How atmospheres are created

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How atmospheres are created

• Direct capture from solar nebula
• Outgassing
• Evaporation/sublimation
• Bombardment

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How atmospheric gasses are lost

• Thermal escape
• Stripping via solar wind/cosmic rays
• Atmospheric cratering
• Condensation
• Chemical reactions

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Thermal escape

• Escape velocity
• Temperature
• The mass of the gas particles

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Discussion
Even though the Earth and the Moon have very
similar composition, the Earth has an atmosphere
while the Moon does not. Why?
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The Moons atmosphere
Because the Moon has no global magnetic field,
solar wind particles constantly strike the
daylight side of the Moon, causing bits of the
surface to fly off. These particles can travel
10s of kilometers and come to rest when they get
to the night side of the Moon.
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The Moon may contain a large deposit of frozen
water
The deepest excavation on the Moon is the Aitken
basin which is over 12 km deep and lies near the
south pole. Because the Sun never shines in this
basin it remains perpetually cold and may contain
6 billion tons of ice, delivered from impacts
with comets.
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Mercurys atmosphere
Mercury is too hot and has too little surface
gravity to hold much of an atmosphere. But there
are trace amounts of oxygen, sodium, and helium.
These must be constantly replenished.
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Mercury Moon
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Discussion
What does the clumpy nature of Mercurys
atmosphere tell you about its origin?
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Mercury may also contain water ice?
Mercury has radar bright areas near the poles
which are very similar to reflections from
Jupiters moons and the polar ice caps on Mars.
Mercurys rotation axis is nearly perpendicular
to its orbit. Thus within 6.5 degrees of the
poles, crater floors are deep enough that the Sun
never shines there allowing temperatures to stay
below freezing.
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305 m Arecibo radio telescope
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Because some of the atmosphere is constantly
escaping into space, atmospheres have no edge.
They just get thinner and thinner.
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Discussion
Can the Earth hold hydrogen and helium gas in its
atmosphere? Explain how you know?
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Discussion
Is it possible that any of the other terrestrial
planets or the Moon can hold hydrogen or helium
gas in their atmospheres? Explain why.
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Early atmospheres
All of the planets started out with atmospheres
captured directly from the solar nebula
containing primarily hydrogen and helium. But
these gases were lost into space by thermal
escape.
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Each of the terrestrial planets then acquired an
atmosphere through a combination of bombardment
and volcanic outgassing. In the case of
Mercury and the Moon, this atmosphere was also
lost into space over time.
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On Earth volcanic eruptions outgas carbon
dioxide, water vapor and sulfur dioxide.
Volcanoes on Venus and Mars probably emit the
same gases.
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Discussion
If the terrestrial planets atmospheres came from
volcanic outgassing, why is Earths so different
from the atmospheres of Venus and Mars? Where
did all the nitrogen and oxygen in Earths
atmosphere come from?
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The origin of life on Earth
The oldest surviving rocks on Earth date from 3.8
billion years ago and show that life in the form
of blue-green bacteria already existed on Earth.
These bacteria used photosynthesis to convert CO2
into O2.
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Chemical reactions remove O2 from atmosphere
Bacteria extract oxygen from nitrates in the soil
and emits N2 into atmosphere. N2 is combined with
O2 by lightning to form nitrogen oxides. The
nitrogen oxides are then absorbed by rain water
and re-deposited in the soil.
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Rust
Iron compounds also like to combine with oxygen
molecules.
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Oxygen buildup
Approximately 2 billion years ago, oxygen could
no longer be absorbed chemically and started to
build up in atmosphere.
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Discussion
Venus has the thickest atmosphere of the
terrestrial planets. Would you expect this? Why
or why not?
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Structure of Earths atmosphere
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Troposphere
The layer of the atmosphere where convective
processes are dominate and most of the clouds and
weather take place.
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Circulation of heat
Because the Sun heats the equator more than the
polar regions, hot air rises near the equator,
travels toward the poles where it sinks back to
the surface and travels back toward the equator.
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Discussion
Suppose you are on a rotating merry-go-round
sitting near the center and you roll a ball
toward the edge. Describe how the ball moves as
it rolls outward. If you sat at the outer edge
and rolled the ball toward the center how would
it move?
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The Coriolis Effect
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Discussion
How does the rotation of Venus differ from that
of the Earth? How would you expect the
circulation of heat to differ on Venus?
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Because the rotation of Venus is so slow and the
atmosphere so thick, the Hadley cells are very
stable and efficient at circulating heat around
the planet. The result is a near constant
temperature from equator to pole and there are no
storms on Venus.
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Venus has huge troposphere, extending 60 km above
the surface.
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Between 30 and 60 km above the surface there are
a number of cloud layers. These are composed
primarily of sulfuric acid (H2SO4) droplets.
It is too hot to rain sulfuric acid on Venus.
Below 30 km the atmosphere is clear on Venus
and wind speeds measured by the Verona landers
were 2-11 miles per hour.
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It snows on Venus
The tops of high mountains on Venus are coated
with a shiny substance, much like high peaks on
Earth covered with snow. On Venus, which is
too hot for water snow, it snows lead and
magnesium sulfides which collect on high mountain
peaks which are cooler than the surface.
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Discussion
In general, temperature falls with height. Why
does the temperature rise in Earths stratosphere
and thermosphere?
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Discussion
Why doesnt the temperature rise in the
stratosphere of Venus, (or more precisely why
doesnt Venus have a stratosphere)?
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Verona 13
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Discussion
Why are the pictures from Verona 13 red?