Planetary Atmospheres: the Terrestrial Worlds


Defining an atmsophere:
A survey of atmospheres:

World
 Composition
 Pressure
(bars)
 Weather?
 Clouds?
Mercury
 helium, sodium, oxygen
 10-14
 none
 none
Venus
 96% CO2
3.5% N2
 90
 slow winds, acid rain
 sulfuric acid clouds
Earth
 77% N2
21% O2
 1
 lots of it!
 H2O
Moon
 helium, sodium, argon
 10-14
 none
 none
Mars
 95% CO2
2.7% N2
 0.007
 winds, dust storms
 H2O, CO2, dust

What causes the variety?

The Structure of an Atmosphere

Structure: how temperature, density, and composition of an atmosphere change with height. It depends strongly on heating from the Sun.

First, think about a planet with no atmosphere. What determines its surface temperature?
World
 Sun Distance
(AU)
 Albedo
 Day Length
(Earth days)
 No Atmosphere Temp
 Actual Avg Temp
Mercury
 0.38
 0.11
 176
 440K
 100K - 700K
Venus
 0.72
 0.72
 117
 230K
 740K
Earth
 1.00
 0.36
 1
 250K
 288K
Moon
 1.00
 0.07
 28
 273K
 100K - 400K
Mars
 1.52
 0.25
 1
 218K
 223K



The structure of a planetary atmosphere is determined in large part by how it interacts with the Sun's light:
The structure of the Earth's atmosphere


Visible light at the Earth's surface:
Infrared light and the greenhouse effect:
The Earth's surface heats up from the Sun's visible light, and emits thermal radiation which peaks in the infrared. Some of this infrared radiation is trapped by greenhouse gases in the Earth's atmosphere.
  • Why is this called a greenhouse effect?
  • Why are clear nights often the coldest?
  • What happens if you increase the amount of greenhouse gases?


Ultraviolet light and the stratosphere:

Ultraviolet light is absorbed in the stratosphere by ozone (O3) -- ozone is dissociated and the ions gain kinetic energy, ie the atmosphere is heated here.
  • What would the temperatures be like if there were no ozone?
  • What would we be like if there were no ozone?

Higher up, there isn't enough ozone to absorb UV light, so the temperature drops again as you move higher.
 X-rays and the thermosphere:
X-rays are very easily absorbed by most any atom or molecule, so the Sun's X-rays are pretty much absorbed as soon as they enter the upper atmosphere.

When atoms absorb X-rays, they become ionized -- ie lots of free electrons are running around up there (in the ionosphere). The ionosphere reflects radio waves, so we can communicate by bouncing signals off the ionosphere.