The Earth as a Planet

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The Earth as a Planet
Basic Properties (size, density,
composition) Interior structure Interior
dynamics Atmosphere Formation and Evolution
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How to find the basic parameters?
We find that the bulk density of the Earth is
much higher than that of surface rocks. It must
have something denser inside.
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Why is the Earth still hot?
The heat was generated by the violent
accumulation of planetesimals which created the
Earth. Further heat is added by decay of
radioactive elements. The main source is the
crystallization of our iron core from liquid to
solid. The heat has trouble getting out because
of the large volume to surface area ratio.
Smaller planets cool faster.
Heat is carried out from the interior by
convection. Hot rock rises up to the surface
where it radiates its energy and cools, sinking
back down. This causes the dynamics on our crust.
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Structure of the Earths Interior
We can use seismic waves generated by earthquakes
to see inside the Earth. We find that it has a
liquid iron/nickel core, surrounded by a
plastic mantle. The crust of solid rock is
thinner than an eggshell in relative terms. Thus,
the Earth is still basically a molten ball.
The denser stuff is in the middle because it sunk
there differentiation.
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Motions of the Earths crust
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The Earths Atmosphere
Our atmosphere is about 80 molecular nitrogen
and 20 molecular oxygen, with water and carbon
dioxide in small amounts. The temperature changes
direction several times as you go up.
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The Greenhouse Effect

1. In equilibrium, a planet must re-radiate all the
   energy it absorbs.
2. Solar energy tends to be converted from visible
   to infrared radiation.
3. Some gases are transparent to visible radiation,
   but opaque to infrared radiation.
4. When radiation is blocked, you need a bigger
   temperature gradient to push the energy through.

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Carbon Dioxide is a critical factor
Without subduction and water, the atmosphere and
climate on the Earth would have been very
different. A little more conversion of CO2 to
rock, or lack of subduction and vulcanism, and we
would have snowball Earth. Less conversion
could make us more like Venus (human greenhouse
gases not really enough for that). Life
has played an important role by using CO2 in
plant respiration and production of shells which
sink to the ocean bottom.
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Carbon Dioxide is Increasing due to Humans
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The Ozone Hole is Different (and less critical)
Ozone blocks solar UV, which causes tans and skin
cancer. The hole is due to CFCs (a chemical)
which is now being controlled. The atmosphere
should recover in about 50 years (we hope).
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The Origin and History of the Moon
The surface of the Moon has craters, highlands,
and maria (dark lava basins). Its composition
is similar to that of the Earths mantle, with
little or no iron core.
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Craters on the Moon
The craters on the Moon are almost all impact
craters. They tell of an early violent history,
one which is also recorded around the rest of the
Solar System. The first half-billion years were
filled with flying debris (some of it
planet-sized).
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Getting Ages from Half-lives
If you know the starting composition of a
material, and you know the time it takes for half
of it to change into a decay product, you can
figure out how old it is by measuring the ratio
of original material to decay product.
Start
1 half-life
2 half-lives
3 half-lives
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The Origin of the Moon A Stupendous Impact
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Tides
There are 2 high and 2 low tides every day.
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Why Tides Happen
Tides are caused by the difference in the pull of
gravity from one body on the near and far side of
another (1/r3). They are important in all kinds
of astronomical contexts. They cause the Earth to
slow down and the Moon to recede.
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Phases of the Moon
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Where to find the Moon in the Sky
The angle between the Sun and the Moon determines
its phase.
A crescent Moon must be near the Sun A half Moon
(first or last quarter) must be overhead when the
Sun is rising or setting The full Moon must rise
when the Sun sets, and be on the meridian near
midnight The waning Moon will tend to be seen in
the daytime
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Phase Movie
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Lunar Eclipses
A lunar eclipse occurs when the Moon goes into
the Earths shadow. Everyone who can see the Moon
then sees the eclipse. It looks red because
sunlight is refracted through our air.
The Moon must be full.
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Solar Eclipses
Solar eclipses happen when the Moon passes
between us and the Sun. The eclipse can only be
seen where the alignment is perfect. The sky goes
dark, the stars come out, and the solar corona
becomes visible. It only lasts a few minutes in
any given spot. Although there are 1 or 2 per
year, any given place has a long wait to see one.
The Moon must be new.
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ShadowDance
The Moon just happens to have about the same
angular size as the Sun. This is a coincidence of
the present, the distant past and future will not
get perfect solar eclipses because the Moons
distance is growing.
Eclipses usually dont happen, because the Moons
orbit is tilted.
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Astro Quiz
Under which condition below would there be a
total solar eclipse every month?

1. The moons orbit crosses the ecliptic at 12
   points.
2. The moons orbit appears to move along the
   ecliptic.
3. The moons orbit is perpendicular to the
   celestial equator.