Planetary Atmospheres Earth and the Other Terrestrial Worlds

1. Planetary AtmospheresEarth 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

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 10-23 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 10-27 kg) = 4.68x10-26kg so Vthermal,N2= 0.488 km/s and Vthermal,H2= Vthermal,N2 (mN2/mH2)1/2 = 0.488 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 = 0.166 (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 = 0.786 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 Moon Mercury

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 • Carbon dioxide ice of polar cap sublimates as summer approaches and it condenses at opposite pole Late winter Midspring Early summer

25. Atmospheric Pressure, Composition and Polar Ice Caps • At surface, pressure ranges from 0.005 to 0.008 bar; <P>=0.006 bar • Highest T about 20 C, lowest about -170 C but average is -50 C • 95.3% CO2 2.7% N2 1.6% 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

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 (300--400 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 50-70 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