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Terrestrial atmospheres Overview • Most of the planets, and three large moons (Io, Titan and Triton), have atmospheres Mars • • • • Very thin Mostly CO2 Some N2, Ar Winds, dust storms Venus • • • • Very thick Mostly CO2 Some N2 Sulfuric acid clouds Earth • • • • Mostly N2, O2 Some H20, Ar Only 0.03% CO2. Water clouds 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 CO2 locked up in rocks and shells. • Percent (by weight) added to atmosphere by volcanic outgassing Gas Deep eruptions Continental geysers Required amount H2O 57.8 99.4 92.8 CO2 23.5 0.33 5.1 Cl2 0.1 0.12 1.7 N2 5.7 0.05 0.24 S2 12.6 0.03 0.13 Others <1 <1 <1 Atmospheric compositions • Comparison of total volatile content on Venus, Earth and Mars shows better agreement. Volatile Venus Earth Mars H2O Atmosphere 60 3 0.02 Oceans/caps 0 250,000 5000? Crust 160,000? 30,000 10,000? Total 160,000? 280,000 15,000? Atmosphere 100,000 0.4 50 Oceans/caps 0 0 10 Crust 0 100,000 >900? Total 100,000 100,000 >1000? 2,000 2,000 300 4 11 0.5 CO2 Table 11.2: mass fraction of volatiles (x109). N2 Atmosphere 40Ar Atmosphere 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. dP GM 2 g dr r 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. Physical Structure • Surface temperatures and pressures are very different for the three terrestrial planets But the pressure scale heights are similar Venus Earth Mars Tequil (K) 238 263 222 Tsurf (K) 733 288 215 Psurf (bar) 92 1.013 0.0056 surf (kg/m3) 65 1.2 0.017 H(km) 8.5 18 16 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 Earth: • water concentrated near surface • CO2 locked in rocks, shells • Leads to oxygen-rich atmosphere Chemical Structure Mars • Water and CO2 clouds Venus • Dominated by sulfur, CO2 • Clouds of sulfuric acid at 48-58 km 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 CO2, sulfuric acid. Present-day Mars 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 max T 0.290 cm K L 4R 2Te4 Opacity • Earth’s atmosphere is not transparent Ozone (high altitude) strongly absorbs UV radiation Water and CO2 (lower altitude) strongly absorb infrared radiation • Upper atmosphere heated by incoming solar radiation • Outgoing radiation from ground heats lower atmosphere Earth transparency Earth’s atmosphere is opaque at infrared wavelengths infrared optical 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 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 Heat radiated away Heat returned to surface Clouds heated from below Break Convection Solar heating alone would cause global circulation, as hot air rises and cool air sinks Rotation and the Coriolis force Recall: in a rotating reference frame, objects do not move in a straight line. Applet Rotation and Coriolis force Winds • Coriolis force splits the Hadley cells in each hemisphere into three cells. Diversion: Australian toilets • Does the Coriolis force influence the direction in which a vortex forms? The acceleration due to the Coriolis force is: a 2 v where is the angular velocity of rotation, and v is the velocity of the moving particle Upper atmospheres Upper atmospheres • The thermosphere is heated by energetic photons from the Sun • Low density means light atoms are able to float to the top 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? vesc 2GM R 1/ 2 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? Venus • CO2 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 Mars • Weather dominated by dust storms Very small dust particles (1 micron diameter) can be carried by strong winds (>180 km/h) Titan • N2 rich atmosphere • Little greenhouse effect, cold surface temperatures • Smog-colour from interactions of solar radiation and methane in atmosphere Next Lecture The Giant Planets