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Terrestrial World Atmospheres
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Some terrestrial worlds have atmospheres. Others don't.
Venus and Mars have thick/thin atmospheres respectively made mostly of
Carbon Dioxide.
Earth is the oddball with a largely Nitrogen atmosphere with a substantial
fraction of Oxygen.
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Why Does the Earth have an Atmosphere?
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Two factors are important
Mercury
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Strong gravity sets a high escape velocity for gas molecules.
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“Low” temperatures (relative to gravity) means gas molecules are moving
slowly enough not to escape.
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Consider Mercury vs. Saturn's Moon Titan
Titan
Note that Mercury is much denser so it's surface gravity/escape
velocity is larger as well.
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Why Does the Earth have an Atmosphere?
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Two factors are important

Strong gravity sets a high escape velocity for gas molecules.

“Low” temperatures (relative to gravity) means gas molecules are moving
slowly enough not to escape.

Consider Mercury vs. Saturn's Moon Titan
Quantitatively...
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escape velocity depends on a planet's mass and radius
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gas motion depends on temperature (and the mass of the atom/molecule)
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Hydrogen escapes more easily than carbon dioxide or nitrogen.
2GM planet
V esc =
R planet
2kT
V thermal =
M molecule
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Why Does the Earth have an Atmosphere?
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Where Do Atmospheres Come From?
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Outgassing:
Water and gases
baked out from
within.
Impacts: Water
and gases
delivered from
outside (comets
in particular)
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A Dynamic Solar System
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With computer simulation we can now look at how the distribution of planets
in the Solar System may have evolved over time.
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The distribution of planets today (and even possibly their order) has not
always been that way.
A fairly crazy sounding scenario, that is now becoming more and more
accepted as the “way things happened” explains
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The present day distribution of the major planets
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Why the Moon appears to have been bombarded by large objects shortly
after the crust had solidified, making the Maria.
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Why the Earth – a fairly dry planet – had abundant water and gasses
delivered to its surface.
Basic premise
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Originally all of the major planets were closer to the Sun and there was a
substantial belt of icy outer planetoids/asteroids.
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The gravitational interactions between Jupiter and Saturn reorganized the
orbits of the outer planets and flushed the outer iceballs onto chaotic orbits
which, among other things, bombarded the inner planets.
Watch this...
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A Thin Atmosphere
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A Thin Atmosphere
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Atmospheric Composition on Earth
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78% Nitrogen
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21% Oxygen
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1% other trace gasses
including argon, water
vapor, carbon dioxide,
methane...
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Why So Different?
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Venus and Mars have atmospheres consisting almost entirely of Carbon
Dioxide.

The atmosphere of Venus is nearly 100 times more massive than the
atmosphere of the Earth.
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Mars has an atmosphere 100 times thinner than Earth's, but still mostly made
of CO2
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Two Processes Modified Earth's Atmosphere
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Removal of carbon dioxide by dissolution in liquid water
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Tens of atmospheres of carbon dioxide have been deposited as carbonate
rocks under previous and current seas.
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Two Processes Modified Earth's Atmosphere
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Contamination of the atmosphere with a corrosive pollutant.
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Plant life has increasingly increased the abundance of oxygen.
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Oxygen is so reactive, it must be continuously replenished.
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Two Processes Modified Earth's Atmosphere
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Contamination of the atmosphere with a corrosive pollutant.
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600 million years ago the oxygen level was only 1% of the atmosphere.
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Currently levels of oxygen were reached only in the last 200-300 million
years (the most recent 5% of Earth history).
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Two Processes Modified Earth's Atmosphere
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Biology continues to modify the Earth's atmosphere in an interesting way.
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Two Processes Modified Earth's Atmosphere
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Biology continues to modify the Earth's atmosphere in an interesting way.