Post on 14-Dec-2015
The Exam results ….
Note: Ex1 score = (number correct)/49 You get one ‘bonus’ question due to boo-boo on a question in the 3PMer’s test
The Earth
How it got this way ….
Accretion of planetesimals
Accretion of planetesimals(4.54 billion years ago)
Massive enough to form a sphere
START
Accretion of planetesimals
Gravitational and impact energies melts it
Temp. and pressure increase inwards to
the core
Differentiation
Denser materials sinkLess-dense ones float
IRON CORE
MANTEL
“Solid” crust
Maybe only 10 million years later
Liquid Fe core + rotation =
magnetic field
Must have looked like this …
After some cooling
IRON CORE
MANTEL
Lithosphere“cool, rigid rock”
Meanwhile: “Heavy bombardment period”
IRON CORE
MANTEL
Comets – ICE (water!)
First few hundred-million years
Asteroids
Asteroids
Still cooling,Heavy crateringand Volcanism
Opps!
IRON CORE
MANTEL
“Theia”
(more than 4.4 billion years ago)
Moon forms from the less-dense
outer layers that get blasted off
After some major recovery …. Late bombardment tapers off
IRON CORE
MANTEL
First oceans form about 3.8 billion years ago from comet water!
Note: close moon
After A LONG time …. The present
IRON CORE
MANTEL
Lithosphere“cool, rigid rock”
Convective cooling by the mantel
driven by temp. difference
100 million year time scale.(bottom to top)
Tectonic plates
Tectonic activity
Our Atmosphere
How it got this way ….
Volcanism!
Gasses trapped within earth’s mantel released
Liquid water!!!
Were did they all go?
Were did this come from?
The light stuff: H2 and He drifted off into space.(Low mass -> high speeds > escape velocity)
Were did they all go?
The semi-light stuff: CH3 and NH3 broken down in the atmosphere by UV radiation. (No ozone layer back then!!)
Water and CO2 ?? We get very lucky…
Greenhouse gasses!
Temperature warm&cool enough for water to condense and rain!
OCEANS!
CO2 the dissolves in the seas and is locked away in carbonate rocks
(e.g. limestone)
Were did The O2 come from?
Modern stromatolites at Shark Bay, Western Australia. Stromatolites are rock structures built up by layers of cyanobacteria, microorganisms that contribute to the addition of O2 to the atmosphere. Their role in oxygen production dates back billions of years and similar structures to the stromatolites at Shark Bay have been found in rocks as old as 3.5 billion years before present.
Our oxygen is “plant” excretion
Atmospheric cycles – delicate balance of everything
“We’re so lucky” slide:
Earth big enough:
Earth-Sun distance just right:
Slower cooling -- Fe core still molten producing a magnetic field protects us from solar ion wind
We formed just far enough so water would condense, but not freeze.We owe the greenhouse effect for this!
Avoided bad greenhouse by locking most H2O and then CO2 in the oceans
Liquid oceans form the right place for early anaerobic life to evolve.At first O2 was a toxic waste product, building up.
O3 layer which shields later, land-based life from UV.Later life evolved to burn O2 as food.
The “Green-house” effect
Why does something (i.e. a planet) change its temperature?
Energy in Energy out
T
IF Energy in Energy out>Temperature increases!
Energy in Energy out
T
IF Energy in Energy out<Temperature decreases!
Energy in Energy out
T
IF Energy in Energy out=Temperature constant!
EQULIBRIUM
After billions of years cooling, the planets are very near equilibrium!
Only cooling very slowly (if at all)
T
A planet at (surface) temperature T
T
Solar radiation:This is ‘mostly’ in the visible range
ENERGY IN
Reflected (Albedo)
Absorbed
Sun
Ligh
t
Depends on surface compositionCloud reflectivity etc.
Self heating:Radioactive decay!
Decreases with time!
T
Thermal radiation:For (modern) planetary temperatures,This is in the infra-red
ENERGY OUT
SURFACE AT TEMPERATURE T
SURFACE AT TEMPERATURE T
No atmosphere:All IR out to space.
SURFACE AT TEMPERATURE T
Non-greenhouse atmosphere:Gasses do NOT absorb in the IRAll IR out to space.
N2 O2
Ar
SURFACE AT TEMPERATURE T
With a greenhouse atmosphere:Gasses do absorb in the IR
Less IR out to space.
CO2
H2O
Absorbed
Re-emitt
ed
For systems close to equilibrium, small changes in the energy balance can cause the surface temperature/climate to change
New sources of CO2 -> “run away” greenhouse effect.
(cooling system failure)
Nuclear war/asteroid strike -> “Nuclear winter”.
(heating system failure)
(increased albedo)