The Earth's Atmosphere and Oceans
- A thin coating of liquid water covers much of the Earth's surface.
- The Earth's retains a thin atmosphere which consists of
- 78% Nitrogen, 21% Oxygen, 1% Other Stuff
- This mix is very different from other inner planets where
atmospheres consist mainly of CO2.
- Where did the atmosphere and water come from?
- Outgassing from within the planet??
- Volcanic activity today releases substantial water and
gas (mostly carbon dioxide) from the Earth's interior.
- This gas and water is being baked out of the rocks in
- This process has been ongoing throughout the history of the
Earth, and was presumably more active during the early
in Earth history.
- Could all of the atmosphere and oceans have been produced
in this way?,... maybe.
- Bombardment from outside???
- The Earth is continually pelted with debris left over from the formation of the Solar System.
- Such bombardment was heavy during the earliest
stages of solar system formation and has tapered off to a
much slower rate today.
- The impactors are largely either rock (asteroids or
pieces thereof) or water ice (comets).
- If only a few percent of the objects impacting
the Earth's surface since it was formed were comets,
then bombardment could have supplied all of the water in the oceans.
- Both impacts and outgassing probably contributed to the atmosphere and
- Regardless of the source, today the Earth's atmosphere differs
significantly from the other terrestrial planets.
- Liquid water oceans remove CO2
from the atmosphere. Life and other processes deposit this
CO2 as carbonate rocks (e.g. limestone).
- The Earth, like Venus,
probably began with a CO2 atmosphere almost
100 times more massive than the current day atmosphere.
- Virtually all of this CO2 was disolved in the oceans.
- After the CO2 has been removed
the next most abundant gas is Nitrogen.
- Oxygen gas is nowhere to be found in the emissions
from volcanoes (the oxygen is locked in molecules like CO2).
- Life produces oxygen - specifically plants consume
CO2 and release oxygen ( O2 ) via photosynthesis.
- Oxygen is a corrosive gas. If it were not
replenished by plants it would disappear from the atmosphere in
a very short time.
- The first oxidized rocks found in geological
date back only 1.2 billion years (quite recently compared to the
4.6 billion year age of the Earth).
- 600 million years ago oxygen constituted only 1% of the atmosphere.
- Biological activity, specifically plant life, has
increased the oxygen level to the current 20% of the Earth's
- In doing so, the plants created an ecological
niche for a new type of life which consumes the oxygen to derive
energy - the animals.
- The Earth cannot sustain more than about 20%
oxygen in the atmosphere. Otherwise spontaeous fires will occur that
will deplete the oxygen back to the 20% level.
In a Nutshell...
- Earth has strong enough gravity AND low enough temperature to retain an
- Volcanic activity introduced water and gasses (mainly CO2)
into Earth's original atmosphere.
- Impacts of comets also delivered water and gasses to the Earth's
- Water condensed to form oceans which removed the CO2 from
- The second most abundant gas, Nitrogen, was left behind as the
main constituent of the atmosphere.
- Oxygen was rare in Earth's atmosphere until only recently. Now it accouts for 20%.
- With the explosion of plant life in the last 1/2 billion years,
photosynthesis has contaminated the atmosphere with Oxygen creating
an ecological niche for animal life on the surface.
Revised November 5, 2003