reference see: Evolution of the atmosphere (Walker, 1975)

Chemistry of Atmospheres (Wayne, 1991)

Chemistry of the Natural Atmosphere (Warneck, 2000)

 

Primordial

If the atmosphere formed from the solar nebula then its noble gases should be similar to solar noble gases. ® Neon would be 10-100 times more abundant in earth’s atmosphere than it is. (Fig 9.1, Wayne, 1991)

 

Assumption is original solar nebula atmosphere blown away by solar wind, based on the difference of the natural isotopic ratios of Ar/Ne on the inner planets compared to the sun.

 

Present atmosphere began formation ~ 4 x 10­9 years ago (bya) based on abundance of radiogeneic isotopic ratios of Ar/Xe.

 

Noble gases in preCambrian (> 0.6 bya) sediments have same isotopic ratio as in amtosphere today à atomosphere is derived from earth’s mantle.

 

Mantle 3.8 bya (during space bombardment) releases, through geologic activity (volcanoes), N, S, H2­O, Cl, CO2 (based on weathering of pre-Cambrian (> 0.6 bya) rocks.

 

CO2 ~ 100 x > today. This much CO2 compensates for a lower solar constant (25%) to keep the earth’s overall temperature warm enough to preserve liquid water. CO2 is reduced by forming carbonates (limestones) in the oceans.

 

S, Cl also highly water soluble ® trapped in oceans or rained out on surface.

 

But no O2, -  O2 also absent from all other planetary atmospheres.

 

Rise of O­2 in the last 2 by on earth is the mystery.

 

 

Compare with Mars and Venus:

Planet

Distance from sun (AU)

Diam-eter

Mass

Surface pressure

Temperature with no atmsophere (K)

Measured temper-ature (K)

CO2

O2

H2O

Venus

0.72

0.95

0.82

90

330

700

96

.007

0.3

Earth

1.0

1.0

1.0

1.0

255

288

.03

0.2

2

Mars

1.52

0.53

0.11

0.05

218

220

95

0.1

0.03

 

 

Venus – initial temperature too hot to allow liquid water to form ® all water in vapor ® no way to trap the CO2 ® runaway greenhouse effect. High temperature vaporizes many more molecules than on earth ® atmospheric pressure 90 x of Earth’s.

 

Mars – liquid water present at some time in the past ® Mars must have had an atmosphere to capture solar energy. Mars, however, too small, not enough mass, to develop a molten core ® no magnetosphere, and little geologic activity ® early atmosphere stripped away by solar wind, and no geologic activity to replace it.

 

 

Earth - large enough for gravitational foces to form a molten core, and thus produce a magnetic field ® magnetosphere which protects earth’s atmosphere from solar wind. (magnetosphere and solar wind)

 

Liquid water (Goldilocks effect) – initial temperatures of Venus (too warm), Mars (too cold), and Earth (just right) for liquid water to form. With no liquid water Venus and Mars cannot remove CO2 from atmosphere, whereas Earth can, capturing CO2 in carbonates.

 

3.7 bya – oldest known sediments, well rounded pebbles ® liquid water present

 

Unweathered (® no exposure to O2)  pyrite and uranite present in pre-Cambrian (> 0.6 bya, Aechian) sediments (Fig. 12.5, Warneck). These sediments show long transport ® ample exposure to atmosphere.

 

Soil rich in FeO rare until 2 bya, banded iron stones. Continental red beds only appear 2 bya (Fig. 12.5, Warneck).

 

Implications of no O2

 

Many organic compounds required for early life could form.

All early life forms (earliest ~ 3.6 bya) were anoxic.

 

Over first 1.5 by after first life formed life evolved in its ability to capture energy.

 

All life must metabolize (derive energy from environment) which has two functions:

1)      biosynthesis – the synthesis of organic molecules to make cells – building cell material from carbon. Two types of biosynthesis

a)      heterotrophs – depend on a source of ogranic carbon in the environment, all modern heterotrophs depend on autotrophs, because of oxidizing atmosphere. Prior to O2 in atmosphere there was a larger soruce of abiotic organic carbon.

b)      autotrophs – can synthesize organic carbon from inorganic carbon, e.g. CO2

 

2)      provide energy to support bioligic function, including biosynthesis

a)      Radiant energy – phototrophs - absorbed energy is converted into chemical energy. Some of these organism use this energy to synthesize organic moleculse à photosynthetic organisms (derive carbon from inorganic compounds).

b)      Chemical energy - chemoheterotrophs – extract energy from chemical reactions of primarily organic compounds extracted from enviroment. This energy source requires the oxidation of one element (electron donor) and reduction (electron acceptor) of another element.

 

Energy sources for early life forms

Fermentation –

·      e.g. C6­H12O6 ® 2C2H5OH + 2CO2   (Sugar ® aclohol + CO2)

·      Caused no change in overall oxidation

·      Can derive energy from ogranic compounds which are not highly reduced or oxidized

·      Relatively inefficeint energy source < 10% as efficient as aerobic respiration

·      Life (heterotrophs) slow growing, required organic molecules created abiotically ® not a geologic force.

 

First autotrophs

·         Anoxic bacteria – e.g. methane bacteria CO2 + 4H2 à CH4 + 2H2O (obligate anaerobes – cannot tolerate exposure to oxygen – now live in deep water sediments)

·         Note reduction of carbon and oxidation of hydrogen.

·         This process limited by supply of hydrogen: Source - volcanoes, Sinks – escape to space, incorporation in organic matter not susceptible to further dissociation, fermentation.

·         No O2 production.

 

Bacterial photosynthesis

·         Does not produce oxygen

·         Reduced CO2 using H, H2S, and organic molecules (purple and green bacteria examples).

·         Reduced organic compounds resulting from metabolism are lost to the system in sediments and thus volcanoes still the only source of new reduced compounds which have been lost to system.

·         This activity would have reduced hydrogen, allowing a buildup of O2 from the photolysis of water.

·         2H2O + hn (UV) ® 2H2 + O2    H escape to space (escape velocity), O builds up. This reaction alone would require 26 by to create the earth’s O2 layer.

·         Buildup of O2 from photolysis of water à ozone (O3) also appears and this is essential to allow organisms to be exposed directly to atmosphere. Life moves to surface.

 

Green plant photosynthesis – 2 problems to overcome

1)      Considerable input of energy required to dissociate water into hydrogen and oxygen. This process essentially uses water as the electron donor instead of hydrogen. Sunlight can provide this energy.

·         CO2 + H2O + hn (visible) ® O2 + CH2O (organic matter) – Photosynthesis. At present photosynthesis creates 20 bTons/year of O2

 

2)      The dissociation of water requires intermediate compounds such as HO2, H2O2 and OH (free radicals). These compounds, particularly OH are very reactive and would attack organic matter. Thus organisms had to develop methods to suppress the concentrations of these molecules, modern organisms use enzymes to do this.

 

·         Once these problems were overcome life was freed from dependence on volcanoes and became a geologic force in modifying the atmosphere, releasing abundant oxygen.

 

·         Leads to carbon cycle

a)      Plants covert CO2 to organic matter and release O2 to atmosphere and oceans O2 converts dead organic matter back to CO2

b)      Some CH2O gets buried ® carbon lost to the system, at least for a while, and O2 increases

c)      CO2 stored in air, oceans, organic matter (living and dead), limestones, humus, peat, coal, oil.

d)     O2 released stored in air/oceans, oxidized soil and minerals. Source of O2 must supply atmospheric O2 and satisfy all surface sinks, e.g. FeO, CO2, SO2, H2­­O.

 

 

Rise of O2­ - began ~ 2.1 bya, complete ~ 0.5 bya

·         Forced anerobic life to move beneath the surface

 

·         New metabolic opportunity – aerobic respiration – oxidation of organic molecules by oxygen. Extracts much more energy per organic molecule than any process so far à these organisms prosper in environments much poorer in organic matter than previous ogranisms.

 

·         Ozone layer advanced enough by ~ 0.5bya that life moves onto the land.


Brief history of discovery of earth’s atmosphere

 

1774 – N2, O2 identified

 

1840 – Electric sparks create an unusual odor. Schönbien proposed naming this gas ozone after Greek (ozien = to smell)

 

1858 – Ozone discovered in natural atmosphere

 


1863 – Soret descibed molecular structure of ozone  

 

1880 – Chappuis ozone absorption bands

 

1881 – Hartley UV spectra of ozone, l < 300 nm (UV). Also demonstrated that atmospheric limit of solar UV spectra was due to ozone.

 

1918 – Lord Rayleigh determined height of ozone layer to be 40-60 km.

 

1921 – Technique to measure total column ozone, based on measuring UV absorption, developed.

 

1925 – Technique refined and applied (Dobson) based on measurements of the relative intensities at different ls in the 300-340 nm range. In recognition of this work total ozone today is measured in Dobson units.

 

1930 – Chapman explained the formation of the ozone layer.

 

1934 – Götz, Meetham, Dobson corrected height of ozone layer to 15-50 km

 

 

Food Preservation

·         100 BC – 1700s – Drying, salting, fermenting, pickling, cool cellars, spring houses, canning. Cooking ® can eat spoiled meat – spices to hide the flavor

 

·         1803 – Ice Box. Thomas More invented the insulated box, with ice in a separate container above the food storage area. Relied on stores of natural ice from frozen lakes and rivers

 

·         1850s – Methods to artificially produce ice were developed.

 

·         1890 – Warm weather/rain lead to a shortage of natural ice. Spurred the development of mechanical refridgeration.

 

Refridgeration/CFCs

•        1918 - Kelvinator, First refrigerator introduced to American market.

 

•        1920s - Refrigerators used ammonia (NH4), sulfur dioxide (SO2), (toxic, odorous), and methyl chloride (CH3Cl), (toxic, no odor) - silent killer.

 

•        1928 - Thomas Midgley, Fridgidaire, dichlorodifluoromethane (CCl2F2)

 

•        1929 - Fridgidaire and DuPont joined to produce CCl2F2, and CFCl3  as Freon.

 

•        1941 - Automobile air conditioning - Packard

 

•        1943 - Bug bomb used in WWII - Lead to use of CFCs as propellants for aerosol spray cans.

 

•        1940 - 1960 - Uses of Freon: Refrigeration, blowing agents to make plastic foams (polyurethane polystyrene), solvents/cleaning agents for circuit boards (defluxing), aerosol propellants.

 

•        1970s - CFC production 600,000 tons annually growing at 10% per year.

 

•        1971 – Lovelock – Electron capture / Gas chromatography – able to detect molecular concentrations on the order of ppt (10-12).

 

•        1974 - Uses with immediate releases - 66%, Refrigeration - 20%

 

•        1974 – Molina and Rowland – What is the sink for atmospheric CFCs?