Lecture #28: Volcanoes are a Gas

0. Grading for Average of Two Exams: A=78-100,B=67-77,C=53-66,D=45-52
I. History of an atmosphere -- solar nebula could have provided only about 2% of Earth's present atmosphere -- molecules moving slower than the escape speed will be trapped as the rest of the nebula blows off -- trapped particles slowly leak out unless much slower than the escape speed (like neon-- atomic number 10) -- gas giants retain primitive atmospheres, Earth, Venus, and Mars have "secondary" atmospheres -- our atmosphere has about 100 times greater ratio of N and O to Ne -- role of "outgassing" and biological action -- H2O and CO2 are dominant outgassed agents, N2 a distant third -- some volatiles, including water, may also come from comets -- water undergoes phase change and also can get photodissociated (it's hard to get water!) -- CO2 can be dissolved in oceans and locked into rocks -- on Earth, we know where the H2O and CO2 are, due to liquid water and the fact that Earth is near the triple point of H2O. -- on Mars, H2O may be in permafrost, CO2 may have been locked in rocks (also, more leakage into space would have occurred on Mars than Earth) -- on Venus, H2O may have been photodissociated and escaped
II. Importance of the dependence of opacity on wavelength -- concept of opacity -- scattering opacity: Rayleigh scattering and the blue sky -- absorbing opacity: the Greenhouse effect -- water vapor and CO2 have higher opacity in the infrared than visible -- blocking the escape of infrared light heats surface and lower atmosphere -- the thick CO2 and cloudy atmosphere of Venus has a huge Greenhouse effect, whereas it is only a temperate effect on Earth. We are lucky N2 is not a Greenhouse gas-- if it were, we wouldn't be here! -- but all is not peachy-- the Greenhouse effect is unstable: Venus may have the potential for liquid water, if not for the runaway aspect of the Greenhouse effect! -- it's possible that life itself may stabilize the greenhouse effect: witness the long-standing presence of liquid water on Earth through harsh environmental variation, vs. other planets
III. Ozone layer on Earth -- lightning can form O3 molecules out of O2 -- O3 blocks harmful ultraviolet rays in sunlight -- perhaps in "primordial soup", ultraviolet rays helped fracture protein chains, increase diversity, but now it needs to be limited
IV. Phases and atmospheres
-- word "phase" means different things, so watch out -- liquid phase is the least common: examples? -- phase diagram and difficulty for life
V. Why are there atmospheres?
-- difference between a gas and a solid: interparticle forces -- difference between gas pressure and solid-body forces (pressure always wants to expand) -- ideal gas law: PV = nkT -- particle speed, temperature, and molecular weight
VI. Atmospheric scale height
-- connection between isothermal scale height, bouyancy, and ideal gas law -- estimate this for Earth (6 km, about the height of Mt. Everest) -- atmosphere is very thin! ("walking distance" to space) (1/1000 radius) -- so gravity is the barrier to space, not the atmosphere -- climbing Mt. Everest: don't try this at home (cold is the least problem) -- why do airplanes fly so high? Why not even higher?
VII. Atmospheric composition and escape
-- how can chemistry of terrestrial planets be oxidizing, when most of the universe is hydrogen? -- thermal speed (decreases with mass) vs. escape speed (independent of mass) -- hydrogen gas is the fastest of all molecules -- formation of the gas giants
VIII. Primitive atmospheres
-- solar nebula would have been about 2% of Earth's present atmosphere -- molecules moving slower than the escape speed will be trapped as the rest of the nebula blows off -- trapped particles slowly leak out unless much slower than the escape speed (like neon-- atomic number 10) -- gas giants retain primitive atmospheres, Earth, Venus, and Mars have "secondary" atmospheres -- our atmosphere has about 100 times more N and O relative to Ne -- role of "outgassing" and biological action -- H2O and CO2 are dominant outgassed agents, N2 a distant third -- water can freeze or boil and get photodissociated -- CO2 on Mars can freeze or sublimate, and on Earth gets dissolved in ocean -- CO2 remains on Venus and some on Mars, N2 remains on Earth
IX. Importance of the dependence of opacity on wavelength
-- scattering opacity: Rayleigh scattering and the blue sky -- Earth is blue due to oceans and atmosphere combination: to see it, put a white piece of paper in the shade -- Neptune is blue for different reason: some Rayleigh scattering in haze above the clouds, but mostly, methane gas appears blue -- absorbing opacity: the Greenhouse effect -- water vapor and CO2 have higher opacity in the infrared than visible -- the thick CO2 and cloudy atmosphere of Venus has a huge Greenhouse effect, whereas it is only a temperate effect on Earth. We are lucky N2 is not a Greenhouse gas-- if it were, we wouldn't be here! -- but all is not peachy-- the Greenhouse effect is unstable: Venus may have the potential for liquid water, if not for the runaway aspect of the Greenhouse effect! -- it's possible that life itself may stabilize the greenhouse effect: witness the long-standing presence of liquid water on Earth through harsh environmental variation, vs. other planets -- role of microbes and photosynthesis in replacing CO2 with O2
X. Temperature structure and the ozone layer on Earth
-- troposphere, stratosphere, mesosphere -- what makes the mesosphere, a region of increasing T? -- lightning can form O3 molecules out of O2 -- O3 blocks harmful ultraviolet rays in sunlight -- perhaps in "primordial soup", ultraviolet rays helped fracture protein chains, increase diversity, but now it needs to be limited -- way up: ionosphere, radio reflection: lucky for Marconi in 1901!
XI. Planetary atmospheres Great Red Spot (unknown composition) -- Io: lots of sulfur and sodium -- Titan: only moon with thick atmosphere, similar N2 content to Earth, some methane