1. ATMOSPHERES

2. Atmospheric Basics Layer of gas surrounding a planet.
Usually very thin for terrestrial planets (exception Venus).
Affects conditions on the planet.
We would like to understand how each of the terrestrial planets ended up having such different atmospheres.
Venus’s thick atmosphere

3. (mostly carbon dioxide - CO2)
All terrestrial planets probably had minimal (primary: H,He) atmospheres at some point after they formed
These original atmospheres were swept away from the terrestrial planets early in their life.
Current atmospheres are secondary atmospheres, formed primarily by outgassing
(mostly carbon dioxide - CO2)

4. Holding onto an atmosphere requires gravity
The strength of gravity determines the escape velocity from the planet.
The temperature and composition of an atmosphere determines the velocities of atoms and molecules in the atmosphere. (At a given temperature, H and He will have higher velocities than more massive elements or molecules; recall that K.E. = 1/2 mv2)

5. Holding onto an atmosphere requires gravity
The strength of gravity determines the escape velocity from the planet.
The temperature and composition of an atmosphere determines the velocities of atoms and molecules in the atmosphere.
If the constituents of an atmosphere are moving faster than escape velocity, then a planet or moon will be unable to hold onto an atmosphere.

6. Larger (stronger gravity), cooler (slower moving molecules) planets
can hold onto atmospheres better than
smaller (weaker gravity),
hotter (faster moving molecules) planets

7. Moon and Mercury “Airless” worlds
gravity too weak to hold onto an atmosphere
“black sky”
The little atmosphere that exists consists of particles of the solar wind that are temporarily trapped.

8. Mars Mars had standing and running water on its surface in the past.
Very little atmosphere today (mainly 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 beneath its soil

9. Temperature at surface hot enough to melt lead
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
Weather forecast “awful” all the time.

10. Earth A moderate atmosphere today
Mostly nitrogen (N2), with some oxygen (O2: arises from photosynthetic life), carbon dioxide (CO2), etc.
Enough to enable liquid water to exist (temperature and pressure adequate)
Together the air & water produce erosion

11. The Jovian planets (high gravity, cool/cold atmospheres) have very substantial atmospheres.

12. LAYERING OF ATMOSPHERES
Structure is created within an atmosphere through interactions of atmospheric gasses with light

14. Interactions of light with atmospheric gasses
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

15. hottest layer, v. rarified Thermosphere
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) Earth is the only one with it.
Troposphere
absorbs IR (greenhouse); convective; weather

16. A magnetic field creates “magnetosphere” that deflects away solar wind particles.
In the absence of a “magnetosphere”, the solar wind will slowly strip away an atmosphere.

17. 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), thereby heating the lower atmosphere.
greenhouse gasses (e.g., H2O, CO2, CH4 - methane) transmit visible light but absorb infrared light

19. 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

20. EVOLUTION OF ATMOSPHERES

21. The dominant gasses arising from outgassing were most carbon dioxide and water vapor, with minor amounts of nitrogen, sulfer, argon, …
Each terrestrial planet’s outgassed atmosphere was roughly the same at the beginning.
Why do they differ now?

22. Mercury is too small and too hot to hold onto an atmosphere.

23. Mars lost much of its atmosphere because of its small size.
The atmosphere it retains resembles its original atmosphere in composition (essentially CO2).
The fate of its water is still a matter of debate. There appears to be substantial amounts of subsurface frozen water.

24. started with more or less identical atmospheres.
Venus and the Earth:
started with more or less identical atmospheres.
Their atmospheres have subsequently followed very different paths.
WHY?

25. Slightly higher temperatures at Venus’ distance from the Sun made it difficult for water to stay in liquid state.
Liquid water exists in abundance on the Earth
Carbon dioxide dissolves in oceans
Photosynthetic life creates oxygen (oxygen has a short lifetime in the atmosphere - must be constantly replenished).

26. Evolution of Atmospheres: Earth vs. Venus
because of the Earth’s temperature:
 On Earth there are oceans
Original CO2 has dissolved into oceans and is tied up in carbonate rocks,
rocks (carbonates) keep 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

27. 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 thus 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 (H escaped to space, O reacted with minerals) and there was no further CO2 to bake out of the Venus surface

28. This is called the runaway greenhouse effect
It happened on Venus because Venus is closer to the Sun. We do not think it can happen on the Earth.
So - Earth has less atmosphere because most of our CO2 is frozen in rocks (e.g., limestone)

29. 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

30. What Determines a Planet’s Surface Temperature?
With a greenhouse effect.
it increases the energy (heat) in lower atmosphere, keeping the surface warmer
It works like a blanket

31. 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

32. The Origin of the Earth’s Moon
We are now mostly finished with planetary environments, so.. Before proceeding to life… A brief visit to a missed topic…
The Origin of the Earth’s Moon

33. The Earth-Moon
double planet does not fit well into the nebular theory
planetesimal accretion predicts both should have the same chemical composition.
They don’t - there are subtle but significant differences
Moon is composed of less dense material than Earth

34. The general view is that the Earth’s moon was created as a result of the impact of a large object, perhaps as large as Mars, with the Earth very early in its existence.
The moon was formed from the debris of this collision, which included lower density “mantle” material from the Earth.

35. “Life”
How is life defined? What is needed for life? How hard it is for life to form? What environments are suitable for life?