1. (Terrestrial) Planetary Atmospheres II

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

3.  Atmosphere persists because: ◦ Micro-meteorite impacts ◦ Solar wind trapped by weak mag field  Surface atoms set free

4.  Low atmospheric pressure ◦ Mars is small, so less gravity ◦ Liquid water is unstable  Mostly CO 2 ◦ 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 has very extreme seasons ◦ Ellipticity is great enough to influence seasons

6.  Southern hemisphere has hottest summers and coldest winters  Summer pole sublimates while winter pole condenses ◦ This drives winds from summer to winter pole

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

8. Dust Devils

9.  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. How?

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

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

13.  Thick CO 2 atmosphere ◦ 96.5% CO 2  Volcanoes pump CO 2 into the atmosphere ◦ Venus is large so remains geologically active

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

15. Venus’ atmosphere creates an extreme greenhouse effect due to the massive amounts of CO 2 in its atmosphere.

16.  Atmospheric pressure at surface: ◦ 90x that of Earth (LOTS of CO 2 )  Density 10% that of water  “…would feel like a cross between swimming and flying.”

17.  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.  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. So why does Venus have so much carbon dioxide in its atmosphere? (It does have volcanoes, but so does Earth!)

20. Because a planet needs oceans to dissolve CO 2. Venus does not have oceans. Earth has about the same amount of CO 2 as Venus– but most of it is locked away in rocks in the ocean!

21. Why does Venus not have any oceans?

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

23.  Average temperature up by 30 o C  More evaporation  More greenhouse gases, higher temp.  More evaporation  Etc…  Runaway Greenhouse Effect: oceans fueled their own evaporation, did not dissolve and store CO 2 in rocks

24.  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 CO 2 on the surface or in the atmosphere

25.  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.  Temperature low enough for water to condense ◦ So we have oceans  Carbon dioxide dissolved in the oceans ◦ Right balance of greenhouse gases

27.  Nitrogen (77%) and oxygen (21%)  Why so much nitrogen?  Volcanoes outgas CO 2, water, and nitrogen ◦ Water into oceans ◦ CO 2 into rocks ◦ Nitrogen went up

28.  Why so much oxygen?  It came from (and still does) photosynthesis and CO 2 ◦ Air only breathable in last few hundred million years

29.  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. (page 326)

31.  It is stable. ◦ If it warms up:  More evaporation, more rain, more CO 2 taken out of the atmosphere (ends up in rocks) ◦ If it cools off:  Less evaporation, less rain, CO 2 released from volcanoes allowed to build up

32.  Example: Snowball Earth