Evolution of the Atmosphere: The Biological Connection The Earth s Four Spheres How It All Began Or At Least How We Think It Began O.k. it s a good guess Egg of energy The Big Bang splattered radiation in all directions Expansion and cooling; atomic particles formed through conversion of energy to mass. E = mc 2 (energy = mass x speed of light 2 ) Formation of Stars, Galaxies Stars condensed out of the matter produced by the Big Bang and continued to move away from the point of the explosion. Simple elements (hydrogen, then helium) fused to form first stars Heavier elements produced within giant pressure cookers (stars) 1
Solar Nebula Hypothesis; Origin of Solar System Within proto Milky Way Galaxy, clumps of gas and dust accumulated, forming large, irregular, nebulae ( clouds ). Dust and gases in one such cloud settled toward mid-plane of slowly rotating disc due to gravitational/electrostatic attraction (protosolar system, in effect, a dust bunny ) The Nine Planets Neptune Uranus Saturn Jupiter Pluto Jovian planets After some of the matter had collected in centre of disc, nuclear reactions began, forming Sun Remaining gas and dust, moving in circular bands around Sun collided and accreted to form planets. Mercury Mars Earth terrestrial planets asteroid belt Early Evolution of Earth Heating, partial melting of Earth due to: Radioactive decay of unstable elements Gravitational migration (sinking) of heavy elements Impacting of extraterrestrial objects (meteorites, comets) Period of Heavy Bombardment As Earth accreted, frequency of impacts decreased BUT size of impacts increased due to increasing size of impacting bodies Period of heavy bombardment from about 4.5 to 4 billion years ago (end Of Hadean Eon, beginning of Archean Eon) - magma ocean due to widepread melting, so no rock record Oldest rocks around 4 billion years old (Acosta Gneiss, Arctic Canada) indicating presence of solid crust by that time. 2
Evidence of Bombardment by Other Objects Obliquity of Axis The Moon: A Product of Heavy Bombardment? For an iron-rich composition, moon s core represents merely 1 to 3% of the Moon's total mass. In contrast, Earth's core is about 33% of our planet's total mass Most of core of impactor believed to have become part of Earth and while Moon accreted from mostly mantle material. overturned Uranus tipped on its side All planets have at least some tilt Jovian Planet Interior: Jupiter Comparison of Jovian and Terrestrial Atmospheres Jupiter Major: Hydrogen (81 %) Helium (18 %) Minor: Methane and other hydrocarbons Ammonia Phosphorus Water Vapour Major: Carbon Dioxide (96.5 %) Nitrogen (3.5 %) Minor: Sulphur Dioxide (0.015 %) Water Argon Carbon Monoxide Helium Oxygen 3
Comparison of Atmospheres: vs. Earth Major: Carbon Dioxide (96.5 %) Nitrogen (3.5 %) Minor: Sulphur Dioxide (0.015 %) Water Argon Carbon Monoxide Helium Oxygen Earth Major: Nitrogen (78 %) Oxygen (21 %) Argon (0.9 %) Water Vapour (0.05 to 2 %) Minor: Carbon Dioxide (0.03 %) Methane, Ozone Rare Inert Gases Possible Sources of Early Atmospheric Gases Remnant gases from solar nebula: H 2, He (these gases too light to be held in atmosphere by gravitational field) Outgassing from Earth s interior (volcanoes): H 2 O, CO 2, SO 2, NH 3 (ammonia), N 2, NO x (nitric oxides) Material from impacting comets: H 2 O, CO (carbon monoxide), CH 3 OH (methanol), CH 4 (methane), C 2 H 2 (acetylene), C 2 H 6 (ethane), HCN (cyanide), Ar (argon) Earth s Atmosphere Major: Nitrogen (78 %) Oxygen (21 %) Argon (0.9 %) Water Vapour (0.05 to 2 %) Minor: Carbon Dioxide (0.03 %) Methane, Ozone Rare Inert Gases Why the change in atmospheric composition? VOLATILES OUTGASSED TO FORM OCEANS & ATMOSPHERE N 2 (molecular nitrogen) primarily derived from volcanic outgassing. CO 2 initially scrubbed from atmosphere through weathering and later biological activity (cooling effect) H 2 O condensed into oceans upon atmospheric cooling, some remained in atmosphere O 2 produced initially by photodissociation, but mostly by photosynthesis later on accumulates to allow formation of ozone ozone layer Ar (1%) mostly radiogenic (from decay of K - potassium) 4
END OF LECTURE 5