(Terrestrial) Planetary Atmospheres II

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

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

 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

 Mars has very extreme seasons ◦ Ellipticity is great enough to influence seasons

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

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

Dust Devils

 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??

How?

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

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

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

With that in mind, why is Venus so inhospitable?

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

 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.”

 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

 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)

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

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!

Why does Venus not have any oceans?

 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?

 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

 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

 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?”

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

 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

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

 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!

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 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

 Example: Snowball Earth