OCN 201: Earth Structure Eric Heinen Eric H. De Carlo, Carlo: OCN 201, OCN Sp2010 201, Fall 2004
Early History of the Earth Rapid accretion of Earth and attendant dissipation of kinetic energy caused tremendous heating. Earth possibly melted completely. In molten state, differentiation would occur.
Core Formation Heavy elements (mainly Fe) would sink inward. Lighter elements would migrate outward. HOWEVER Heavy elements may have already been concentrated at the center because they fell inward during accretion. Thus the core may have formed by heterogenous accretion at Earth s formation or soon after if Earth formed by homogenous accretion. The latter would have been accompanied by extensive outgassing, the process that ultimately formed the oceans. Core formation was largely complete in <100 Ma.
Origin of Oceans/Atmosphere: I Accretion and differentiation of Earth would have created an atmosphere by outgassing Our atmosphere is thus of secondary origin: derived by outgassing of interior rather than directly from solar nebula Evidence: Earth is depleted in noble gases Alternate hypothesis: heterogeneous accretion of late veneer, possibly from asteroids (and small amount of comets) NOTE: we are simplifying the event sequence that led to the formation of the Earth and oceans
Origin of Oceans/Atmosphere: II Water as a volatile substance would have been outgassed from early Earth but retained by its gravity BUT the Earth surface was too hot for a liquid ocean Is Earth fully outgassed today??? Estimates range from 20% to nearly complete Outgassing was likely faster from early Earth Radioactive decay (causes heat) was 4-5 X greater than today, thus outgassing would have been commensurately faster
Origin of Oceans/Atmosphere: III Outgassing of Earth continues now... Evidence comes from 3 He in ocean near MOR 3 He released from Earth interior by volcanic processes, Sp2010
3 He in Pacific Ocean
Composition of Volcanic Gases TODAY. 80% H 2 O 10% CO 2 5% SO 2 1% H 2 Trace, N 2, HCl Major gases are in oxidized form now Only H 2 is reduced Early volcanic gases were likely in reduced form (H 2, CH 4, H 2 S, NH 3 ) Early atmosphere: Free O 2 would have been absent. CO 2 and CH 4 were probably abundant. The CO 2 would have eventually reacted with rocks (in water): (H 2 O) + CO 2 + CaSiO 3 CaCO 3 + SiO 2 + (H 2 O)
Early Atmosphere Free O 2 was absent Any O 2 was quickly used to oxidize reduced materials in rocks O 2 also used to oxidize reduced Fe (which was likely very abundant in early seawater)
Solar Luminosity Luminosity of Sun has increased by 30% over 4.5 Ga. Change in luminosity has altered Earth T Early Earth T = 248K (-13 o F, -25 o C) Current T = 288K (59 o F, 15 o C) Faint early sun paradox : why did Earth s early oceans not freeze over? CO 2 and CH 4 methane in the early atmosphere may be the answer., Sp2010
Structure of the Earth Inner core: solid Fe-Ni Outer core: liquid Fe-Ni Mantle: rocky: Mg-Fe silicate Crust: rocky: Mg-Fe-Al-Ca silicate Oceans: H 2 O with dissolved salts Atmosphere: N 2, O 2, Ar
Internal Structure of the Earth: I Mean density of Earth is 5.5 g/cm 3 (mass/vol) Density determined by shape, size, mass, and moment of inertia of Earth Earth structure determined in large part by physical measurements (seismic methods) Discontinuities in seismic velocities are due to changes in bulk density/composition
Seismic Waves: I Generated by earthquakes (or explosions) Two types: P and S waves
Seismic Waves: II P-waves Primary waves Faster than S waves Compressional Like a spring compressing and dilating Travel through solid or liquid
Seismic Waves: III S-waves Secondary waves Slower than P waves Shear Like undulation of string Do not propagate through a liquid No restoring force in liquid
Seismic Waves: IV
Seismic Waves: V Waves bend in response to changes in properties of material. S-wave shadow zone P-wave shadow zones, Sp2010
Internal Structure of the Earth: II (based on chemical properties) Inner Core: 5100-6370 km, solid Fe + 6% Ni (16 g/cm 3 ) Outer Core: 2900-5100 km, liquid Fe-Ni (12 g/cm 3 ) Core is 32% of Earth mass, 16% of its volume Mantle: ~10-2900 km, solid Mg-Fe-silicates (4.5 g/cm 3 ), 68% of Earth mass, 83% of its volume Crust: the skin of Earth: 0.4% of Earth mass and <1% of its volume.
Earth s Crust The crust represents only 0.4% of the mass of Earth, and <1% of its volume. There are two types: Oceanic and Continental Oceanic Only 6 km thick Made of basalt (like Hawaiian Islands) 2.9 g/cm 3 Continental 35 km thick Made of granite (really granodiorite or andesite) 2. 7 g/cm 3
Internal Structure of the Earth: III (based on physical properties) Use viscosity and strength to describe outer layers: Lithosphere: 0-100 km = mantle + crust Asthenosphere:100-700 km = mantle Mesosphere:700-2900 km = mantle
Internal Structure of the Earth: IV Lithosphere is cool, rigid, can support loads and includes the crust and uppermost mantle Asthenosphere is near its melting point, deforms plastically Upper asthenosphere (100-230 km) is a low velocity zone thought to contain ~1% melt Upper asthenosphere is the zone of isostatic compensation and a zone of magma generation for igneous rocks The mesosphere (most of mantle) extends to the core and is more rigid than the asthenosphere
Bulk Composition of Earth (wt %) Elements are NOT distributed uniformly wt.% Fe 36.0 O 28.7 Mg 14.8 Si 13.6 Subtotal: 93.1% 85 ± 4 % of Fe is in the core (metallic) (Upper) Mantle: rocky: mainly Mg-silicates. Ni 2.0 Ca 1.7 S 1.7 Al 1.3 Most of the Ni is in the core Al is mostly in alumino-silicates TOTAL: 98-99%
Summary why do we have oceans? Lots of liquid water available but why? 1) rapid accretion of cold, icy, water rich planetesimals (allowed retention of volatile H 2 O after ice melted) 2) outgassing of interior of Earth brought H 2 O to surface 3) moderate distance from Sun moderate temperature allowed H 2 O to be present in liquid form Do other bodies in the Solar System have oceans? YES! Mars probably had oceans in the distant past. Europa (moon of Jupiter) may have oceans under thick ice. Titan, a moon of Saturn, may have liquid hydrocarbon oceans, with continents of rock, H 2 O ice, and CO 2 ice.
Earth and its nearest neighbors All 3 planets have similar noble gas abundance ratios, implying grossly similar composition and outgassing history.
Comparative Planetology Distance Surface Surface ATMOSPHERE: Mass Radius Density from Sun Temp. Press. H 2 O CO 2 N 2 O 2 10 26 g km g/cm 3 10 6 km (K) (atm) % % % % Venus 49 6050 5.3 108 750 100 0 96.5 3.5 0 Earth 60 6370 5.5 150 288 1 <1 0.04 78 21 Mars 6.4 3390 3.9 228 210 0.006 0.1 96 2.5 0.25 Inventory of CO 2, in units of 10 20 moles: Earth, in crust, atmosphere, and oceans: 75 in mantle: 150 Venus, in atmosphere: 120 H 2 O + CO 2 + CaSiO 3 CaCO 3 + SiO 2 + H 2 O
Venus has outgassed a similar amount of CO 2 as Earth, but it stayed in the the atmosphere, causing a runaway greenhouse. On Earth this CO 2 is locked up in rocks! High temperatures caused Venus to lose all of its H 2 O (260 atm-worth!) by photodissociation followed by loss of H 2 to space. Venus would lose an Earth Oceans worth of H 2 O in 30 to 300 million years.
Comparison of Atmospheres: Venus vs. Earth N 2 has remained in atmosphere on Venus just like it has on Earth Low relative concentration on Venus results from dilution by the highly abundant CO 2 in the Venusian atmosphere., Sp2010
Comparison of Atmospheres: Mars vs. Earth Mars atmosphere is 1/150 that of Earth s. Mars is small and cold! N 2 was lost to space: ~1 atm in 4.5 billion years., Sp2010 H 2 O and CO 2 are frozen in the polar caps and regolith (soil).
SUMMARY Fate of Planetary Gases (volatile compounds) Earth Venus Mars H 2 O oceans H space ice O rocks (1 ocean in 30-300 million years) CO 2 rocks atmosphere ice N 2 atmosphere atmosphere space (1 atm in 4.5 billion years) O 2 atmosphere none none
Evolution of Atmosphere-Ocean System, the Rise of Free Oxygen: I Earth is chemically reducing (much reduced Fe) To make free O 2 reducing material must be isolated/separated from oxidizing material Core formation did much but not enough Two theories for rise of O 2 in atmosphere: 2 H 2 O = 2 H 2 + O 2 Photodissociation of H 2 O and loss of H 2 to space (alone this would produce current levels in 4.5 Gy) CO 2 + H 2 O = CH 2 O + O 2 Photosynthesis combined with burial of 0.1% of OM (this would produce current levels in 4 My)
Evolution of Atmosphere-Ocean System, the Rise of Oxygen: II Before O 2 could accumulate in atmosphere enough needed to be produced to oxidize large surface reservoirs of reduced material, e.g., Fe 2+ dissolved in early oceans Free oxygen began accumulating about 2.4 Gy bp and present levels were likely reached around 800 My bp Multicellular organisms evolved later because of development of ozone layer, they eventually migrated from sea to land
Possible evolution of Earth s atmosphere over geologic time, Sp2010
Early Earth was a violent place, yet life originated there!, Sp2010