1 the terrestrial planets ii venus and mars. 2 topics l atmospheres l planetary atmospheres l earth...
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AtmospheresAtmospheres
Atmospheres Consist of atomsatoms and moleculesmolecules moving about in
random directions, that is, gasesgases.
Pressure Is the force per unit area. It arises from collisions
between the atoms and molecules
Temperature Is a measure of the average kinetic energykinetic energy of the
particles, that is, the energy of motion.
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Energies of Gas MoleculesEnergies of Gas Molecules
Kinetic energy (m = mass, v = speed)
mv2 / 2
The average kinetic energy per gas molecule is
3kT / 2
kk = 1.381 x 101.381 x 10-23-23 J / K J / K is Boltzman’s constant T T is the absolute temperature in Kelvin
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TemperatureTemperature
In science temperature is measured in Kelvin (K) The scale starts at the lowest possible
temperature, called absolute zeroabsolute zero 0 0C corresponds to 273 K 27 0C corresponds to 300 K 100 0C corresponds to 373 K
Temperatures in the universe range from 3 K to several billion K
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What is the Speed of Gas Molecules?What is the Speed of Gas Molecules?
Speed of Molecules Equate the formula for
kinetic energy to that for the average kinetic energy per molecule
Example: At T=290 KT=290 K oxygen molecules move at ~ 500 m/s500 m/s
Lower mass molecules move faster than higher mass ones
kTmv 232
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m
kTv
3
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Escape of Planetary AtmospheresEscape of Planetary Atmospheres
The Escape Speed The escape speed, V = V = √(√(2GM/R)2GM/R), is the minimum
initialinitial speed required for an object to escape from another of mass MM and radius RR.
For Earth, this speed is 11km/s11km/s.
Therefore, if a molecule is moving upwards at a speed greater than 11 km/s, then unless it is impeded it will escape into space.
We expect that planets, like Earth, with large escape speeds will have denser atmospheres than those with low escape speeds such as Mars.
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Planetary AtmospheresPlanetary Atmospheres
Terrestrial Planets The atmospheres of the TerrestrialTerrestrial planets are rather
different from that of the Sun, which is composed mostly of hydrogen and helium.
It is likely that these planets began with hydrogen-rich atmospheres, called primary atmospheresprimary atmospheres, which somehow were replaced by those present today, called secondary atmospheressecondary atmospheres.
Jovian Planets Jupiter and Saturn retained their primary
atmospheres, while Venus, Earth and Mars did not. This suggests that a planet’s mass is an important
factor in determining a planet’s atmospheric composition.
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Observed Atmospheric CompositionsObserved Atmospheric Compositions
Venus Earth Mars
Surface Pressure (bars) 92 1.0 0.007
Surface Temperature (K) 737 288 210
Carbon dioxide (%) 96.5 0.033 95.3
Nitrogen (%) 3.5 78.178.1 2.7
Oxygen (%) 0.0 20.920.9 0.13
Water (%) 0.02 0.1 to 3 0.03
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Atmospheric Compositions Atmospheric Compositions If All If All COCO22 was Gaseouswas Gaseous
Venus Earth Mars
Carbon dioxide (%) 96.5 98.5 95.3
Nitrogen (%) 3.5 1.1 2.7
Oxygen (%) 0.0 0.3 0.13
Other (%) 0.5 0.1 2.17
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Why are Venus and Earth so Different?Why are Venus and Earth so Different?
Venus and Earth They have similar masses and radii, but their
atmospheres are quite different.
Volcanism On both planets, volcanic activity released huge
amounts of COCO22 and HH22OO to form their primordial primordial secondary atmospheressecondary atmospheres.
Distance from Sun But Venus is closer to the Sun than Earth and so
receives more energy per unit area. The difference in heating could explain the different evolution of the two atmospheres.
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Evolution of AtmospheresEvolution of Atmospheres
Earth Water rained down to form the oceans Erosion and water chemistry removed some of the
CO2 from the atmosphere to form carbonates (in rocks such as limestone)
The evolution of life further altered the atmosphere
Venus Being hotter, water re-evaporated, leaving the
surface very dry and the CO2 and water still in the atmosphere. This triggered a runaway greenhouse effect
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The Greenhouse EffectThe Greenhouse Effect
Sunlight When sunlight falls on an object the latter
heats up and radiates some of the energy as infrared radiation.
Infrared Radiation Infrared radiation cannot pass easily through
certain substances, such as carbon dioxide.
Therefore, carbon dioxide in an atmosphere tends to trap heat near a planet’s surface.
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The Greenhouse Effect – IIThe Greenhouse Effect – II
Greenhouse Effect is Good! Were it not for the greenhouse effect, the
Earth would be about 3333oo C C cooler.
But! If the concentration of carbon dioxide is too
great this triggers a runaway greenhouse runaway greenhouse effecteffect. Venus, with its dense atmosphere of carbon dioxide, appears to have suffered a greenhouse catastrophe that has pushed its surface temperature to about 750 K750 K.
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MarsMars
1877 Giovanni Schiaparelli announced he saw canalicanali on Mars. This
was mistranslated from Italian into English as canals.canals.
1892 The Percival Lowell built an observatory at Mars Hill, Flagstaff
Arizona to look for life on Mars.
1897 H.G. Wells published the novel The War of the WorldsThe War of the Worlds.
1938 Just before the outbreak of the Second World War a broadcast
by Orson Welles frightened millions of Americans who believed that Martians were in fact invading!
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MarsMars
1907 – Alfred Russell Wallace critiqued Lowell's work:
“Any attempt to make that scanty surplus [of water], by means of overflowing canals, travel across the equator into the opposite hemisphere, through such terrible desert regions and exposed to such a cloudless sky as Mr. Lowell describes, would be the work of a body of madmen rather than of intelligent beings. It may be safely asserted that not one drop of water would escape evaporation or insoak at even a hundred miles from its source.”
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MarsMars
1976July 20, Viking 1 landed on Mars after a
one-and-a-half year journey of about 100 million km.