Planetary
Atmospheres
Chapter 10
Atmospheric basics
• What is an atmosphere
An atmosphere is a layer of gas that surrounds a planet or satellite.
It can be very thin. In the case of the Earth, 2/3 of the atmosphere are within
the first 10 km.
An atmosphere is composed of several gases and molecules. The gases
present in the terrestrial atmosphere are molecules such as N₂, O₂, H₂O
and CO₂
• Atmospheric pressure
The molecules and atoms have kinetic energy. They move fast and collide
with atoms and creates pressure. At room temperature oxygen and
nitrogen have speeds of 500 m/s. Billions of collisions take place in a few
seconds. This collisions creates pressure in all directions.
Atmospheric basics
• Where does the atmosphere end?
If we increase the altitude in the atmosphere, the pressure decrease. The
density of the atmosphere decreases and the collision of molecules are
less frequent. Around 60 km, the pressure and density are low and it is
considered the “edge of space”. But it doesn’t mean that there no air
molecules. It is just very thin. Since there is very little air molecules, the
sky looks dark. There is very little scatter of light in the few molecules.
Satellites, and the Space Station (Altitude about 400 km) travel through a very
thin atmosphere that produces drag . Because of the drag, they lose
altitude. They need a boost to increase their altitude or they will enter the
more dense part of the atmosphere and will burn
The atmosphere of the terrestrial planets
Atmospheric basics
• How atmospheres affect planets?
•
•
•
•
It creates pressure. The pressure determine for example if liquid water
can exist on the surface
The atmospheres can scatter and absorb light. Scatter of light produces
daytime bright skies! It absorb high energy radiation (X-rays and Gammarays) and protect and make possible life.
Atmosphere produces wind and create weather.
Atmosphere can make the surface temperature warmer through presence
of greenhouse gasses.
The greenhouse effect
• The energy received from the Sun by the Earth, a planet or
body in part is absorbed by the planet or it is reflected back
into space.
• The surface absorb the energy and warm up. Since the surface
has low temperature, it emit at IR wavelengths.
• Some gases present in the atmosphere can absorb the IR
radiation, trapping it. These are called the greenhouse gases.
Some of these gases are CO₂, H₂O and methane (CH₄). Their
molecules rotates and vibrates when they absorb IR photons.
But they don’t keep the energy. They reemit another IR
photon which can be absorbed by the surface or other
molecules. It slow the escape of the IR photons.
Surface Temperature
• What determines a planet’s surface temperature?
R
Incident
energy
Reflected
energy
Sun
Energy re-radiated
from warm surface
Absorbed energy
warms surface
On the Earth, without atmosphere the average temperature ~ -18 C
With the atmosphere the average temperature ~ 15 C
WHY?
(Water and CO₂ are opaque to infrared radiation  trap heat)
Albedo
•
•
•
•
The fraction of energy reflected (not absorbed) by the surface is called the
albedo A (0<A<1).
The Albedo is the ratio: energy reflected/energy received
An albedo of 0 means no energy is reflected, all energy is absorved. An
albedo of 1 means all energy is reflected, no energy is absorbed.
This is also called reflectivity. A planet with an albedo of 1 will look bright
Data from NASA's Terra and Aqua satellites
Areas colored red show the brightest, most reflective regions (high albedo); yellows and
greens are intermediate values; and blues and violets show relatively dark surfaces (low
albedo).
The greenhouse effect
The greenhouse effect in the terrestrial planets
Earth Atmosphere
• Composition
– 78 % Nitrogen, 21 % Oxygen, 1 % Argon, 0.03 % CO2, water
• Atmosphere protects the surface by blocking
– UV radiation from Sun
– cosmic rays and radiation
– small impacts
• Regulates surface temperature
The structure of the Earth’s atmosphere
Effect of light at different wavelenghts when it
strikes gases in the atmosphere
Atmosphere light scatter
Temperature profile terrestrial planets
Venus (-40 C)
Earth (-16 C)
The arrows show the
temperatures if there were no
greenhouse effect
Earth Magnetic Field
• Remember…The Earth’s inner core is
solid, while the outer core is liquid.
• Earth’s core is made up of metals
(Iron & Nickel)
• In metals, electrons (negative
charge particles) move freely
• Magnetism is caused by moving
charges.
• The Earth inner core spins almost
frinctionless within the liquid outer
core creating a magnetic field
• It spins faster than the rest of the
Earth!
The Giant Magnet
The terrestrial magnetic field the solar wind and
the aurora
Earth Magnetic Field
• Earth’s magnetic field protect us from cosmic rays
Auroras
• Some charged particles from the solar wind get trapped in the Earth’s
magnetic field lines, they spiral toward the magnetic poles where they
precipitate hitting the gasses the Earth atmosphere and releasing
energy. The atoms of the atmosphere are excited and emit light
Weather and climate
• Weather refers to a combination of winds, clouds,
temperature and pressure. It characterize the
conditions that make days hotter, colder, warmer.
• Climate is the long term average of the weather
conditions. It characterize the conditions that prevail
over long periods of time in a location.
• Wind, rain, temperature, clouds, pressure depends
on the energy in the atmosphere
Earth wind patterns
Circulation on the Earth atmosphere
Atmospheric heating creates circulation cell
The height of the cell is not to scale.
Wind circulation in N and S hemispheres
Coriolis effect
Water cycle in the Earth atmosphere and surface
Major factors affecting long term climate changes
Changes in axis tilt
A large tilt produces extreme season.
A small tilt keep the polar regions colder and the
equatorial regions warmer
Changes in greenhouse gases
abundance
An increase of gasses retains more IR
radiation and increases the temperature. A
decrease has a cooling effect
Melting, evaporating and sublimating
• Melting : the transition from solid to liquid
• Evaporating: the transition from liquid to gas
• Sublimating: the transition from solid to gas without
going through the fluid phase. An example is carbon
dioxide (dry ice) at normal sea level pressure and
temperature. Example: Dry ice goes from solid at -78
degrees C to gas at 20 degrees C.
Where do planetary atmospheres
come from?
• Three primary sources
– Primordial (solar nebula: H and He)
– Outgassing (trapped gases)
– Later delivery (mostly comets)
Terrestrial planet atmospheres are not primordial
Why not?
Gas loss (lighter elements  higher velocity  velocity
higher than escape velocity)
How does a planet gain atmospheric
gases?
•
-
Gain and the sources of atmospheric gases
Outgassing
Evaporation and sublimation
Surface ejection
How an atmosphere gain gases
How does a planet lose atmospheric
gases?
•
-
Losses of atmospheric gas
Condensation
Chemical reactions
Solar wind stripping
Thermal escape
How an atmosphere loses gases
The atmosphere of Mercury and the Moon
•
•
•
•
•
•
•
Mercury and the Moon have a very, very low density of gases above their
surface. Nothing that can be considered an atmosphere, at least not a
permanent atmosphere.
They may have had some gases released by volcanic outgassing in the
past.
At the present there is no volcanic activity in any of them
Because of Mercury high surface temperature, it lost those gases fast
The Moon has lower temperature but its low mass cannot retain gases
and they escape fast
The only ongoing source of gas is surface ejection when meteorites, solar
wind or high energy photons hit the surface and release atoms and
molecules from it surface.
These atoms and molecules are ejected at high speed, fast enough to
reach the escape velocity of these two bodies
Important Concepts
Why some planets or satellites retain an atmosphere and other
don’t?
• Surface gravity: Strength of the gravitational force at
the planet surface. This determine the escape
velocity
• Escape velocity: Velocity required for an object to
escape the gravitational pull of another
• The thermal velocity of the gas molecules
The equations for escape velocity and molecular
speed
Vesc 11.2 M / R
Vesc = Escape velocity in km/s
M = Mass of body in Earth masses
R = Radius of body in Earth radius
Vmol 0.157 T / W
Vmol = Speed of molecules in km/s
T= Temperature in K
W = Molecular mass in Hydrogen atom masses
Thermal escape of gases from an atmosphere
(Mathematical Insight 10.2)
The case of the Moon as an example. T = 400 K, mean thermal velocity = 0.5
km/s of sodium atoms, Escape velocity = 2.4 km/s
V thermal is the peak thermal velocity
T is temperature in K
m is the mass of a single atom
k is the Boltzmann’s constant
The seasons in Mars
The elliptical orbit of Mars make seasons more extreme
Axial tilt = 25.2 degrees
Mars polar caps
Recent evidence of Mars frozen water
Phoenix lander robotic arm carved a trench and uncovered white material.
This material melted or sublimated in a few days: Water ice
A high resolution image of a Mars runoff channel
Evidence of flow of liquid water in the past
Water on Mars
•
•
•
•
•
•
Except for the flow down the slopes of craters (gullies), there is no other
evidence of liquid water on Mars at the present. There is evidence of
frozen water in the polar caps and under the ground surrounding the
poles
The Mars Reconnaissance Orbiter using radar has been able to detect
water ice in layer surrounding the polar regions
The Phoenix Lander landed near the polar regions and was able to detect
water ice in the ground under and around the landing site where the top
soil was blown away by the landing rockets
Salty water may have melted and run down (gullies) the slopes of some
craters. This may be attributed to sublimation of dry ice (frozen carbon
dioxide)
There is plenty of evidence of flow of liquid water in the past. Runoff and
outflow channel are found in many places.
Liquid water and possibly rain may have existed about 3 billion years ago
when Mars was warmer and wetter
Why Mars climate changed
•
•
•
•
•
The conclusion is that Mars had had a warmer and wetter climate in the
past (3 billions years ago). The temperature and pressure was enough to
allow liquid water to be stable and even forming lakes or oceans
Calculations show that enough carbon dioxide may have existed so the
density of the atmosphere could have been 400 times larger than now.
Under those conditions, liquid water may have been stable and be able to
form oceans hundreds of meter deep
The extra carbon dioxide provided a bigger warming effect but the Sun
was dimmer at that time so probably some other greenhouse gases were
present in the atmosphere may have contributed to the greenhouse effect
The big question: What happened to the atmosphere that lost most of its
gases?
The lost of carbon dioxide and the connection
with the lost of magnetic field
•
•
•
•
•
•
The lost of the carbon dioxide may have a close link to the lost of the
global magnetic field
Mars may have had a stronger global magnetic field.
Its core was melted and had convection. Because it has a rapid rotation,
these two components created a magnetic field
Because Mars is smaller, it cool off faster and the convection in the liquid
core ceased. Without that, the magnetic field weakened or disappeared.
Without magnetic field, Mars atmosphere lost the protection
The solar wind was able to strike the atmosphere and surface blowing
away into space the carbon dioxide or any other gases
Dissociation of water molecule in the
atmosphere of Mars
• Another factor that contributed to the lost of water in the
past is that there was no layer of ozone to protect the
atmosphere.
• Under this condition, the UV radiation from the Sun will
dissociate the water molecules into H and O.
• Hydrogen can be lost quickly due to its low mass and the
relatively high thermal velocity (and the low escape velocity
due to the mass of the planet)
• Oxygen can combine with iron to form iron oxide (rust) which
is what give now the reddish color to the surface
Mars magnetic field and retention of atmosphere: Early
Mars and Mars today
A summary about the atmosphere of Mars
•
•
•
•
•
•
•
The size and low mass of Mars are the reason for the present state of its
atmosphere.
It was big enough to have volcanism which provided outgassing and
release plenty of water and carbon dioxide and also to have global
magnetic field.
But it was small, cool off fast and was not able to maintain the internal
source of heat.
Once it cool off, the volcanic activity ceased and the magnetic
disappeared.
The relatively weak gravity and the solar wind caused the existing gas to
be tripped away to space
About 3 billions years ago it had most of the condition for life to exist:
liquid water, warm temperatures and thicker atmosphere. But it turned
into a frozen and desert planet.
Any life may be extinct now or some may be hidden underground where
temperatures due to some remaining volcanic activity can make liquid
water to exist
The atmosphere of Venus
•
•
•
•
•
•
Even if the size and mass of Venus are similar, their atmospheres differs
radically
The high temperature (740 K, 878 F) and the high pressure (90 times
higher than that on Earth) make life practically impossible.
The atmosphere is entirely carbon dioxide with no more than traces of
other gases.
There are no seasons. The rotational axis is tilted close to 180 degrees
respect to the orbital plane.
The atmospheric circulation (only one cell due to low Coriolis force) makes
the poles temperatures similar to other regions of the planet
There is no precipitation on the surface. The droplets of sulfuric acid
(H₂SO₄) may precipitate in the upper layers of the atmosphere but they
evaporate before they reach the surface (about 30 km above the surface)
due to the high temperature
A comparison between Earth and Venus
atmosphere
•
•
•
•
•
•
Venus atmosphere has about 200,000 times more carbon dioxide than
Earth’s atmosphere. This causes a strong greenhouse effect.
Both planets had a lot of outgassing due to volcanic activity. Venus lost all
the water gas but keep the carbon dioxide.
The Earth lost most of the carbon dioxide gas and water gas .
The water is now in liquid form. Water condensed and precipitated and
formed ocean
The carbon dioxide transformed into solid. Carbon dioxide dissolved in
water where it goes a chemical reaction and is converted into carbonates
rock (such as limestone)
Venus doesn’t have water. Too hot to retain water. Carbon dioxide cannot
dissolved and it doesn’t have an effective way to react to form carbonates.
Even if it could form carbonates, the carbonates will dissociates due to
high temperatures
A comparison between Earth and Venus
atmosphere
•
•
•
•
The disappearance of water in Venus invokes the process of UV radiation
from the Sun which dissociated the water molecules into H and O. H
escaped into space due to high temperatures. O combined with surface
rocks and part was lost into space by the solar wind.
Venus does not have a magnetic field (why?) that can protect the
atmosphere from solar wind.
An analysis of the H left in the atmosphere reflect a higher concentration
of deuterium, an isotope of H. A small part of water molecules contain
deuterium. Deuterium is twice heavier than H (it has a neutron) and was
not able to escape. The concentration of deuterium is a hundred times
higher in Venus than on Earth. This support the idea of the lost of water if
the water was in the form of gas.
But an interesting question is why water did not condense into liquid like it
did on the Earth?
The greenhouse effect, a runaway process
•
•
•
The positive feedback effect. A change in one parameter or property will produce a
change in other parameters or properties of a system that amplifies the behavior
of those parameters and increases more the change in the original parameter.
Let’s consider the example of increasing the temperature of the Earth, more water
in the oceans will evaporate increasing the water gas in the atmosphere. But water
gas will trap more IR emission rising the temperature. The increase in temperature
will cause more evaporation in the oceans, increasing the water vapor content
which will trap even more IR radiation. This will continue until all the water will
evaporate. This will produce a runaway greenhouse effect. If the temperature
reach a high values, the carbon dioxide trapped in rocks may be released,
increasing the greenhouse gas.
In the case of Venus, with no magnetic field, the water will dissociate and the H
will be lost. No possibility to reform water anymore and only the carbon dioxide
will be left!
The Earth’s atmosphere
•
•
•
•
•
Earth was able to retain water because the temperature was low enough to allow
water to condense. It may have had oceans 4.3-4.4 billions years ago
Carbon dioxide dissolved in the oceans and a chemical reaction turned into
carbonates. The carbonates rock contains thousands more times carbon dioxide
than the atmosphere.
The high content of nitrogen in the terrestrial atmosphere can be explained by the
removal of the carbon dioxide and the water condensing into liquid leaving N as
the main component.
Oxygen is not part of the gases released by outgassing. It came from plants and
microorganism that remove carbon dioxide and release oxygen as part of the
photosynthesis. It may have taking a billion years to build the oxygen. Finally the
oxygen content may have reached a level that allow us to breath in the last few
hundred million years
The oxygen that reached the upper part of the atmosphere is dissociated by UV
emission and forms ozone (ozone molecule is formed by three oxygen atoms).
Ozone absorb UV radiation
Origin of oxygen and ozone in Earth’s
atmosphere
The carbon dioxide cycle
•
•
•
•
The Earth temperature changes over time. Earth had gone through ice
ages and warm periods. The radiation of the Sun has been increasing but it
also had some period of low solar activity which created some cold winter
(Maunder minimum, 1645-1715)
But Earth is able to maintain a temperature high enough to keep liquid
water.
It appears like the greenhouse effect is somehow self regulated.
This is accomplished by the carbon dioxide cycle which is illustrated in the
following figure.
The Earth’s carbon dioxide cycle
The carbon dioxide cycle
The carbon dioxide cycle and the Earth
“snowball” effect
How is human activity changing the Earth?
• Global warming
- The greenhouse effect
- Burning fossil fuel is increasing the greenhouse gases. The concentration
of carbon dioxide is about 30% higher than in the last million years
- Climate models incorporating the effect of humans seems to match the
observed trend
• The ozone layer
- A groups of chemicals known as CFCs, choroflourocarbons are very
efficient to combine with the Ozone molecule (O₃), destroying the ozone
molecule. CFC were used in A/C, refrigerators
Earth temperature and CO₂ changes in the last 400,000 years
Rapid increase in the last 50 years
Terrestrial global warming in the last thousand year
Increase of carbon dioxide and temperature
Earth temperature changes
Comparison of models with and without human increase in greenhouse gases
The growing Ozone hole
•
•
•
•
•
•
A groups of chemicals known as CFCs, choroflourocarbons are very
efficient to combine with the Ozone molecule, destroying the ozone
molecule.
The CFC molecule dissociate in the upper atmosphere by solar radiation
and releases chlorine which combine with ozone and forms molecular
oxygen
The chlorine acts as a catalyst. It participate in the reaction but it is
released.
CFCs were widely used in A/C, refrigerators, car A/C, propellant in
aerosol cans and other uses.
Substantial cuts in the use and production of CFC’s has improved the
situation.
The ozone hole was discovered around 1980’s over the Antartica.
Evolution - Outgassing
• Second atmosphere from outgassing
– Volcanoes emit CO₂, SO₂, H₂, N₂, water, methane
• Removing the carbon dioxide
– Dissolves in the oceans
– Ends up in rocks
• Formation of N₂ and CO₂
– UV sunlight breaks up methane and ammonia
– Nitrogen from ammonia – CO from methane
Oxygen in the Atmosphere
• Very little primordial oxygen
– Almost no oxygen 2 billion years
ago
– 10 % of present amount 1 billion
years ago
– Sudden increase 600 million
years ago
• Biological activity started 2 billion
years ago
• Plants convert CO₂ into O₂ and trap
carbon