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 their 2nd 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 KThe 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

effectLike 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 Earths’ atmosphere

makes up less than 0.04% of

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

MagnetosphereThe 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

gasesUnfortunately, 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