Life on Earth. I.
Life on Earth. I.
EarthTerrestrial Planet1 AU from the Sun
Equilibrium temperature: 247 K (-26C)Actual mean temperature: 287 K
Differentiated into•Inner core•Outer core•Mantle•Crust
CompositionCrust:
•Oxygen 47%•Silicon: 28%•Aluminum: 8%•Iron% 5%•Calcium: 4%
Overall:•Iron: 35%•Oxygen: 30%•Silicon: 15%•Magnesium: 13%•Nickel: 2%
Atmosphere:•N2: 77%•O2: 21%•Ar: 1%•H2O: <1%
SurfaceWater: 71%
Land: 29%
StructureInner core: 1.6% of mass 5150-6378 kmOuter core: 31% 2700-5150 kmMantle: 67% 40-2700 kmCrust: 0.4% upper 40 kmOceans: 0.02%Atmosphere: 9x10-5%
The Earth is 4.5 billion years old.
How do we know?
Radiometric DatingHow old is the Earth?
K40 half-life:1.25 billion years
Radioactive DecayThe change of an atomic nucleus into a lower energystate
• decay: proton neutron + positronAtomic number decreases
•Inverse decay: neutron proton + electronAtomic number increases
•Fission: breakup into 2 smaller nuclei
Inherently probablistic
Radioactive Dating. II.Useful radioisotoptes
C14 N14: t1/2 = 5730 years
Al26 Mg 26: t1/2 = 717,000 years
K40 Ar40: t1/2 =1.25 billion years
U238 Pb 206: t1/2 = 4.47 billion years
Rb87 Sr 87: t1/2 = 49.4 billion years
Radioactive Dating. III.How it works:
•Measure abundance of parent atom•Measure abundance of daughter atom•Correct for original abundance of daughter•Ratio of parent to daughter (corrected) givesnumber of half lives
Age of the Solar System: 4.57 GyrAge of the Earth:
•Oldest rocks: 4 Gyr•Zircon grains: 4.4 Gyr
• Parent– Original abundance: A0– Half life: – Current abundance: A = A0 (1/2)
• Daughter– Original abundance: B0– Half life: – Current abundance: B = B0 + A0 (1-(1/2) )
• 1 half life: down by a factor of 2• 2 half lives: down by a factor of 2x2 = 4• n half lives: down by a factor of 2n
Birth of a Planet
Protoplanetary Disks
• Formation of a flattened disk is anatural consequence of theconservation of angular momentum in acollapsing, rotating system.
• Such disks are observed.
Planet Formation
Protoplanetary Disks
Birth of the Earth• Inner protoplanetary disk is hot• Terrestrial planets lack volatiles
Noble gases65CH4120NH3150H2O175FeS680MgSiO31000Silicates1200Fe, Ni1300Fe2O3, FeO, Al2O31500CondensateTemperature (K)
Birth of the Earth• Small dust grains collide and stick• Once grain becomes large enough,
gravity takes over• Runaway accretion ensues.
I Feel the Earth Move Under My Feet
Plate Tectonics
Driven by internal heat:•Radioactive decay•Differentiation•Accretion
Internal heat drives convection
Convection currents in mantle drive continental drift
Plate Tectonics
Continental crust (lithosphere) floats on the denser mantleCrust is broken into about a dozen platesThe continental plates move independently
Continental Plates
Plates in Motion
Plates Tectonics andHabitability
Subduction removes carbonates into mantleOtherwise greenhouse CO2 accumulates
Vulcanism ejects gas, including H2OReconstitutes atmosphere
Vulcanism provides new land and raises mountainsCounters weathering/erosion
What Planets are TectonicallyActive?
All planets have internal heat.
Convection depends on the heat gradient (Tcore)
The heat content is proportional to the volume (R3)
Heat loss is proportional to the surface area (R2)
Planets stay warm for a time R3/R2, so R
Big rocky planets (like Earth)
An Advantage of aTectonically Active Planet
Molten iron cores are convective, and generate amagnetic field through dynamo action (much likethe Sun).
Magnetic Fieldsdivert chargedparticles.
The Crust
Low density - floats on the mantle
Is eroded and weathered
Records impact history
Preserves the fossil record
The Early Earth
The Early Earth
Formed from circumstellar disk at its present radius.Surface remelted in collision that formed Luna.
Differentiated about 4.5 Gyr.Zircon grains dated to 4.4 GyrOldest rocks: 4 GyrOldest continental crust 3.9 Gyr (in Greenland)
(Lunar rocks are 4.5 Gyr old)
The Hadean Earth4.5 - 3.9 GyrImpacts melt the surface.Volatiles escape to space
Source of atmosphere, oceans: outgassing and impactsEarly atmosphere: CO2, H2O, N2, H2S, SO2, H2
Oceans exist by 4.4 Gyr
Impacts:•4.5 Gyr•Late Heavy Bombardment at 3.9 Gyr
Lunar crater counts give this dating
Life and the Hadean EarthThere is no fossil record.
There is nochemical record.
Impacts might havekept the surface ina molten/sterilestate.