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.