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AST 111 Lecture 19

AST 111 Lecture 19. (Terrestrial) Planetary Atmospheres II. Moon and Mercury. Atmospheres consist of exospheres only Take either of their atmospheres, could “almost store them in a dorm room” No volcanic outgassing Geologically dead. Moon and Mercury. Atmosphere persists because:

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AST 111 Lecture 19

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  1. AST 111 Lecture 19 (Terrestrial) Planetary Atmospheres II

  2. Moon and Mercury • Atmospheres consist of exospheres only • Take either of their atmospheres, could “almost store them in a dorm room” • No volcanic outgassing • Geologically dead

  3. Moon and Mercury • Atmosphere persists because: • Micro-meteorite impacts • Solar wind trapped by weak mag field • Surface atoms set free

  4. Mars’ Atmosphere • Low atmospheric pressure • Mars is small, so less gravity • Liquid water is unstable • Mostly CO2 • Not enough to produce much greenhouse effect • Pressure is 13,000x greater on Venus • No oxygen, no ozone layer • UV light hits surface • Thin atmosphere

  5. Mars’ Atmosphere • Mars has very extreme seasons • Ellipticity is great enough to influence seasons

  6. Mars’ Atmosphere • Southern hemisphere has hottest summers and coldest winters • Summer pole sublimates while winter pole condenses • This drives winds from summer to winter pole

  7. Mars’ Atmosphere • These winds cause dust storms over the whole surface! • Previous astronomers thought they saw changes in vegetation!

  8. Mars’ Atmosphere Dust Devils

  9. Mars’ Atmosphere • Surface features and erosion suggest heavy rainfall • Used to have thick atmosphere • Otherwise water (which there was lots of) would be unstable • Used to be geologically (…perhaps biologically?) thriving. What happened??

  10. Mars’ Atmosphere How?

  11. Mars’ Atmosphere • Core temperature dropped • Became geologically dead • Volcanoes stopped • Due to low gravity on Mars, some atmosphere escaped • Not replenished by volcanoes

  12. Mars’ Atmosphere As Mars became geologicallydead, its core solidified. Mars lost its magnetic field and the solar wind stripped away most of its atmosphere.

  13. Atmosphere of Venus • Thick CO2 atmosphere • 96.5% CO2 • Volcanoes pump CO2 into the atmosphere • Venus is large so remains geologically active

  14. Atmosphere of Venus With that in mind, why is Venus so inhospitable?

  15. Atmosphere of Venus Venus’ atmosphere creates an extreme greenhouse effect due to the massiveamounts of CO2 in its atmosphere.

  16. Atmosphere of Venus • Atmospheric pressure at surface: • 90x that of Earth (LOTS of CO2) • Density 10% that of water • “…would feel like a cross between swimming and flying.”

  17. Atmosphere of Venus • Venus has thick clouds of sulfuric acid • Volcanoes blast sulfur dioxide • Reaches upper atmosphere • UV converts it to sulfuric acid • Sulfur compounds give Venus its yellowish color

  18. Atmosphere of Venus • Slow rotation, surface at same temperature • Not much wind • Sulfuric acid rain evaporates 10 miles above the surface • Poles just about as hot as the equator • Greenhouse effect • No seasons (no axial tilt)

  19. Atmosphere of Venus So why does Venus have somuch carbon dioxide in its atmosphere? (It does have volcanoes, but so doesEarth!)

  20. Atmosphere of Venus Because a planet needs oceansto dissolve CO2. Venus does not have oceans. Earth has about the same amountof CO2 as Venus– but most of it is locked away in rocks in the ocean!

  21. Atmosphere of Venus Why does Venus not have any oceans?

  22. Atmosphere of Venus • It did! And so did Earth. • Venus closer to the Sun than Earth (50 oF hotter) • Oceans began to evaporate • What kind of gas is water vapor?

  23. Atmosphere of Venus • Average temperature up by 30 oC • More evaporation • More greenhouse gases, higher temp. • More evaporation • Etc… • Runaway Greenhouse Effect: oceans fueled their own evaporation, did not dissolve and store CO2 in rocks

  24. Atmosphere of Venus • Venus has no magnetic field • Solar wind broke up water molecules in the atmosphere • Hydrogen flies off, oxygen reacts on surface • No water left to dissolve the CO2 on the surface or in the atmosphere

  25. Atmosphere of Venus • P. 323: “We’ve seen that moving Earth to Venus’ orbit would cause our planet to become Venus-like. If we could somehow move Venus to Earth’s orbit, would it become Earth-like?”

  26. Atmosphere of Earth • Temperature low enough for water to condense • So we have oceans • Carbon dioxide dissolved in the oceans • Right balance of greenhouse gases

  27. Atmosphere of Earth • Nitrogen (77%) and oxygen (21%) • Why so much nitrogen? • Volcanoes outgas CO2, water, and nitrogen • Water into oceans • CO2 into rocks • Nitrogen went up

  28. Atmosphere of Earth • Why so much oxygen? • It came from (and still does) photosynthesis and CO2 • Air only breathable in last few hundred million years

  29. Stability of Earth’s Climate • No runaway greenhouse effect (like Venus has) • No freezing (like Mars has) • Self-stabilizing greenhouse effect • Requires two parts: Volcanoes and oceans • Venus has one but not the other!

  30. Carbon Dioxide Cycle (page 326)

  31. Carbon Dioxide Cycle • It is stable. • If it warms up: • More evaporation, more rain, more CO2 taken out of the atmosphere (ends up in rocks) • If it cools off: • Less evaporation, less rain, CO2 released from volcanoes allowed to build up

  32. Carbon Dioxide Cycle • Example: Snowball Earth

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