Life on Earth. I.
Earth
Terrestrial Planet
1 AU from the Sun
Equilibrium temperature: 247 K (-26C)
Actual mean temperature: 287 K
Differentiated into
•Inner core
•Outer core
•Mantle
•Crust
Composition
Crust:
•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%
Surface
Water: 71%
Land: 29%
Structure
Inner core: 1.6% of mass
Outer core: 31%
Mantle:
67%
Crust:
0.4%
Oceans:
0.02%
Atmosphere: 9x10-5%
5150-6378 km
2700-5150 km
40-2700 km
upper 40 km
The Earth is 4.5 billion years old.
How do we know?
Radiometric Dating
How old is the Earth?
K40 half-life:
1.25 billion years
Radioactive Decay
The change of an atomic nucleus into a lower energy
state
• decay: proton  neutron + positron
Atomic number decreases
•Inverse  decay: neutron proton + electron
Atomic 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) gives
number of half lives
Age of the Solar System: 4.57 Gyr
Age 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 a
natural consequence of the
conservation of angular momentum in a
collapsing, rotating system.
• Such disks are observed.
Planet Formation
Protoplanetary Disks
Birth of the Earth
• Inner protoplanetary disk is hot
• Terrestrial planets lack volatiles
Temperature (K)
Condensate
1500
Fe2O3, FeO, Al2O3
1300
Fe, Ni
1200
Silicates
1000
MgSiO3
680
FeS
175
H2O
150
NH3
120
CH4
65
Noble gases
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 mantle
Crust is broken into about a dozen plates
The continental plates move independently
Continental Plates
Plates in Motion
Plates Tectonics and
Habitability
Subduction removes carbonates into mantle
Otherwise greenhouse CO2 accumulates
Vulcanism ejects gas, including H2O
Reconstitutes atmosphere
Vulcanism provides new land and raises mountains
Counters weathering/erosion
What Planets are Tectonically
Active?
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 a
Tectonically Active Planet
Molten iron cores are convective, and generate a
magnetic field through dynamo action (much like
the Sun).
Magnetic Fields
divert charged
particles.
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 Gyr
Oldest rocks: 4 Gyr
Oldest continental crust 3.9 Gyr (in Greenland)
(Lunar rocks are 4.5 Gyr old)
The Hadean Earth
4.5 - 3.9 Gyr
Impacts melt the surface.
Volatiles escape to space
Source of atmosphere, oceans: outgassing and impacts
Early 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 Earth
There is no fossil record.
There is no
chemical record.
Impacts might have
kept the surface in
a molten/sterile
state.