Lecture 24

Terrestrial planets: atmospheres

Atmospheres are created by three processes:

Outgassing

Evaporation/sublimation

Impacts

All planets probably had minimal (primary. H,He) atmospheres at some point after they formed

How do you keep an atmosphere?

Gravity attracts gas towards the planet. Thermal motions of the gas can cause it to escape. Gravity determines the escape velocity Temperature determines the thermal velocity.

Large planets hold onto atmospheres easier than small planets

Cooler planets hold onto atmospheres easier than hot ones.

Original primary atmospheres (H,He) were swept away from the terrestrial planets early in their life.

Jovian planets still have these primary atmospheres

Present day terrestrial atmospheres are secondary atmospheres, formed primarily by outgassing (mostly carbon dioxide - CO2)

or, in the case of the Earth, a tertiary atmosphere of oxygen and nitrogen (secondary atmosphere modified by life and presence of liquid water)

Which moons have atmospheres?

Jupiter's Io (tenuous sulfur dioxide) Jupiter's Europa (tenuous oxygen) Saturn's largest moon Titan (dense nitrogen &

methane) Neptune's largest moon Triton (tenuous nitrogen &

methane).

Moon and Mercury

“black sky”

The little atmosphere that exists consists of particles of the solar wind that are temporarily trapped.

Mars

Very little atmosphere today (CO2)

Mars had standing and running water on its surface in the past.

Therefore, it must have had a more substantial atmosphere in the past

Does it have water today? Yes - frozen in polar ice caps and possibly beneath its soil

Earth

A moderate atmosphere today

Mostly nitrogen (N2), with some oxygen (O2), carbon dioxide (CO2), etc.

Enough to enable liquid water to exist (temperature and pressure adequate)

Together the air & water produce erosion

Venus

Densest atmosphere of all Terrestrials

Mostly CO2

Temperature at surface hot enough to melt lead

Pressure at the surface ~ 90 times that on Earth

Perpetual cloud cover, sulfuric acid rain

How do we account for these differences?

Composition of outgassed gasses

The dominant gasses arising from outgassing

were carbon dioxide and water vapor, with minor

amounts of nitrogen, sulfer, argon, …

Each terrestrial planet’s outgassed atmosphere

was roughly the same.

How do we account for these differences?

Composition of outgassed gasses

(Carbon dioxide and water vapor)

Temperature appropriate for liquid water?

Distance from the sun, amount of greenhouse

gasses in atmosphere.

Oceans absorb carbon dioxide -> carbonate rocks

How do we account for these differences?

Composition of outgassed gasses

(Carbon dioxide and water vapor)

Temperature appropriate for liquid water?

(Carbon dioxide dissolves in oceans)

Interaction of light and atmospheres

Dissociation of water and ammonia molecules

by UV light, and warming by the greenhouse effect

How do we account for these differences?

Composition of outgassed gasses

(Carbon dioxide and water vapor)

Temperature appropriate for liquid water?

(Carbon dioxide dissolves in oceans)

Interaction of light and atmospheres

(dissociation and the greenhouse effect)

Photosynthetic life - generation of oxygen

• X rays

• ionize atoms & molecules

• dissociate molecules

• absorbed by almost all gases

• Ultraviolet (UV)

• dissociate some molecules

• absorbed well by O3 & H2O

• Visible (V)

• passes right through gases

• some photons are scattered

• Infrared (IR)

• absorbed by greenhouse gases

Interactions of light with atmospheric gasses

Exosphere

hottest layer, v. rarified

Thermosphere

absorbs X-rays, ionized, ionosphere, reflects some radio, aurora

Mesosphere• weakly absorbs UV

Stratosphere• strongly absorbs UV,

ozone (O3), stratified (no convection)

Troposphere• absorbs IR (greenhouse);

convective; weather

The greenhouse effect Planets heat up by absorbing the Sun’s visible light

Planets cool off by radiating infrared out to space

Greenhouse gasses trap infrared radiation in

troposphere (lowest level of atmosphere), heating lower

atmosphere.

greenhouse gasses (e.g., H2O, CO2, CH4 -

methane) transmit visible light but absorb

infrared light

Greenhouse effect raises temperature of lower atmosphere

Greenhouse effect is critical to the existence of life on Earth – it raises temperatures to “habitable” level, permits liquid water

Terrestrial planets: atmospheres Question: Why does Venus have so much more

atmosphere than Earth? The answer is found in what Venus’

atmosphere is made of: CO2 (carbon monoxide)

What happens if there is a lot of CO2 in a planet’s atmosphere?

Due to the large amount of CO2 in its atmosphere, the surface temperature on Venus is over 700 K, instead of the 230 K that it should be at this distance from the Sun.

Does this also explain why Venus has so much atmosphere???

YES!

because water can exist in liquid form On Earth there are oceans

Original CO2 has dissolved into oceans,

rocks (carbonates) which keeps levels of CO2 just balanced in

atmosphere keeps planet WARM but not HOT

if planet were hotter, CO2, H2O would

be boiled out of oceans and baked out of rocks more CO2, H2O enter Atmosphere

Evolution of Atmospheres:

Earth vs. Venus

Evolution of Atmospheres:

Earth vs. Venus

Liquid water may have existed early in Venus’ history – but most vaporized into atmosphere: T was hotter on Venus

H2O vapor is a greenhouse gas - trapped energy making planet hotter; eventually T so high that water boils

‘runaway’ because more H20 goes into the Atmosphere as it evaporates; no water left on planet to dissolve CO2 – out of balance!

eventually stabilized when H20 broken down by UV sunlight and no further CO2 to bake out of the Venus surface

Earth has about the same amount of CO2 as Venus

Much of the Earth’s CO2 is ‘frozen’ into the rocks

However, if we could raise the temperature of our atmosphere a little bit, it would release a little bit more CO2 into the air

This would trap a little bit more heat, raising the temperature a little bit more…

This would release a little bit more CO2…

…which would trap a little bit more heat…

…which would raise the temperature a little bit more…

…which would release a little bit more CO2…

…which would trap a little bit more heat…

…which would raise the temperature a little bit more…

…which would release a little bit more CO2…

You get the idea!

This is called the runaway greenhouse effect

It happened on Venus because Venus is closer to the Sun

So - Earth has less atmosphere because most of our CO2 is still frozen in rocks

Global warming concern

There has been a large Increase in atmospheric CO2

Human activity is responsible for this increase.

CO

2 is a strong greenhouse gas

There is a strong correlation between the increase in carbon dioxide in the atmosphere and the global temperature.

Essentially all evidence indicates that human activity is a significant contributor to this trend (50%, 100% ???).

Can it be halted? reversed?

red) Ozone is broken apart by ultraviolet radiation.

What Determines a Planet’s Surface Temperature?

In the absence of the Greenhouse Effect:

the planet's distance from the Sun the planet’s overall reflectivity

• the higher the albedo (reflectivity), the less light absorbed planet cooler

What Determines a Planet’s Surface Temperature?

Greenhouse Effect. cannot change incoming Sunlight, so it cannot

change the total energy returned to space it increases the energy (heat) in lower atmosphere,

keeping the surface warmer It works like a blanket

Greenhouse Effect on the Planets

Greenhouse Effect warms Venus, Earth, & Mars on Venus: it is very strong on Earth: it is moderate on Mars: it is weak avg. temp. on Venus & Earth would be freezing

without it

Global warming concern

There has been a large Increase in atmospheric CO2

Human activity is responsible for this increase.

CO

2 is a strong greenhouse gas

There is a strong correlation between the increase in carbon dioxide in the atmosphere and the global temperature.

Essentially all evidence indicates that human activity is a significant contributor to this trend (50%, 100% ???).

Can it be halted? reversed?