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Planetary Atmospheres
What has the Most Craters?
Evolution of Terrestrial Planets
After the condensation and accretion phases of planet formation,
terrestrial bodies can go through 4 different stages of evolution.
(The rates of evolution can vary greatly.)
 Differentiation – in a molten planet, heavy materials sink
 Cratering – left over bodies impact the planet’s surface
 Flooding – water, lava, and gases trapped inside the
planetcome to the surface and cover the terrain.
 Erosion – surface features are destroyed due to running
water, atmosphere, plate tectonics, and geologic motion.
What has the Most Craters?
Mercury
a) Mercury
Venus
Earth
Mars
Why is the Earth’s Core Hot?
Radioactivity
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Contraction
Accretion
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c) Heat from radioactivity
What is the Density of Kuiper Belt Objects?
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b) similar to water.
Which Heating Source Doesn’t Decline?
Accretion
Solar
Contraction
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Radioactivity
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c) Heating by the Sun
At What Wavelength Can You Observe
Planet Formation?
d) in the infrared part of the spectrum
The Atmospheres of the Solar System
Planet
Mercury
Venus
Earth
Mars
Titan
Jupiter, Saturn, Uranus, Neptune
Atmosphere
none
thick CO2
N2, O2, [CO2]
thin CO2
N2, CH4, NH3
H2, He, CH4, NH3
The Atmosphere of Jovian Planets
Jovian planets are similar to the Sun. Due to their smaller
mass, their central pressures and temperatures are not great
enough to fuse hydrogen. They are thus cooler, so hydrogen
can react with other atoms to form molecules.
H + H  H2
4H + C  CH4 3H + N  NH3
Since the inside of Jupiter is hot (due to the pressure), while the
cloud tops are cool, the composition of the atmosphere changes
with depth.
The fast rotation rate of the Jovian planets also drives strong
currents and storms, similar to the trade winds and hurricanes
on Earth.
The Structure of Jupiter’s Atmosphere
The inside of Jupiter is
extremely hot and, in fact,
Jupiter shines (in the
infrared) by gravitational
contraction.
Jupiter’s Trade Winds
Jupiter’s equator is moving faster than the poles (it has farther
to go in a day). This drives a network of very strong winds and
storms.
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Formation of an Atmosphere
When a terrestrial planet enters its flooding stage, gases
trapped inside the during formation (or created as a result of
radioactive decay) will be outgassed. These gases include
 H2, He, H2O, N2, CO2, and probably CH4 and NH3
As the planet cools, water vapor condenses out of the
atmosphere and falls as rain. Oceans form. But will the
planet be able to keep this atmosphere?
Temperature versus Gravity
Escape velocity: the speed a particle must have to escape the
gravity of a body and not come back
Temperature:
the average kinetic energy of an atom or
molecule
The kinetic energy of an
atom or molecule depends
both on its speed, and its
mass. Light particles
move quickly; heavy
particles move slowly.
It’s easier for a body to hold onto heavy gases than light gases.
The Masses of Gases
Atom # protons # neutrons Total weight
H
He
C
N
O
1
2
6
7
8
0
2
6
7
8
1
4
12
14
16
Molecule
Weight
Molecule
Weight
H2
CH4
N2
CO2
2
16
28
44
He
NH3
O2
4
17
32
Mercury versus Titan
Mercury and Titan are both low-mass bodies. But …
 Mercury is close to the Sun, so it is hot. Its gravity is not
strong enough to keep its gases from escaping into space.
 Titan is in the outer solar system and is cold. The
molecules are moving slowly, so the moon can hang onto
its atmosphere (except for the lightest gases of H2 and He).
Titan’s Lakes
The atmospheric pressure at the surface of Titan is about
twice that on Earth. In fact, Titan’s surface is not all solid
-- it has lakes (of liquid methane and ammonia).
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The Atmosphere of Mars
The composition of Mars’ atmosphere is determined by
 The mass of the planet. Since Mars is only about 0.1 M,
it does not have the gravity to hold onto H2 and He. It
can barely hold onto N2.
 Proximity to the Sun. Gases such as CH4 and NH3 are
destroyed by ultraviolet light. Mars’ atmosphere is not
thick enough to shield itself from ultraviolet photons.
 Chemistry. Oxygen (O2) reacts with almost anything
(i.e., minerals in rocks), so it cannot stay free.
Consequently, Mars’ atmosphere is primarily CO2 with a little
bit of N2.
Carbon-Dioxide and Mars
Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes
seasons, just like the Earth. In winter at the pole, CO2 freezes
out and becomes dry ice. In summer, this ice evaporates and
becomes part of the atmosphere. This cycle produces strong
winds and dust storms.
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Carbon-Dioxide and Mars
Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes
seasons, just like the Earth. In winter at the pole, CO2 freezes
out and becomes dry ice. In summer, this ice evaporates and
becomes part of the atmosphere. This cycle produces strong
winds and dust storms.
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Venus and Earth
The similarities:
 The planets have similar masses (0.82 M versus 1.0 M)
 The planets have similar compositions (density 4.2 vs. 5.5)
 The planets’ distances from the Sun are similar (0.72 A.U.
versus 1.0 A.U.)
 Neither planet can hold onto light gases (H2 and He)
 Neither planet can keep large amounts of CH4 and NH3 in
its atmosphere (due to ultraviolet light from the Sun)
The main difference:
 The Earth’s temperature is between 50° C and +50° C,
while Venus’ temperature is +470° C
Properties of Carbon-Dioxide
CO2 has two interesting properties:
 CO2 dissolves into liquid water (H2O) to create H2CO3
(carbonic acid). Carbonic acid (i.e., the fizz in soda)
then reacts with any number of minerals. For instance
H2CO3 + Ca  H2 + CaCO3 (limestone)
The result is that, if liquid water is around, CO2 will be
removed from the air, and locked up in rocks.
 CO2 is a greenhouse gas. It is transparent to optical
light, but it absorbs infrared light. So sunlight can make
it through CO2, but the heat it brings cannot get out.
Runaway Greenhouse Effect
Venus and Earth both started out with similar atmospheres. But
because Venus is slightly closer to the Sun …
 Venus was a bit warmer, and had a bit less liquid water
 With less liquid water, less CO2 dissolved away
 With more CO2 in the atmosphere, the greenhouse effect
was more effective
 The warmer temperature caused more water to evaporate
 With even less liquid water, even less CO2 dissolved away
 As all the water evaporated and the temperature increased,
outgassing of greenhouse gases (CO2 and CH4) became
easier. CO2 was “baked out” of the rocks
 Ultraviolet light destroyed the CH4, NH3, and H2O in the
atmosphere, leaving a thick atmosphere of CO2
The Atmosphere of Earth
Venus and Earth both started out with similar atmospheres. But
because the Earth is slightly farther away from the Sun …
 Earth was a bit cooler, and had a bit more liquid water
 With more liquid water, more CO2 dissolved away
 With less CO2 in the atmosphere, the greenhouse effect was
less effective
 With more liquid water and a comfortable environment,
photosynthetic life developed
 Photosynthesis removed even more CO2 from the
atmosphere, replacing it with O2. (When dinosaurs lived,
there was 5 times more CO2 in the air!)
 Lightning plus atmospheric O2 created ozone, which
shielded the Earth from ultraviolet light. Water molecules
in the atmosphere survived longer (along with life)
Today on Earth and Venus
A small change in the conditions now can lead to large
changes later on!
There is a Tide …
The Earth-Moon System
Tides are the difference in
gravity from one side of a
body to the other. On
Earth, the tides draw the
water out towards/away
from the Moon.
But the Earth is constantly
rotating, pulling the tidal
bulge out of alignment. As
a result, the water is
continually moving in the
opposite direction of the
Earth’s rotation.
Tidal Friction
The movement of the water on Earth has two effects:
 It slows down the Earth’s rotation. When dinosaurs
roamed the Earth, a day was 22 hours long.
 It pulls the Moon along a bit faster, slinging it out further
from the Earth.
Tides on the Moon
The movement of the water will eventually stop the rotation of
the Earth. But what about the Earth’s tidal force on the Moon?
Since the Earth is about 80 times more massive than the Moon,
its tidal force is 80 times greater. Tidal friction of flowing
rocks (lava) has long since locked the Moon to the Earth.
Jupiter’s Moons
Jupiter is much more massive than the Earth, so the tidal effect
on its moons is much greater. Recall the 4 Galilean satellites …
Moon
Mass
(lunar)
Density
Distance
(water = 1) (1000 km)
Period
(days)
Io
1.21
3.5
422
1.769
Europa
0.65
3.0
671
3.551
Ganymede
2.01
1.9
1071
7.155
Callisto
1.47
1.8
1884
16.689
Io
Io’s density is that of rock. It
has no impact craters no
visible water, and is entirely
molten, except for a thin
crust that is constantly being
resurfaced by volcanism.
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Europa
Europa’s icy crust is thin.
Below the crust is (probably)
liquid water. There are few
(if any) impact craters on
Europa.
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Ganymede
Ganymede has many craters,
but also a network of grooves
that lie on top of the craters.
These are mostly likely caused
by expansion and contraction
of ice layers.
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Callisto
Callisto has an extremely old, icy
surface covered with impact craters.
It is essentially unchanged since the
time it was formed.
Jupiter and the Sun
There is very little difference between Jupiter and a star.
 The composition of Jupiter is similar to that of a star.
 Jupiter formed in a mini-nebula, just like the solar nebula.
 During formation, Jupiter shined by gravitational
contraction, just like a star.
 Jupiter’s luminosity prevented light elements from
condensing on its inner moons, just like the Sun.
The only difference between Jupiter and a star is that Jupiter
hasn’t been able to fuse hydrogen.
Jupiter’s Moons
The four Galilean moons of Jupiter show a range of
properties:
 Io is entirely molten, except for a thin crust. Volcanos
are erupting all the time, covering the surface with
lava.
 Europa is warm enough under its surface to have
liquid water.
 Ganymede has rills and grooves on its surface, as if ice
has been warmed and cooled.
 Callisto is an old, cold moon, with no sign of evolution
since it was formed.
Why the difference?
Jupiter and Tides
The tidal force of Jupiter on its moons is much stronger than the
tides of the Earth-Moon system. These objects should be tidally
locked to Jupiter. But …
 Io, Europa, and Ganymede orbit in a 1:2:4 resonance. Io is
constantly being perturbed by its neighbors.
 Io’s orbit is elliptical – its speed changes during its orbit.
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Io can’t become tidally locked!
Heat and the Moons of Jupiter
As a result of Jupiter’s tides …
 Io is continually stressed by the tides of Jupiter. Its
interior is kept entirely molten.
 Europa feels some tidal stress as well. However, since it
is further away, the stress is less. Europa’s interior is
probably warm enough to melt ice into liquid water.
 Ganymede has been thermally stressed in the past, either
by heat from Jupiter’s gravitational contraction, or by
tides. The grooves in its surface are probably due to ice
expansion and contraction. It is now tidally locked.
 Callisto is far enough away from Jupiter to be thermally
unaffected. It is a cold body.