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The Terrestrial PlanetsThe Terrestrial PlanetsIIII

Venus and Mars

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TopicsTopics

Atmospheres Planetary Atmospheres

Earth Venus

Mars

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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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Fig. 6-41, p.122

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Fig. 6-43, p.123

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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.

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Fig. 6-48a, p.127

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From Near Space To DeepFrom Near Space To Deep

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Fig. 6-52, p.129

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Fig. 6-54, p.130