Chapter 9:
Atmospheres
of the
Terrestrial
Planets
Terrestrial Atmospheres
Only Earth, Venus and Mars have a
substantial atmosphere. The Moon
and Mercury only have traces of
gases around them.
The primary atmosphere
was mostly H and He
H and He were captured
during formation. Since
all the terrestrial worlds
have small masses, their
gravity wasn’t strong
enough to hold on to
these gases and they
escaped to space. Only
massive planets like
Jupiter can hold on to a
primary atmosphere.
Venus, Earth
and Mars are on
nd
their 2
atmosphere
after having lost
their first one
Play with Gas Retention
Simulator on ClassAction
website in Resources Menu
Most secondary atmospheres
come from volcanoes and comets
The gases are mostly
carbon dioxide, carbon
monoxide, sulfur dioxide,
water vapor and nitrogen
The Greenhouse Effect is important
on Venus, Earth and Mars
A balance is established between the incoming energy
and the outgoing energy. Since blackbody radiation
depends on temperature, the balance point depends on
the temperature of the planet
On Venus a runaway greenhouse
effect baked the planet. The
balance point is almost 750 K
The clouds of Venus
give it a very high
albedo (0.65). It’s
temperature would be
below freezing if not
for the greenhouse
effect. The thick
atmosphere of CO2
causes an extreme
greenhouse effect.
The
atmosphere
of Mars is too
thin to have
much of a
greenhouse
effect
Like Venus, it’s mostly
CO2 but it’s so thin
there just isn’t much
gas to absorb infrared
radiation from the
ground
Earth’s
atmosphere
managed to
lock away
almost all
the CO2
If all the CO2 locked up
in Earths’ rocks were
released, Earths’
atmosphere would be
98% CO2. As it is, CO2 makes up less than 0.04% of
Earths’ atmosphere
How did our atmosphere get
this way?
Most of the CO2, CO and SO2 got locked up by the
oceans as rocks like limestone. That left mostly nitrogen
and smaller amounts of CO2. Life created the oxygen.
Earth’s
Atmosphere
is layered
like an
onion
The layers are due to
how the temperature
changes with altitude.
Mars and Venus don’t
show the same kind
of layering.
The way temperature changes
is due to energy transport
In the troposphere
convection is driven by
heat from the ground so
the temperature decreases
with altitude
In the stratosphere and
thermosphere energy is
absorbed directly from
the Sun so the
temperature increases
with altitude
Beyond the atmosphere,
Earth has a Magnetosphere
The
magnetosphere
shields the
Earth’s
atmosphere
from the solar
wind.
When particles trapped in the
magnetic field collide with the
upper atmosphere we get auroras
The weak or non-existent magnetic
fields of Venus and Mars lead to
strong erosion by the solar winds
The stronger gravity of Venus has been able to hold on
to its atmosphere but Mars lost most of its atmosphere
to erosion by the solar wind.
Just as in the interior of the
planets, convection is
important to atmospheres
Rotation and the Coriolis effect are
also important
Convection in the upper
atmosphere is influenced by the
Coriolis Effect
Winds in the
upper
atmosphere
of Venus
also show
strong
convection
Despite its thin atmosphere,
convection on Mars is
important to its global winds
Convection is also important
on a smaller scale
Thunderstorms
are driven by
convection
Static electricity developed
by the convection creates
lightning
Mars also shows convection in
its clouds and storms
Martian Global Dust Storm
Dust Storm Erupting out of
North Pole of Mars
Dust Devils
are small
scale
convection
Dust devils on Mars can be several kilometers tall
Dust devils are found in dry
places on Earth
Weather
on
Venus?
The ESA’s Venus Express is
now taking a close look at the
atmosphere of Venus
Is mankind changing Earth’s
atmosphere?
Man?
Each spring in Antarctica a hole
develops in the ozone layer
The Ozone Hole changes from
year to year
Ozone depletion is caused by
CFC’s
CFC’s are Chlorofluorocarbon molecules which are
man made compounds. They do not occur naturally
The solution: stop using CFC’s
The 1987 Montreal Treaty gradually reduces the
production and consumption of CFC’s worldwide
Is the Ozone Hole getting
worse or better?
Our actions have led to an
increase in the level of a
number of greenhouse gases
Unfortunately,
we can’t stop
producing
these gases
as easily as
we stopped
producing
CFC’s
Deforestation removes the
trees that remove CO2
The cut trees are
usually burned which
adds more CO2 to the
atmosphere
There is a direct relationship
between the level of CO2 and the
global average temperature
Our actions may be keeping us
from another ice age
All climate models predict that
it will get warmer
We are performing an experiment
on our atmosphere. What the
outcome will be we don’t yet know