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ASTRONOMY 202 Spring 2007: Solar System Exploratio n
Instructor: Dr. David Alexander Web-site: w
ww.ruf.rice.edu/~dalex/ASTR202_S07
Class 28: Planetary Atmospheres [3/28/07]
� Announcements
� Atmospheric Structure
� Weather and Climate
� Global Wind Patterns� Precipitation� Long-term climate
change
� Gain and Loss of Atmospheres
� Retaining a planet’s atmosphere� Losing and atmosphere
� Comparative Climates
� Martian Weather� Venusian Weather� Importance for Earth
Now Playing: Airbag - Radiohead
Chapter 10Chapter 10
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Announcements
Homework
Homework #6 is due now please
Homework #7 is available online and due Mon Apr 2
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Atmosphere
All the planets of the Solar System have an atmosphere to some
degree.
Life would be virtually impossible without an atmosphere. Apart
from providing Oxygen to breathe, atmospheres protect a planet from
harmful UV radiation from the Sun and collisions from
micrometeorites, regulate the temperature of the planet, and
provides rain giving clouds.
Venus
Earth
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Terrestrial Atmospheres
The atmospheres of the terrestrial worlds vary in their
composition, density and pressure. It is the atmospheric pressure
that generally defines the main characteristic of an
atmosphere.
Unit of pressure is the bar : 1 bar is equivalent to 14.7 pounds
per square inch at sea level on Earth
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Role of an Atmosphere
Get better picture of Earth’s atmosphere
2/3 of Earth’s atmosphere lies within 10 km
but can have an impact on
satellites as high as several
hundreds of kilometres.
Atmospheres provide a crucial function in the development of a
planet’s geology and more importantly on its ability to sustain
life.
• Atmospheres make planet surfaces warmer(Greenhouse Effect)
• Atmospheres absorb and scatter light (including solar UV and
X-rays)
• Atmospheres create pressure (allowing liquid water to form
)
• Atmospheres create wind and weather (controlling long-term
climate changes)
• Atmospheres can interact with planetarymagnetic fieldscreating
magnetospheres
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The Greenhouse Effect
The most important effect of an atmosphere is to regulate the
surface temperature of the planet. It does this via the Greenhouse
Effect.
Not all gases absorb infrared radiation.
The main Greenhouse gases are:
Water vapour (H2O)
Carbon Dioxide (CO2)
Methane (CH4)
Molecules comprised of different elements are more efficient
absorbers of infrared radiation
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The Greenhouse Effect
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Greenhouse Effect on Terrestrial Planets
Without a Greenhouse Effect, the balance of energy input and
output of a planet would result in much colder surface temperatures
since the radiated energy escapes completely.
No atmosphere: Temperature regulated by distance from Sun and
reflectivity of the planet (albedo).
