AST 111 Lecture 19(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

Moon and Mercury

Atmosphere persists because:◦ Micro-meteorite impacts◦ Solar wind trapped by weak mag field

Surface atoms set free

Moon and Mercury

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

Mars’ Atmosphere

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

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

Mars’ Atmosphere

These winds cause dust storms over the whole surface!

Previous astronomers thought they saw changes in vegetation!

Mars’ Atmosphere

Mars’ Atmosphere

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??

Mars’ Atmosphere

Mars’ Atmosphere

How?

Core temperature dropped

Became geologically dead

Volcanoes stopped

Due to low gravity on Mars, some atmosphere escaped◦ Not replenished by volcanoes

Mars’ Atmosphere

Mars’ Atmosphere

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

Thick CO2 atmosphere

◦ 96.5% CO2

Volcanoes pump CO2 into the atmosphere

◦ Venus is large so remains geologically active

Atmosphere of Venus

Atmosphere of Venus

With that in mind, why is Venus so inhospitable?

Atmosphere of Venus

Venus’ atmosphere creates an extreme greenhouse effect due to the massive

amounts of CO2 in its atmosphere.

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.”

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

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)

Atmosphere of Venus

Atmosphere of Venus

So why does Venus have somuch carbon dioxide in its

atmosphere?

(It does have volcanoes, but so doesEarth!)

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!

Atmosphere of Venus

Why does Venus not have any oceans?

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?

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

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

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?”

Atmosphere of Venus

Temperature low enough for water to condense◦ So we have oceans

Carbon dioxide dissolved in the oceans◦ Right balance of

greenhouse gases

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

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

Atmosphere of Earth

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!

Stability of Earth’s Climate

Carbon Dioxide Cycle

(page 326)

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

Carbon Dioxide Cycle

Example: Snowball Earth

Carbon Dioxide Cycle