Download The Earth

The Earth and Its Moon
The Earth
•Solid inner core, liquid outer core
•atmosphere - 50km thick
•magnetosphere - charged
particles caught in Earth’s
magnetic field
The Moon
•Moon has no hydrosphere,
atmosphere or magnetosphere
•same basic interior regions as
Earth
The Tides
•Earth is the only planet with large quantities of liquid water on the surface
•Most coastal locations experience 2 low and high tides each day
•what causes this?
The moon pulls the
water
The moon also pulls the
Earth
This causes two bulges one on the side facing
the moon and one on the
opposite side where the
water is “left behind”
The moon is tidally locked to the Earth - the same side of the
moon is always facing us (moon rotation period is the same as its
orbital period)
Is there a side which is always dark?
Is there a side which we (on Earth) never see?
M
M
Earth
M
M
Earth’s Atmosphere
•Protects the surface
•Regulates temperature
•nitrogen (78%)
•oxygen (21%)
•argon (0.9%)
•carbon dioxide (0.03%)
Ozone
absorbs UV
light
Convection
occurs here
Convection
Warmer air travels up and cooler air comes
down to take its place
•Results in convection cells which heat the atmosphere
•creates surface winds and is responsible for most types of weather
The Greenhouse Effect
•Sunlight not reflected by clouds reaches the Earths surface (about 50%)
•The heated earth re-radiates this light in the form of infrared radiation
•Infrared light is partially blocked by the Earth’s carbon dioxide (and water vapor)
•So, only part of the IR light goes into space, part goes back to earth
The average Earth
temperature is about
40K hotter because
of greenhouse effect
than it would be
without it! Why is
this important?
Why does Earth retain its atmosphere
(and the moon has none)?
Gravity
•Gas molecules are in constant motion - hotter gas, faster motion
•The fact that the atmosphere is heated keeps it from falling onto Earth
•Escape speed is the speed an object (in this case, a molecule) must travel
to escape another object’s (Earth, the Moon) surface
•If a planet’s escape speed is at least 6 times greater than the molecules’
velocity, molecules of that type will not “escape” in significant quantities
Earth’s escape speed = 11.2 km/s
Oxygen and nitrogen molecular speed = 0.6 km/s
Moon escape speed = 2.4 km/s
Seismology
•Earth’s interior structure is probed by studying how waves
travel through it (we can only drill so far! - 10km)
•Earthquakes generate seismic waves
•Certain types of waves reflect off different materials and travel
through these materials at different speeds (higher density faster)
Mantle - 3000 km thick (80% of planet volume)
Crust - 15 km thick (8 km under ocean - 20-50
km under continents)
Density and temperature increase with depth
High central density suggests the core is mostly
nickel and iron
Density “jumps” between mantle and core –
sudden change in material
No jump in density between inner and outer
core - material is the same and just goes from
liquid to solid
Evolution of the Solid Earth
•Accretion- material comes together to
make the planet 4.6 Billion years ago
(age of Sun). Earth was bombarded by
interplanetary debris which made it hot.
•Differentiation - different densities and
compositions to the earth - Earth was
molten, allowing higher-density
material to sink to the core (this core
material still has temperatures like that
of the Sun!)
•Crustal Formation - cooling and
thickening of crust about 3.7 Billion
years ago
The Surface of the Earth
•The Earth is still active today: earthquakes, volcanoes…
•Sites of activity outline surface plates - plate tectonics
•Continental drift - few cm/year
•Plates collide head on (mountains) or shear past (earthquakes)
•Some plates are separating (under Atlantic) - new mantle material
wells up between them
Sites of
earthquakes or
volcanoes in the
past 100 years
What causes the forces that move the plates?
Each plate is crust+mantle
Warm mantle rock (softened by high temps) moves up cools - moves down
Convection!
Lunar Surface - lack of atmosphere and water preserves surface features
Maria – mantle material
•“seas” - darker areas resulting
from earlier lava flow
•Basaltic, iron rich, high density
(3300 kg/m3).
Highlands – crust material
•elevated many km above maria
•Aluminum rich, low density
(2900 kg/m3).
Craters
Caused by meteoroid impacts
• pressures to the lunar surface heats the rock and
deforms the ground
• explosion pushes rock layers up and out
•The ejecta blanket surrounds the crater
A 1-kg object hitting the Moon’s surface
at 10 km/s would release as much energy
as the detonation of 10 kg of TNT!
•Craters can be up to 100km in diameter
•A new 10km crater is formed every 10 million years
•A new 1m crater is formed each month
•A new 1cm crater is formed every few minutes!
The rate of cratering on the moon is determined from the known
ages of the highland and maria regions.
The Moon (and solar system?) experienced a sharp drop in the
rate of meteoritic bombardment ~ 3.9 billion years ago.
The rate of cratering has been roughly constant since that time.
What happened then?
The end of the accretion process
The Earth’s Magnetosphere – space influenced by Earth’s magnetic field
•Magnetic field lines run from the south to north magnetic poles
•Magnetic poles are close to (but not the same as) the axis poles
•The field is distorted by the solar wind
The Van Allen belts make up the inner magnetosphere
•2 belts at 3000km and 20000km above Earth
•Highly charged particles (electrons and protons) from the solar
wind become trapped by the magnetic field lines
Aurora Borealis
Northern Lights – caused
when the charged particles
escape the magnetic field and
collide with Earth’s
atmosphere near the poles
What causes the Earth’s magnetic field?
The rotation of the planet coupled with the electrically
conducting liquid metal core = dynamo effect
Formation of the Moon - theories
1. The moon was a sister planet - formed together with Earth
But, the moon has a very different chemical composition
2. The moon formed somewhere else and was captured
But, the moon’s mantle is too similar to the Earth
Impact Theory
•Mars-sized body hit
the molten Earth
•Parts of the mantle
blew off and later
formed the moon
•Earth had already
differentiated so little
iron was in the mantle