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Earth
Astronomy 311
Professor Lee Carkner
Lecture 12
Terra -- The Earth Goddess

We use the old
English names for
the Earth, Sun and
Moon, rather than
the Latin ones
Earth Facts
Size: 12700 km diameter

Orbit: 1 AU (1.5 X 108 km)
Description: wet, temperate, inhabited
Earth’s Atmosphere
Composition:

21% O2

Very different from other atmospheres

Outer planets: mostly H2
Early Atmosphere
Where did the original atmosphere
come from?

Bombardment of icy planetesimals
(comets)
Early composition:

Water (H2O)

Ammonia (NH3)
Formation of Atmosphere

Ultraviolet light breaks up some
molecules (four key elements: C,H,O,N)

CO2 dissolves in H2O, H2O rains out to
form oceans, N2 left behind
Life (plants) forms and produces O2
Final atmosphere -- O2 and N2
The Habitable Zone
In order to support life a planet must be in the
habitable zone

Width of zone determined by the effectiveness of
the carbonate-silicate cycle
Inner Edge -- where water is lost by UV dissociation
Outer Edge --

Note: planet needs to be large enough to have
atmosphere and plate tectonics
Too hot,
water is destroyed
can’t remove CO2
Too cold,
try to warm up
with more CO2
but CO2 forms
clouds and blocks
sunlight
Just right,
temperature
kept stable
at ~273 K
(water is liquid)
Hypothetical Habitable Zone
Planet Temperature
Can estimate a planet’s temperature by
radiation balance:

Emit radiation back into space

Note: this assumes the planet is just a big
uniform ball with no atmosphere or climate

Remember 0 K is absolute zero and 300 K is about
room temperature

TF = 1.8TK - 460
Radiative Power
Hot objects absorb and emit energy
based on the Stefan-Boltzmann law
P = sAT4
Where:

s is a constant = 5.67 X 10-8 (W/(m2 K4))

for a sphere A = 4pr2
T is the temperature of the object (in K)
Energy Balance
If we compare the energy the Earth
emits with the energy it gets from the
Sun, we can find its temperature
TE = [RS/(2 DS)]½ TS
Where:
TE
TS is the temperature of the Sun
RS
DS is the distance from the Earth to the Sun
Plate Tectonics
The two top layers of the Earth are the crust and
the mantle

Crust is hard and rigid

Plates move around and crash into each other
forming trenches and mountains

Plate tectonics and water resurface the Earth
Most other planets resurfaced by volcanism and
cratering
How Plate Tectonics Work
Plate Boundaries
Plate Collision -- The Himalayas
Seismic Waves
Types of waves:
P waves: pressure or compression wave
example:
S waves: shear waves
example:
The different densities of the inner
earth refract the waves

When an earthquake occurs we can
measure the strength of S and P waves
all over the Earth
Earthquake Studies of the
Earth’s Interior
Seismic Waves and the Earth’s
Interior
No S waves detected on opposite side of
Earth

Core must be liquid
There is a shadow zone where no P or S
waves are detected

Very faint P waves detected in shadow zone
Refracted by solid inner core
Structure of the Earth
Structure of the Earth
Crust:
surface to 35 km

Outer core
2900-5100 km

Mantle
35-2900 km
composed of silicates
and heavier material

Inner core
5100-6400 km
composed of solid
iron
Next Time
Read Chapter 8
Just the moon parts
Summary
Earth is unique for at least two reasons
Large amounts of liquid water
constantly reshapes the surface
Large amounts of free oxygen
produced by life
Earth has liquid water and life because
it is in the habitable zone
Summary: Atmosphere
Earth’s initial atmosphere composed of
CHON
H and O form water -- oceans
C and O form carbon dioxide -- rock
N stays in atmosphere
Plants produce oxygen
Mild temperature maintained by
carbonate-silicate cycle
Summary: Surface
Solid iron inner core, liquid iron outer
core, solid mantle and crust
Crust is broken up into plates which
slide around on the upper mantle
Plate tectonics and erosion constantly
alter surface