1. Terrestrial atmospheres

2. Overview Most of the planets, and three large moons (Io, Titan and Triton), have atmospheres Mars Very thin Mostly CO 2 Some N 2, Ar Winds, dust storms Venus Very thick Mostly CO 2 Some N 2 Sulfuric acid clouds Earth Mostly N 2, O 2 Some H 2 0, Ar Only 0.03% CO 2. Water clouds

3. Secondary atmospheres Can calculate how many volatiles had to be added to the atmosphere to get present surface conditions  Not just current atmosphere content, but also the oceans and CO 2 locked up in rocks and shells. GasDeep eruptions Continental geysers Required amount H2OH2O57.899.492.8 CO 2 23.50.335.1 Cl 2 0.10.121.7 N2N2 5.70.050.24 S2S2 12.60.030.13 Others<1 Percent (by weight) added to atmosphere by volcanic outgassing

4. Atmospheric compositions Comparison of total volatile content on Venus, Earth and Mars shows better agreement. VolatileVenusEarthMars H2OH2O Atmosphere6030.02 Oceans/caps0250,0005000? Crust160,000?30,00010,000? Total160,000?280,00015,000? CO 2 Atmosphere100,0000.450 Oceans/caps0010 Crust0100,000>900? Total100,000 >1000? N2N2 Atmosphere2,000 300 40 Ar Atmosphere4110.5 Table 11.2: mass fraction of volatiles (x10 9 ).

5. Physical Structure Use the equation of hydrostatic equilibrium to determine how the pressure and density change with altitude, in an isothermal atmosphere. You may neglect the change in gravitational force with altitude.

6. Physical Structure Pressure decreases with increasing altitude Atmospheres are compressible, so density decreases with altitude Compare with the pressure structure of the oceans, where the density remains approximately constant.

7. Physical Structure Surface temperatures and pressures are very different for the three terrestrial planets  But the pressure scale heights are similar VenusEarthMars T equil (K)238263222 T surf (K)733288215 P surf (bar)921.0130.0056  surf (kg/m 3 )651.20.017 H(km)168.518

8. Thermal structure The thermal structure of the terrestrial atmospheres are similar There is at least one temperature minimum  Caused by heat being trapped by cloud layers Temperature gradient depends on heat transport  Radiation – depends on the opacity of the atmosphere  Convection  Conduction – important near the surface

10. Chemical Structure Earth: water concentrated near surface CO 2 locked in rocks, shells Leads to oxygen-rich atmosphere Venus Dominated by sulfur, CO 2 Clouds of sulfuric acid at 48-58 km Mars Water and CO 2 clouds

11. Atmospheres and Water Lots of evidence that liquid water existed on Mars’ surface  Liquid water requires higher temperatures and pressures  Must have been a much denser atmosphere at one point Venus: too hot for liquid water to exist  Water evaporates, H and O dissociate  H is lost, and O forms CO 2, sulfuric acid. Present-day Mars

12. Opacity Consider a completely transparent atmosphere  No radiation (sunlight) is absorbed  Optical sunlight hits the ground and heats it up  Earth reradiates this energy in the infrared  No effect on atmosphere

13. Opacity Earth’s atmosphere is not transparent  Ozone (high altitude) strongly absorbs UV radiation  Water and CO 2 (lower altitude) strongly absorb infrared radiation Upper atmosphere heated by incoming solar radiation Outgoing radiation from ground heats lower atmosphere

14. Earth transparency Earth’s atmosphere is opaque at infrared wavelengths infrared optical

15. Greenhouse effect Optical radiation strikes the Earth and heats it up Infrared radiation is absorbed by lower atmosphere and reradiated in all directions  Including back to the ground, heating it further Surface and lower atmosphere heat up until the amount of IR radiation escaping the atmosphere is equal to the amount of solar radiation coming in

16. Greenhouse Effect Assume a simple model of the greenhouse effect, where a cloud layer (with same surface area as the Earth) is completely transparent to optical radiation, and completely opaque to infrared radiation. Calculate by how much the surface temperature increases when the cloud layer is present. Solar heating Clouds heated from below Heat returned to surface Heat radiated away

17. Break

18. Convection Solar heating alone would cause global circulation, as hot air rises and cool air sinks

19. Rotation and the Coriolis force Recall: in a rotating reference frame, objects do not move in a straight line. Applet

20. Rotation and Coriolis force

21. Winds Coriolis force splits the Hadley cells in each hemisphere into three cells.

22. Diversion: Australian toilets Does the Coriolis force influence the direction in which a vortex forms? where  is the angular velocity of rotation, and v is the velocity of the moving particle The acceleration due to the Coriolis force is:

23. Upper atmospheres

24. The thermosphere is heated by energetic photons from the Sun Low density means light atoms are able to float to the top

25. Escape of atmospheres Particles with velocity greater than the escape velocity will leave the atmosphere (if the density is low enough that they will not collide with another atom) What is the average velocity of a particle at temperature T? How high a T do you need for a particle with this velocity to escape?

26. Escape of atmospheres Particles with velocity greater than the escape velocity will leave the atmosphere (if the density is low enough that they will not collide with another atom) At a given temperature, particles of a given mass have a Maxwellian velocity distribution, with a long tail to high velocities: At T~2000 K (top of thermosphere), how much faster than the average velocity must a hydrogen atom be moving to escape? How about for a Neon atom?

27. Venus CO 2 rich atmosphere  Leads to a strong greenhouse effect, at high surface temperatures Clouds made mostly of sulfuric acid Motion of upper atmosphere is due to convection, as a result of the strong temperature gradient.  Almost no wind or weather at the surface, due to the slow rotation of Venus

28. Mars Weather dominated by dust storms  Very small dust particles (1 micron diameter) can be carried by strong winds (>180 km/h)

29. Titan N 2 rich atmosphere Little greenhouse effect, cold surface temperatures Smog-colour from interactions of solar radiation and methane in atmosphere

30. Next Lecture The Giant Planets