Atmospheric Properties and Earth's Climate

1. Press Release Talks Reminder: Please tell me who your partner is and what your topic is by Nov. 1. If you do not have a partner, let me know and I will find one for you.

2. Exam #2 Approaching Exam #2 will be Thursday, November 1. Same format as Exam #1. We will play AstroJeopardy on Tuesday, October 30. You can add onto your formula sheet.

3. Why do atmospheric properties vary with altitude?

4. Light’s Effects on Atmosphere • X-rays and UV light can ionize (remove electrons from atoms) and dissociate (break apart molecule into atoms) molecules. • Molecules tend to scatter blue light more than red (that’s why the sky is blue). • Molecules can absorb infrared light.

5. Earth’s Atmospheric Structure • Troposphere: lowest layer of Earth’s atmosphere -- ~12 km from surface • Temperature drops with altitude (less greenhouse effect with altitude). • Warmed by infrared light from surface and convection

6. Earth’s Atmospheric Structure • Stratosphere: layer above the troposphere – 12-80 km • Temperature rises with altitude in lower part, drops with altitude in upper part. • Warmed by absorption of ultraviolet sunlight breaking apart (disassociating) ozone (O3)

7. Earth’s Atmospheric Structure • Thermosphere: layer at about 80 – 200 kilometers altitude • Temperature rises with altitude. • X-rays from the Sun heat and ionize gases (N2 and O2). • Ionisphere located here – important for radio communication

8. 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

9. Atmospheres of Other Planets • Earth is only planet with a stratosphere because of UV-absorbing ozone molecules (O3). • Those same molecules protect us from Sun’s UV light. “No greenhouse” temperatures

10. Earth’s Magnetosphere • Magnetic field of Earth’s atmosphere protects us from charged particles streaming from Sun (the solar wind) – some particles get funneled in thru Earth’s magnetic poles

11. Aurora (Northern Lights) • Charged particles from solar wind energize the upper atmosphere near magnetic poles, causing an aurora.

12. The 1859 Carrington Event In 1859, a tremendous solar flare dubbed the “Carrington Event” knocked out telegraph stations everywhere. Telegraph operators were shocked and telegraph paper set on fire! Northern lights as far south as Cuba and Honolulu! If it happened today, estimated damage to communication satellites and transformers, and its effects on communication would be $1-2 trillion.

13. What creates wind and weather?

14. Weather and Climate • Weather is the ever-varying combination of wind, clouds, temperature, and pressure. • Local complexity of weather makes it difficult to predict. • Climate is the long-term average of weather. • Long-term stability of climate depends on global conditions and is more predictable.

15. Global Wind Patterns • Global winds blow in distinctive patterns: • Equatorial: E to W • Mid-latitudes: W to E • High latitudes: E to W

16. Circulation Cells: No Rotation • Heated air rises at equator. • Cooler air descends at poles. • Without rotation, these motions would produce two large circulation cells.

17. Circulation Cells: No Rotation • On Venus, thick atmosphere distributes heat evenly  no big difference between equator and poles • On Mars, thin atmosphere does not distribute heat well  large temperature differences between equator and poles

18. Coriolis Effect • Conservation of angular momentum causes a ball’s apparent path on a spinning platform to change direction.

19. Myth of Fact? In the southern hemisphere, water goes down sinks or toilets in the opposite direction as in the northern hemisphere because of the Coriolis effect. Myth! The Coriolis effect only matters on very large (hundreds of km) length scales, not the diameter of your sink.

20. Coriolis Effect on Earth • Air moving from a pole to the equator is going farther from Earth’s axis and begins to lag behind Earth’s rotation. • Air moving from the equator to a pole moves closer to the axis and travels ahead of Earth’s rotation.

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

22. Circulation Cells: No Rotation • Heated air rises at equator. • Cooler air descends at poles. • Without rotation, these motions would produce two large circulation cells.

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

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

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

26. 1) Solar Brightening • The Sun very gradually grows brighter with time, increasing the amount of sunlight warming the planets. This is a long-term effect (noticeable over tens of millions of years).

27. Sun’s luminosity was 70% of its current value when it was born.

28. 2) Changes in Axis Tilt • Greater tilt creates more extreme seasons, while smaller tilt keeps polar regions colder. • Greater tilt  warmer summers  less ice  less reflectivity  warmer planet • Noticeable over thousands to millions of years.

29. Changes in Axis Tilt • Small gravitational tugs from other bodies in solar system cause Earth’s axis tilt to vary between 22° and 25°. • Cyclical interplay between tilt and climate called Milankovitch cycles.

30. 3) Changes in Reflectivity • Higher reflectivity tends to cool a planet, while lower reflectivity leads to warming. • Small drops in temperature measured after volcanic eruptions (aerosols reflect light).

31. Mount Pinatubo Eruption in 1991 • Injected 17 millions tons of SO2 into atmosphere • Decreased sunlight reaching Earth’s surface by 10% • Lowered northern hemisphere temperature by 1° F and globally by 0.7° F • Raised temperature in stratosphere by a few degrees (absorption by aerosols).

32. 4) Changes in Greenhouse Gases • An increase in greenhouse gases leads to warming, while a decrease leads to cooling  most important factor.

33. How does a planet gain or lose atmospheric gases?

34. Sources of Gas Outgassing from volcanoes Evaporation of surface liquid; sublimation of surface ice Impacts of particles and photons more important less important (on Earth)

35. Losses of Gas Thermal escape of atoms* Sweeping by solar wind* Condensation onto surface Chemical reactions with surface (e.g., rusting) *gas gone for good

36. Thermal Escape – Important Concept

37. Do the Moon and Mercury have any atmosphere?

38. Exospheres of the Moon and Mercury • Sensitive measurements show that the Moon and Mercury have extremely thin atmospheres. • Gas comes from impacts that eject surface atoms (exospheres only). Earth’s Moon Mercury

39. What is Mars like today?

40. Seasons on Mars • The relatively high ellipticity of Mars’s orbit makes seasons more extreme in the southern hemisphere.

41. Polar Ice Caps of Mars CO2 + H2O • Carbon dioxide (dry) ice of polar cap sublimates as summer approaches and condenses at opposite pole. CO2 Late winter Mid-spring Early summer

42. Polar Ice Caps of Mars • Residual ice of the polar cap remaining during summer is primarily water ice – water has a higher sublimation temperature than carbon dioxide. • If all water ice melted, and ocean 11-meters deep over the entire planet could be formed.

43. Dust Storms on Mars • Seasonal winds can drive dust storms on Mars. • Dust in the atmosphere absorbs blue light, sometimes making the sky look brownish-pink.

44. Why did Mars change?

45. Climate Change on Mars • Mars has not had widespread surface water for 3 billion years. • Greenhouse effect probably kept the surface warmer before that. • Somehow Mars lost most of its atmosphere - low mass of Mars allowed gases to diffuse away via high velocity tail of Maxwellian velocity distribution. - little/no additional gas replenishment from outgassing once volcanic activity declined

46. What is Venus like today?

47. Atmosphere of Venus • Venus has a very thick carbon dioxide atmosphere with a surface pressure 90 times that of Earth. • Slow rotation produces a very weak Coriolis effect and little weather.

48. Atmosphere of Venus • Reflective clouds contain droplets of sulfuric acid. • The upper atmosphere has fast winds (50x Venus’ rotational speed!) that remain unexplained.

49. How did Venus get so hot?

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