Schedule

1
Planetary Atmospheres
2
The Atmospheres of the Solar System
3
Formation of an Atmosphere

• When a terrestrial planet enters its flooding
  stage, gases trapped inside the planet during
  formation (or created as a result of radioactive
  decay) will be outgassed. These gases include
• H2, He, H2O, N2, CO2, and probably CH4 and NH3
• As the planet cools, water vapor condenses out of
  the atmosphere and falls as rain. Oceans form.
  But will the planet be able to keep this
  atmosphere?

4
Formation of an Atmosphere
5

• http//www2.biglobe.ne.jp/norimari/science/JavaAp
  p/Mole/e-gas.html

6
What determines if a planet will keep an
atmosphere?

• Planet mass a planet with higher mass exerts
  more gravity and can hold onto an atmosphere more
  easily

Gas mass gas that are lighter (like H and He)
escape the planets gravity more easily Gas
temperature if the temperature is higher, the
gas particles are moving faster, and can escape
the planets gravity
Its easier for a heavy body to hold onto
heavy/cold gases.
7
Mercury versus Titan

• Mercury and Titan are both low-mass bodies. But
  
• Mercury is close to the Sun, so it is hot. Its
  gravity is not strong enough to keep its gases
  from escaping into space.
• Titan is in the outer solar system and is cold.
  The molecules are moving slowly, so the moon can
  hang onto its atmosphere (except for the lightest
  gases of H2 and He).

8
The Atmosphere of Mars

• The composition of Mars atmosphere is determined
  by
• The mass of the planet. Since Mars is only about
  0.1 M?, it does not have the gravity to hold onto
  H2 and He. It can barely hold onto N2.
• Proximity to the Sun. Gases such as CH4 and NH3
  are destroyed by ultraviolet light. Mars
  atmosphere is not thick enough to shield itself
  from ultraviolet photons.
• Chemistry. Oxygen (O2) reacts with almost
  anything (i.e., minerals in rocks), so it cannot
  stay free.
• Consequently, Mars atmosphere is primarily CO2
  with a little bit of N2.

9
Carbon-Dioxide and Mars

• Mars pole is tipped 24 from the ecliptic. It
  therefore undergoes seasons, just like the Earth.
  In winter at the pole, CO2 freezes out and
  becomes dry ice. In summer, this ice evaporates
  and becomes part of the atmosphere. This cycle
  produces strong winds and dust storms.

10
Venus and Earth

• The similarities
• The planets have similar masses (0.82 M? versus
  1.0 M?)
• The planets have similar densities (4.2 versus
  5.5)
• The planets distances from the Sun are similar
  (0.72 A.U. versus 1.0 A.U.)
• Neither planet can hold onto light gases (H2 and
  He)
• Neither planet can keep large amounts of CH4 and
  NH3 in its atmosphere (due to ultraviolet light
  from the Sun)
• The main difference
• The Earths temperature is between -50 C and
  50 C, while Venus temperature is 470 C

11
Properties of Carbon-Dioxide

• CO2 has two interesting properties
• CO2 dissolves into liquid water (H2O) to create
  H2CO3 (carbonic acid). Carbonic acid (i.e., the
  fizz in soda) then reacts with any number of
  minerals. For instance
• H2CO3 Ca ? H2 CaCO3 (limestone)
• The result is that, if liquid water is around,
  CO2 will be removed from the air, and locked up
  in rocks.
• CO2 is a greenhouse gas. It is transparent to
  optical light, but it absorbs infrared light. So
  sunlight can make it through CO2, but the heat it
  brings cannot get out.

12
Properties of Carbon-Dioxide
13
Runaway Greenhouse Effect

• Venus and Earth both started out with similar
  atmospheres. But because Venus is slightly
  closer to the Sun
• Venus was a bit warmer, and had a bit less liquid
  water
• With less liquid water, less CO2 dissolved away
• With more CO2 in the atmosphere, the greenhouse
  effect was more effective
• The warmer temperature caused more water to
  evaporate
• With even less liquid water, even less CO2
  dissolved away
• As all the water evaporated, and the temperature
  increased, outgassing of greenhouse gases (CO2
  and CH4) became easier, CO2 was baked out of
  the rocks
• Ultraviolet light destroyed the CH4, NH3, and H2O
  in the atmosphere, leaving a thick atmosphere of
  CO2

14
The Atmosphere of Earth
Venus and Earth both started out with similar
atmospheres. But because the Earth is slightly
farther away from the Sun

• Earth was a bit cooler, and had a bit more liquid
  water
• With more liquid water, more CO2 dissolved away
• With less CO2 in the atmosphere, the greenhouse
  effect was less effective
• With more liquid water and a comfortable
  environment, photosynthetic life developed
• Photosynthesis removed even more CO2 from the
  atmosphere, replacing it with O2
• Lightning plus atmospheric O2 created ozone,
  which shielded the Earth from ultraviolet light.
  Water molecules in the atmosphere survived longer
  (along with life)

15
Today on Earth and Venus

• A small change in the conditions now can lead to
  large changes later on!

16
The Structure of Jupiters Atmosphere

• Since the inside of Jupiter is hot (due to the
  pressure), while the cloud tops are cool, the
  composition of the atmosphere changes with depth.

17
The Structure of Jupiters Atmosphere
18
Jupiters Trade Winds

• Jupiters equator is moving faster than the poles
  (it has farther to go in a day). This drives a
  network of very strong winds and storms.

19
Summary

• Planetary Atmospheres
• Jupiter very strong winds and storms
• Mercury vs. Titan easier to hold cold/heavy
  gases
• Mars thin, mostly CO2
• Venus vs. Earth greenhouse properties of CO2