Planetary atmospheres

 

Primary Atmosphere

–      Gas captured during the formation of the planet

–      Terrestrial planets likely lost theirs quickly

 

Secondary Atmosphere

-        Gas released during internal differentiation of Terrestrial planets

       Volcanoes – CO2, H20

-        Impacts of comets

AST0702.swf

 

What it takes to hang on to an atmosphere

Atmospheres are lost from the top.

3 primary factors:

1) Gravitational attraction of planet – bigger is better

2) Speed (related to T) of the molecule – hotter is faster

3) Mass of the molecule – lighter gets lost

(points 2+3 > kinetic energy, see excursion 7.1)

 

In order of increasing atmosphere:

Mercury, Mars, Earth, Venus

 

Earth and Venus: Life vs. The Greenhouse effect

Early atmospheres rich in CO2

 

Earth:

Life responsible for rise in O2

AST0705.swf

CO2 into oceans > limestone

moderate greenhouse effect (35K warmer)

Atmospheric generalities - AST0706.jpg

 

Venus:

Lost water early = lost ability to remove CO2 from atmosphere

Lots of CO2, H2O vapour = runaway greenhouse effect (surface T≈ 460° C)

AST0703.swf

 

We’ll look at atmospheres of individual planets when we discuss them in detail