1
Terrestrial World Atmospheres
●
●
●
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.
2
Why Does the Earth have an Atmosphere?
●
Two factors are important
Mercury
–
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
Titan
Note that Mercury is much denser so it's surface
gravity/escape velocity is larger as well.
3
Why Does the Earth have an Atmosphere?
●
●
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...
–
escape velocity depends on a planet's mass and radius
–
gas motion depends on temperature (and the mass of the
atom/molecule)
●
√
Hydrogen escapes more easily than carbon dioxide or
nitrogen.
2GM planet
V esc =
R planet
√
2kT
V thermal =
M molecule
4
Why Does the Earth have an Atmosphere?
5
Where Do Atmospheres Come From?
●
●
Outgassing:
Water and
gases baked
out from within.
Impacts: Water
and gases
delivered from
outside
(comets in
particular)
6
A Dynamic Solar System
●
With computer simulation we can now look at how the distribution
of planets in the Solar System may have evolved over time.
–
●
●
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
–
The present day distribution of the major planets
–
Why the Moon appears to have been bombarded by large objects
shortly after the crust had solidified, making the Maria.
–
Why the Earth – a fairly dry planet – had abundant water and
gasses delivered to its surface.
Basic premise
–
Originally all of the major planets were closer to the Sun and there
was a substantial belt of icy outer planetoids/asteroids.
–
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...
7
A Thin Atmosphere
8
A Thin Atmosphere
9
Atmospheric Composition on Earth
–
78% Nitrogen
–
21% Oxygen
–
1% other trace
gasses including
argon, water vapor,
carbon dioxide,
methane...
10
Why So Different?
●
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.
–
Mars has an atmosphere 100 times thinner than Earth's, but still
mostly made of CO2
11
Two Processes Modified Earth's Atmosphere
●
Removal of carbon dioxide by dissolution in liquid water
–
Tens of atmospheres of carbon dioxide have been deposited as
carbonate rocks under previous and current seas.
12
Two Processes Modified Earth's Atmosphere
●
Contamination of the atmosphere with a corrosive pollutant.
–
Plant life has increasingly increased the abundance of oxygen.
–
Oxygen is so reactive, it must be continuously replenished.
13
Two Processes Modified Earth's Atmosphere
●
Contamination of the atmosphere with a corrosive pollutant.
–
600 million years ago the oxygen level was only 1% of the
atmosphere.
–
Currently levels of oxygen were reached only in the last 200-300
million years (the most recent 5% of Earth history).
14
Two Processes Modified Earth's Atmosphere
●
Biology continues to modify the Earth's atmosphere in an
interesting way.
15
Two Processes Modified Earth's Atmosphere
●
Biology continues to modify the Earth's atmosphere in an
interesting way.