Presentation on theme: "FORMATION OF CRUST AND ATMOSPHERE Planets of solar system probably formed from remnants of supernovas, i.e., disc-shaped clouds of hot gases (solar nebula)."— Presentation transcript:
FORMATION OF CRUST AND ATMOSPHERE Planets of solar system probably formed from remnants of supernovas, i.e., disc-shaped clouds of hot gases (solar nebula). Condensing vapors formed solids that emerged to form planetesimals (small bodies). Accretion of planetesimals lead to formation of inner planets (Mercury-Venus-Earth-Mars). Large outer planets condensed from gases at much lower temperatures.
ACCRETION OF EARTH As accretion built up the Earth to its present mass, it heated up owing to: –radioactive decay of unstable isotopes –kinetic energy from impacts Heating melted Fe and Ni which sank to the center forming the core. Subsequent cooling permitted solidification of remainder to form mantle of roughly Mg-Fe- silicate composition. Crust, hydrosphere and atmosphere formed from upper mantle during the early history of Earth.
THE CRUST Crust is shell of volume <0.0001% of total Earth volume; a small but important part (to us). Crust evolved through time as “incompatible” elements were removed from mantle by partial melting. Relative elemental abundances O > Si > Al > Fe > Ca > Na > Mg.
THE ATMOSPHERE The smallest of Earth’s geological reservoirs; particularly susceptible to pollution. Mixing time very short, so contamination very quickly spreads throughout the globe, but is also diluted rapidly. Bulk composition of atmosphere very similar world wide. Lower atmosphere (troposphere) is well mixed by convection.
At 15-25 km, the atmosphere is heated by absorption of UV by O 2 and O 3 (stratosphere). This is where the ozone layer occurs. Above ~ 120 km, mixing is so weak that gas molecules can separate gravitationally - H 2 and He are enriched at the top, O 2 and N 2 at bottom. Definitions: –Heterosphere: where gravitational separation occurs –Homosphere: well-mixed part of atmosphere –Turbopause: separates heterosphere and homosphere
THE ATMOSPHERE Volatile elements escaped from mantle during crust formation (e.g., volcanic degassing). Some were retained to form atmosphere. Capture of volatile-rich planetesimals (from orbits beyond Jupiter), which contained water, ammonia and methane. Primitive atmosphere, probably CO 2 + N 2 with minor H 2 and H 2 O(vapor). Modern atmosphere had to await evolution of life.
COMPOSITION OF THE ATMOSPHERE Water and CO 2 are somewhat variable in abundance. Most gases are relatively constant in abundance.
OZONE O 2 (g) + h 2 O(g) ( < 242 nm) O 2 (g) + O(g) O 3 (g) These ozone production reactions are balanced with ozone destruction reactions to maintain a steady state. O 3 (g) + h O 2 (g) + O(g) O 3 (g) + O(g) 2O 2 (g)
ADDITIONAL OZONE DESTROYING REACTIONS NO(g) + O 3 (g) O 2 (g) + NO 2 (g) NO 2 (g) + O(g) NO(g) + O 2 (g) O 3 (g) + O(g) 2O 2 (g) OH(g) + O 3 (g) O 2 (g) + HO 2 (g) HO 2 (g) + O(g) OH(g) + O 2 (g) O 3 (g) + O(g) 2O 2 (g) Chlorine from ChlorFlourCarbonhydroxides (CFC’s) gives rise to: Cl(g) + O 3 (g) O 2 (g) + ClO(g) ClO(g) + O(g) Cl(g) + O 2 (g) O 3 (g) + O(g) 2O 2 (g)
THE HYDROSPHERE Bulk of water at Earth’s surface is in oceans (>97%) and in polar ice-caps and glaciers. <1% is continental fresh water, most stored as groundwater. Source of water not well known: –Water bound as OH in meteorites? –Water-rich comets –Water-rich planetesimals? As Earth’s surface cooled to 100°C, water could condense. From existence of old sedimentary rocks, we know the ocean existed by 3.8 B.Y.
Differentiation of the Hydrosphere Oceans selectively remove elements from seawater: “The fate of all elements and compounds dissolved in the ocean is to be removed from it” Dissolved constituents in the oceans can be divided into two groups Conservative Non-conservative
Conservative elements Li +, Na +, K +, Rb +, Cs +, Mg 2+, Ca 2+, Sr 2+, Cl -, F -, Br -, I -, (SO 4 ) 2-, borate species B(OH) x and (UO 2 ) 2+ -Enriched in seawater in comparison to river water (average factor 775) -Occur in constant proportions throughout the oceans -Mean oceanic residences times (MORT) = Factor of million years