1. Planetary Atmospheres Earth and the Other Terrestrial Worlds

2. What is an atmosphere?
An atmosphere is a layer of gas that surrounds a world

3. Earth’s Atmosphere
Most gas within about 10 km of surface (height of tallest mountains)
Consists mostly of molecular nitrogen: N % and oxygen: O %
Most of the rest is Argon: Ar -- 1%
That’s dry air: up to ~4% water vapor is held in the air too

4. Atmospheric Pressure
Gas pressure proprotional to both density and temperature.
P T
Adding air molecules increases the pressure in a balloon.
Heating the air also increases the pressure.

5. Atmospheric Pressure
Pressure and density decrease with altitude because the weight of overlying layers is less
Earth’s pressure at sea level is
1.03 kg per sq. meter
14.7 lbs per sq. inch
1 bar

6. Where does an atmosphere end?
There is no clear upper boundary
Most of Earth’s gas is < 10 km from surface, but a small fraction extends to >100 km
Altitudes >60 km are considered “space”: sky is black, not blue

7. Where does an atmosphere end?
Small amounts of gas are present even at > 300 km

8. Effects of Atmospheres
Create pressure that determines whether liquids (like water) can exist on surface
Absorb and scatter light
Create wind, weather, and climate
Interact with solar wind to create a magnetosphere
Can make planetary surfaces warmer through greenhouse effect

9. Pop Quiz Print your name neatly (1 pt)
Draw cutaway diagrams through the Earth and Mars, with relevant layers illustrated and labeled in approximately correct scales. (6 pt)
What is the radius of Mars in km? (2pt)
If the temperature at the surface of the earth goes up by 10% while the density of that gas stays constant, what happens to the pressure of that gas? (2pt)

10. Quiz Answers 2)
3) RM= 0.53  RE = 0.53  6378 km = 3380 km RM= 3397 km, but 0.5  6400 km = 3200 km is OK
4) P rises by 10% since pressure = density  temperature

11. Greenhouse Effect
Visible light passes through atmosphere and warms planet’s surface
Surface heats up, emits infrared light
Gases in atmosphere absorb IR light from surface, trapping heat: its reemitted, but in random directions, so temporarily trapped
CO2, H2O & CH4
So atm acts like a blanket, slowing escape of heat
Greenhouse Effect Applet

12. Planetary Temperature
A planet’s surface temperature is determined by balance between the energy of sunlight it absorbs and the energy of outgoing thermal radiation
Thermal Equilibrium Applet

13. Temperature and Distance
A planet’s distance from the Sun determines the total amount of incoming sunlight
Temperature vs Radius Applet

14. Temperature and Rotation
A planet’s rotation rate affects the temperature differences between day and night
Temperature vs Day Length Applet

15. Temperature and Reflectivity
A planet’s reflectivity (or albedo) is the fraction of incoming sunlight it reflects
Planets with low albedo absorb more sunlight, leading to hotter temperatures
Temperature vs Albedo Applet

16. “Without Greenhouse” Temperatures
Venus would be 510°C colder without greenhouse effect!
Earth would be 31°C colder (below freezing on average)
Mars only 6 °C colder

17. Thought Question
What would happen to Earth’s temperature if Earth were more reflective?
a) It would go up.
b) It would go down.
c) It wouldn’t change

18. Thought Question
What would happen to Earth’s temperature if Earth were more reflective?
a) It would go up.
b) It would go down.
c) It wouldn’t change

19. Thought Question
If Earth didn’t have an atmosphere, what would happen to its temperature?
a) It would go up a little (less than 10° C).
b) It would go up a lot (more than 10° C).
c) It would go down a little (less than 10° C).
d) It would go down a lot (more than 10° C).
e) It would not change.

20. Thought Question
If Earth didn’t have an atmosphere, what would happen to its temperature?
a) It would go up a little (less than 10° C).
b) It would go up a lot (more than 10° C).
c) It would go down a little (less than 10° C).
d) It would go down a lot (more than 10° C).
e) It would not change.

21. Computing Planet Temperatures
With no greenhouse gases we have the temperature depending on planet’s distance from Sun, d (in AU), & reflectivity (albedo), :
Logical: T goes down if d goes up and T goes down if more light is reflected ( goes up)
Example: For Mars, d = and  = 0.16, so
T = 280 K  [(1-0.16)/(1.5242)]1/4 = 280 K  [0.84/2.32]1/4
T = 280 K  [0.362]1/4 = 280 K  = 217 K = -56 C

22. Why do atmospheric properties vary with altitude?

23. Light’s Effects on Atmosphere
Ionization: Removal of an electron
Dissociation: Destruction of a molecule
Scattering: Change in photon’s direction
Absorption: Photon’s energy is absorbed

24. Light’s Effects on Atmosphere
X-rays ionize molecules, so few get through
UV light dissociates molecules
Visible: Molecules tend to scatter blue light more than red
Molecules can absorb infrared light

25. Earth’s Atmospheric Structure
Troposphere: lowest layer of Earth’s atmosphere
Temperature drops with altitude up to ~ 12 km
Warmed by infrared light from surface and convection

26. Earth’s Atmospheric Structure
Stratosphere: Layer above the troposphere
Temperature rises with altitude in lower part to about 50 km,
drops with altitude in upper part up to ~80 km
This part is the mesosphere
Warmed by absorption of ultraviolet sunlight by ozone

27. Earth’s Atmospheric Structure
Thermosphere: Layer at about 100 km altitude
Temperature rises with altitude
X-rays and ultraviolet light from the Sun heat and ionize gases
So also called Ionosphere

28. Earth’s Atmospheric Structure
Exosphere: Highest layer in which atmosphere gradually fades into space
Temperature rises with altitude; atoms can escape into space
Warmed by X rays and UV light

29. Thought Question Why is the sky blue?
a) The sky reflects light from the oceans.
b) Oxygen atoms are blue.
c) Nitrogen atoms are blue.
d) Air molecules scatter blue light more than red light.
e) Air molecules absorb red light.

30. Thought Question Why is the sky blue?
a) The sky reflects light from the oceans.
b) Oxygen atoms are blue.
c) Nitrogen atoms are blue.
d) Air molecules scatter blue light more than red light.
e) Air molecules absorb red light.

31. Reminder: why the sky is blue
Atmosphere scatters blue light from Sun, making it appear to come from different directions
Sunsets are red because red light scatters less
Also dimmer, since all blue & green light is gone, along w/ most yellow and some orange & red.

32. Atmospheres of Other Planets
Earth is only planet with a stratosphere because of UV-absorbing ozone molecules (O3).
Those same molecules partially protect us from Sun’s UV light.
Sunburn & skin cancer arise from UV that gets through.
No greenhouse temperatures

33. Earth’s Magnetosphere
Magnetic field of Earth’s atmosphere protects us from charged particles streaming from Sun (solar wind)

34. Aurora
Charged particles can enter atmosphere at magnetic poles, causing an aurora Aurora Borealis Applet

35. Basic Atmosphere Summary
What is an atmosphere?
A layer of gas that surrounds a world
How does the greenhouse effect warm a planet?
Atmospheric molecules allow visible sunlight to warm a planet’s surface but absorb infrared photons, trapping the heat.
Why do atmospheric properties vary with altitude?
They depend on how atmospheric gases interact with sunlight at different altitudes.

36. Weather vs. Climate
Weather is the ever-varying combination of wind, clouds, temperature, and pressure
Local complexity of weather makes it difficult to predict beyond a week or so
Only detailed measurements plus massive computer models make predictions that good
Climate is the long-term average of weather
Long-term stability of climate depends on global conditions and is more predictable

37. Global Wind Patterns Global winds blow in distinctive patterns
Equatorial: E to W
Mid-latitudes: W to E
High-latitudes: E to W
In both troposphere and boundary of stratosphere (jet stream)

38. Circulation Cells: No Rotation
Heated air rises at equator
Cooler air descends at poles
Without rotation, these motions would produce two large circulation cells just several km high

39. Coriolis Effect
Conservation of angular momentum causes a ball’s apparent path on a spinning platform to change direction
Counterclockwise spin means ball deflected right, regardless of going in or out
Start here on 11/12/09