How to build a habitable planet
Why is Earth habitable, why is life like it is, and
what has been it’s impact on the Earth?
Eight lectures will address these questions.
Lectures 1 and 2
1. Where do the elements come from ?
2. What controls their distribution in the solar system ?
3. What controls their distribution on Earth?
4. Why is the Earth inhabitable when other planets
aren’t?
galaxy
Composition of the solar system
Plasma disc allows compositional variation with distance,
controlled by temperature
The composition of the Earth
The abundance of elements
in the Universe
and the partitioning on these
elements within the solar
system
determine the building blocks
of the planet
Lecture 2
1. What controls elemental distribution on Earth?
- segregation into core, mantle, crust, hydrosphere,
atmosphere
2. Why is the Earth inhabitable when other planets
aren’t?
- liquid water, retention of atmosphere, temperature
regulation by inorganic processes
The segregation of the Earth
Timing the segregation of the Earth
129
Xenon
produced by 129I,
which has a halflife of 16 Ma
Iodine segregates
into early
mantle
Early heating gravitational + short lived radionuclides,
eg 26Al, half-life 0.73 Ma
The segments of the Earth
Atmosphere
Hydrosphere/biosphere
Crust
Mantle
Core
Core
Largely iron - nickel alloy
Outer core molten
Evidence from earthquake
waves S-waves pass through solids,
P-waves through solids and
liquids.
Velocity depends on density
and hence on composition
Diffraction occurs at velocity
jumps
Mantle
Largely Fe-Mg silicates
Contains most long lived radionuclides 40K, 238U, 235U, 232Th
These maintain heat and produce convection cells.
This geological activity forms crust and is vital to
global environmental processes.
Crust
Segregated into thick, old, Si-rich
continental crust and thin, young, Mg-Fe
rich oceanic crust.
Topography a function of this density
disparity.
Both a function of plate tectonics
Plate tectonics
Partial melting of the
convecting mantle
generates
oceanic crust.
Partial melting of oceanic
crust and its associated
sediments generates
continental crust which has
too low a density to return
to
the mantle.
Plate tectonics
Volcanoes, the main route of degassing volatiles,
earthquakes and significant topography are related to
plate boundaries.
Plate tectonics
Previous configurations
of continents and
oceans have
affected climate
through differences
in topography and
circulation patterns.
Changes in sea level
are generated by
variations in the
rate of formation
of oceanic crust.
Atmosphere
Volatiles were degassed
as the planet segregated.
Subsequent modifications to
the atmosphere have been a
result of interactions
with space, volcanic
emissions and biotic
influences.
The atmosphere has
a major role in
climate control
Atmosphere
Hydrogen is easily lost from Earth’s atmosphere, but
water is retained.
Water denatures to H2 and O in the stratosphere, but this
is cold and hence dry, so that most of Earth’s primitive
water has been conserved.
Climate control: albedo
If Earth absorbed all incoming
radiation = 50C
Earth currently radiates 33%
of incoming heat
If Earth had present albedo as
only control = -20 0c
Ice has high albedo - if planet
froze it would stay frozen
Actual average temp. = 20 0C
Climate control: greenhouse gases
Outgoing energy is trapped by any atmospheric
molecule with > 3 atoms.
Most significant are H2O, CO2, CH4, N2O
If oceans evaporated, planet would stay hot.
Climate control: greenhouse gases
CO2 cycles through atmosphere and crust.
Solid form is CaCO3. Rate of formation depends on
supply of Ca, which is controlled by weathering rates.
Higher temperatures raise weathering rates and increase
drawdown of CO2 from atmosphere.