1. Planetary Atmospheres Earth and the Other Terrestrial Worlds

2. 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

3. Coriolis Effect on Earth
Air moving from pole to equator is going farther from axis and begins to lag Earth’s rotation
Air moving from equator to pole goes closer to axis and moves ahead of Earth’s rotation
These create low pressure systems: chance of storms: in troposphere only -- need convection & surface heating
Coriolis Effect on Earth Applet

4. Coriolis Effect on Earth
Conservation of angular momentum causes large storms to swirl
Direction of circulation depends on hemisphere
N: counterclockwise
S: clockwise

5. Circulation Cells with Rotation
Coriolis effect deflects north-south winds into east-west winds
Deflection breaks each of the two large “no-rotation” cells breaks into three smaller cells

6. Prevailing Winds
Prevailing surface winds at mid-latitudes blow from W to E because Coriolis effect deflects S to N surface flow of mid-latitude circulation cell

7. Clouds and Precipitation
Sunlight evaporates water from oceans (and lakes)
Driven upward by convection & droplets condense at those lower temperatures & form clouds
When droplets grow enough rain or snow can form

8. What factors can cause long-term climate change?

9. Solar Brightening
Sun very gradually grows brighter with time, increasing the amount of sunlight warming planets
When Solar System was young the Sun was about 73% as bright as today

10. Changes in Axis Tilt
Greater tilt makes more extreme seasons, while smaller tilt keeps polar regions colder

11. Changes in Axis Tilt
Small gravitational tugs from other bodies in solar system cause Earth’s axis tilt to vary between 22° and 25°
This nutation has a period of about 40,000 years
Bigger tilts make more extreme seasons
Extra summer heat reduces ice build-up so overall planet temperature goes up

12. Changes in Reflectivity
Higher reflectivity tends to cool a planet, while lower reflectivity leads to warming
So more clouds & ice lead to yet more clouds & ice
Volcanoes & pollution insert aerosols into atmosphere, increasing albedo and cooling the earth

13. Changes in Greenhouse Gases
Increase in greenhouse gases leads to warming, while a decrease leads to cooling

14. Sources of Gas
Evaporation of surface liquid; sublimation of surface ice
Impacts of particles and photons eject small amounts
Outgassing from volcanoes: H2O,
CO2, N2, H2S, SO2

15. Losses of Gas Thermal escape of atoms Sweeping by solar wind
Condensation onto surface; and thermal escape
Chemical reactions with surface--rusting; and solar wind sweeping
Large impacts blast gas into space

16. Atmospheric Compositions
Structure and composition of any atmosphere is a battle between pressure gradients and gravity.
This is true for ALL PLANETS and STARS (and for their interiors too) Stable situations require HYDROSTATIC EQUILIBRIUM: the difference between pressures pushing out and in are exactly balanced by gravity.
At a given temperature, a molecule will have an average random velocity of:
where k = 1.38 x J K-1 is Boltzmann's constant.

17. Boltzmann (Thermal) Distribution
Thermal vs Escape Velocities Applet

18. Thermal Velocity vs Escape Velocity
Example: For the Earth, Tatm = 270 K and mass of N2= 2 x 14 x (1.67 x kg) = 4.68x10-26kg so Vthermal,N2= km/s and Vthermal,H2= Vthermal,N2 (mN2/mH2)1/ = km/s (28/2)1/2 = 1.83 km/s
BUT, there is a wide Boltzmann distribution in velocities at a given Temperature so a particular type of molecule will EVENTUALLY ESCAPE if Vthermal > Vescape/6 = (2GM/R)1/2
For earth, Vesc = 11.2 km/s so N2 stays but H2 goes.
While for Mercury, Tmax= 700K so Vth,N2,Mercury=(700K/290K)1/2 Vth,N2,Earth = km/s But, Vesc,Mercury=4.2 km/s < 6 x 0.79 km/s SO, even N2 leaves Mercury

19. Weather & Climate Summary
What creates wind and weather?
Atmospheric heating and Coriolis effect
What factors can cause long-term climate change?
Brightening of Sun
Changes in axis tilt
Changes in reflectivity
Changes in greenhouse gases
How does a planet gain or lose atmospheric gases?
Gains: Outgassing, evaporation/sublimation, and impacts by particles and photons; also trapped solar wind
Losses: Condensation, chemical reactions, blasting by large impacts, sweeping by solar winds, and thermal escape

20. Do the Moon and Mercury have any atmospheres?

21. Exospheres of Moon and Mercury
Sensitive measurements show Moon and Mercury have extremely thin atmospheres: so technically, yes; practically, no.
Gas comes from impacts that eject surface atoms
Mercury’s gravity also slows down some solar wind particles

22. Martian Atmosphere: What is Mars like today?

23. Seasons on Mars
The eccentricty (0.09) of Mars’s orbit makes seasons more extreme in the southern hemisphere:
closest during the southern summer, so hotter
furthest during southern winter, so colder
Tilt close to Earths, so similarities, but 1.88 times as long

24. Polar Ice Caps of Mars
Late winter
Midspring
Early summer
Carbon dioxide ice of polar cap sublimates as summer approaches and it condenses at opposite pole

25. Atmospheric Pressure, Composition and Polar Ice Caps
At surface, pressure ranges from to bar; <P>=0.006 bar
Highest T about 20 C, lowest about -170 C but average is -50 C
95.3% CO % N % Ar
Residual ice of polar cap during summer is primarily water ice

26. Atmospheric Structure

27. Dust Storms on Mars Seasonal winds can drive big dust storms on Mars
Dust in the atmosphere absorbs blue light, sometimes making the sky look brownish-pink
Features hidden for months over most of surface

28. Changing Axis Tilt
Calculations suggest Mars’s axis tilt ranges from 0° to 60° over long time periods (torques & S hemisphere is bigger)
Such extreme variations cause dramatic climate changes
These climate changes can produce alternating layers of ice and dust

29. Climate Change on Mars
Mars has not had widespread surface water for 3 billion years
Greenhouse effect probably kept surface warmer before that
Once hotter & denser; liquid water dissolved much of CO2
Somehow Mars lost most of its atmosphere!

30. Climate Change on Mars
Magnetic field may have preserved early Martian atmosphere when hot enough for molten metallic core
Solar wind may have stripped atmosphere after field decreased because of interior cooling & so dynamo ended

31. Summary: Martian Atmosphere
What is Mars like today?
Mars is cold, dry, and frozen
Strong seasonal changes cause CO2 to move from pole to pole, leading to dust storms
Why did Mars change?
Its atmosphere must have once been much thicker for its greenhouse effect to allow liquid water on the surface
Somehow Mars lost most of its atmosphere, perhaps because of declining magnetic field
Extreme shift of rotation axis might also have led to high enough temperatures to drive off most gas

32. What is the main source of the original atmospheres of the terrestrial planets?
Gas accreted from the solar nebula
Comets
Gas released from interior rocks (outgassing)
Evaporation from ice
None of the above
Answer: C

33. What is the main source of the original atmospheres of the terrestrial planets?
Gas accreted from the solar nebula
Comets
Gas released from interior rocks (outgassing)
Evaporation from ice
None of the above

34. Atmosphere of Venus
Venus has a very thick carbon dioxide atmosphere with a surface pressure 90 times Earth’s
Slow rotation produces very weak Coriolis effect (so just two large cells) and little weather
Very small tilt and eccentricity: little seasonal variation
Venus Express images: day side in
visible (left) & night in IR (right)

35. Greenhouse Effect on Venus
Thick carbon dioxide atmosphere produces an extremely strong greenhouse effect
Earth escapes this fate because most of its carbon and water is in rocks and oceans

36. VENUS'S ATMOSPHERE
Data from Pioneer Venus and Veneras imply it is MUCH HOTTER & DENSER THAN EARTH'S
PV = 92 PE = 92 atm; P drops to 0.1PV around 30 km up from the surface.
TV = 730 K --- runaway greenhouse effect (hotter than Mercury); Thick atm and Coriolis cells carry heat well: this temperature is nearly uniform around Venus and there's little difference between day- and night-side T's.
Fast winds ( km/h) above 70 km; slow surface winds: < 10 km/h But, even those slow winds would FEEL like a hurricane, since the atmospheric density is so high.

37. Pressure and Temperature
ATMOSPHERIC COMPOSITION:
96.5% CO2; most of rest N2
Above 30 km: H2SO4 haze
Clouds, mostly sulfuric acid drops, between km.
They evaporate by 30 km so rain doesn’t hit surface
More H2SO4 clouds seen even higher.

38. Hot Atmosphere of Venus
Reflective clouds contain droplets of sulphuric acid
No little sunlight actually reaches the surface
What does, however, heats surface & heat is trapped because CO2 absorbs IR radiation
Upper atmosphere has fast winds that remain unexplained (circulate in ~4 days)

39. Runaway Greenhouse Effect
Runaway greenhouse effect would account for why Venus has so little water: it outgassed a lot, but hi T meant all of it went into the atm, where UV dissociates it: H escapes, O reacts w/ rocks.
CO2 not trapped in carbonate rocks, as on Earth:  amounts of CO2

40. Thought Question
What is the main reason why Venus is hotter than Earth?
a) Venus is closer to the Sun than Earth.
b) Venus is more reflective than Earth.
c) Venus is less reflective than Earth.
d) Greenhouse effect is much stronger on Venus than on Earth.
e) Human activity has led to declining temperatures on Earth.

41. Thought Question
What is the main reason why Venus is hotter than Earth?
a) Venus is closer to the Sun than Earth.
b) Venus is more reflective than Earth.
c) Venus is less reflective than Earth.
d) Greenhouse effect is much stronger on Venus than on Earth.
e) Human activity has led to declining temperatures on Earth.

42. Summary of Venus’ Atmosphere
What is Venus like today?
Venus has an extremely thick CO2 atmosphere
Slow rotation means little weather
How did Venus get so hot?
Runaway greenhouse effect made Venus too hot for liquid oceans
All carbon dioxide remains in atmosphere, leading to a huge greenhouse effect
Venus might have been temperate when young because Sun was less luminous

43. Why is Earth’s atmosphere so different and wonderful?

44. Four Important Questions
Why did Earth retain most of its outgassed water?
Why does Earth have so little atmospheric carbon dioxide, unlike Venus?
Why does Earth’s atmosphere consist mostly of nitrogen and oxygen?
Why does Earth have a UV-absorbing stratosphere?

45. Earth’s Water and CO2
Earth’s temperature remained cool enough for liquid oceans to form
Oceans dissolve atmospheric CO2, enabling carbon to be trapped in rocks

46. Nitrogen and Oxygen
Most of Earth’s carbon and oxygen is in rocks, leaving a mostly nitrogen atmosphere
Plants and algae release some oxygen from CO2 into atmosphere
Original atmosphere had no free O2
Earliest life was anaerobic but algae produced some oxygen; eventually most life adopted it to burn food
Current 21% for < 5108yr

47. Ozone and the Stratosphere
Ultraviolet light can break up O2 molecules, allowing ozone (O3) to form
Without plants to release O2, there would be no ozone in the stratosphere to absorb UV light (and break back into O2+O)

48. Why does Earth’s climate stay relatively stable?

49. Carbon Dioxide Cycle
Atmospheric CO2 dissolves in rainwater, makes it acidic
Rain erodes minerals which flow into ocean
Minerals combine with calcium to make rocks like limestone on ocean floor
Subduction carries carbonate rocks down into mantle
Rocks melt in mantle and outgas CO2 back into atmosphere through volcanoes

50. Earth’s Thermostat Cooling allows CO2 to build up in atmosphere
Heating causes rain to reduce CO2 in atmosphere
But these take time: up to 400,000 yr to restore T

51. Long-Term Climate Change
Changes in Earth’s axis tilt might lead to severe ice ages sometimes called snowball earth
Widespread ice tends to lower global temperatures by increasing Earth’s reflectivity
CO2 from outgassing will build up if oceans are frozen, ultimately raising global temperatures again
Eventually runaway greenhouse Sun brightens
Start here on 11/17/09

52. How is human activity changing our planet?

53. Some Dangers of Human Activity
Human-made CFCs in atmosphere destroyed ozone, reducing protection from UV radiation until a treaty banned them; ozone holes are slowly reducing.
Human activity is driving many other species to extinction
Human use of fossil fuels produces greenhouse gases that cause global warming

54. Global Warming
Earth’s average temperature has increased by 0.5°C in past 50 years
Concentration of CO2 is rising rapidly
Clearly due to burning of fossil fuels at accelerating rates
An unchecked rise in greenhouse gases will eventually lead to more significant global warming
Gasses like methane are rarer but more powerful greenhouse gas: much from cattle, so also on humans

55. CO2 Concentration
Global temperatures have tracked CO2 concentration for last 500,000 years
Antarctic air bubbles indicate current CO2 concentration is highest in at least 500,000 years!

56. CO2 Increase is Anthropogenic
Most of CO2 increase has happened in last 50 years!
Human population more than doubled & fossil fuel consumption went up much faster

57. Modeling of Climate Change
Complex models of global warming strongly indicate that the recent temperature increase is indeed consistent with human production of greenhouse gases

58. Consequences of Global Warming
Storms more numerous and intense
More droughts and more floods
Rising ocean levels; melting glaciers
Changing ocean currents
Uncertain effects on food production, availability of fresh water
Potential for social unrest: entire low-lying countries probably underwater in years

59. Summary: Earth’s Unique Atmosphere
How did Earth’s atmosphere end up so different?
Temperatures just right for oceans of water
Oceans keep most CO2 out of atmosphere
Nitrogen remains in atmosphere
Life releases some oxygen into atmosphere
Why does Earth’s climate stay relatively stable?
Carbon dioxide cycle acts as a thermostat
How might human activity change our planet?
Destruction of ozone
High rate of extinction
Global warming from production of greenhouse gases