© sierra college astronomy department 1 atmospheres of the terrestrial planets

© Sierra College Astronomy Departmen© Sierra College Astronomy Departmentt

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Atmospheres of the Atmospheres of the Terrestrial PlanetsTerrestrial Planets


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Differences between the planetsDifferences between the planetsMoon and Mercury have virtually no Moon and Mercury have virtually no atmosphere.atmosphere.

Venus has a thick carbon dioxide atmosphere Venus has a thick carbon dioxide atmosphere which completely shrouds the planet.which completely shrouds the planet.

Mars also has a carbon dioxide atmosphere, Mars also has a carbon dioxide atmosphere, but it is very thin.but it is very thin.

Earth’s atmosphere is hospitable for life and is Earth’s atmosphere is hospitable for life and is made mostly of nitrogen and oxygen. It also made mostly of nitrogen and oxygen. It also has varied weather patterns.has varied weather patterns.

Lecture 10: Atmospheres of the Terrestrial WorldsLecture 10: Atmospheres of the Terrestrial Worlds

Atmosphere BasicsAtmosphere Basics

Table10.1

Terrestrialworlds


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What is an atmosphere?What is an atmosphere?For the terrestrial planets it’s a very thin For the terrestrial planets it’s a very thin layer of air which surrounds the planetslayer of air which surrounds the planetsFor the Earth, the atmosphere would be For the Earth, the atmosphere would be as thick as a one dollar bill on a standard as thick as a one dollar bill on a standard globe.globe.The The temperature of thetemperature of the atmosphere of the atmosphere of the terrestrial terrestrial planetsplanets is low enough to is low enough to contain molecular nitrogen (Ncontain molecular nitrogen (N22) and ) and compounds like water (Hcompounds like water (H22O)O)



Thinlayer


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Atmospheric pressureAtmospheric pressureCollisions of atoms and molecules create Collisions of atoms and molecules create pressure – force per unit area, that pressure – force per unit area, that pushes in all directions.pushes in all directions.

Molecules in the room are going 500 m/s.Molecules in the room are going 500 m/s.

Air pressure depends directly on the Air pressure depends directly on the speed of the molecules and the volume in speed of the molecules and the volume in which they are contained.which they are contained.



Balloonanalogy


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Atmospheric pressureAtmospheric pressureGas around a planet is held down by gravity and the Gas around a planet is held down by gravity and the atmospheric pressureatmospheric pressure is highest at the surface from is highest at the surface from the layers of gas above.the layers of gas above.Planetary atmospheres are in a perpetual balance Planetary atmospheres are in a perpetual balance between the downward weight of their gases and the between the downward weight of their gases and the upward push of their gas pressureupward push of their gas pressureWe use the We use the barbar, roughly equal to the Earth’s , roughly equal to the Earth’s atmospheric pressure (14.7 lbs/inatmospheric pressure (14.7 lbs/in22), as the unit of ), as the unit of atmospheric pressure.atmospheric pressure.This “weight” is hardly felt by us because in comes on This “weight” is hardly felt by us because in comes on all directions and is countered by the pressure of our all directions and is countered by the pressure of our bodies outward.bodies outward.



Pillows


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Where does the atmosphere end?Where does the atmosphere end?The atmosphere of the Earth does not The atmosphere of the Earth does not suddenly but thins out with increasing altitude.suddenly but thins out with increasing altitude.On the Earth the atmosphere get so thin at 60 On the Earth the atmosphere get so thin at 60 km that collisions between molecules are rare km that collisions between molecules are rare and the sky would look black even in daytimeand the sky would look black even in daytime

– This is often described as the edge of space.This is often described as the edge of space.

There is still some air above this 60-km level as There is still some air above this 60-km level as satellites feel the atmospheric drag and satellites feel the atmospheric drag and eventuallyeventually spiral downward without periodic spiral downward without periodic boosts.boosts.



Pillows


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How atmospheres affect planetsHow atmospheres affect planetsAtmospheres can determine whether Atmospheres can determine whether something can exist as a liquid on the surface something can exist as a liquid on the surface such as watersuch as waterAtmospheres absorb and scatter light.Atmospheres absorb and scatter light.Atmospheres can create wind and weatherAtmospheres can create wind and weatherInteractions between atmospheric gases, the Interactions between atmospheric gases, the solar wind and the magnetic field of the planet solar wind and the magnetic field of the planet can form a protective can form a protective magnetospheremagnetosphere around around the planetthe planetSome atmospheric gases can make the Some atmospheric gases can make the surface of the planet warmer via the surface of the planet warmer via the greenhouse effectgreenhouse effect




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Earth’s AtmosphereEarth’s Atmosphere

Greenhouse effectGreenhouse effectThe The greenhouse effectgreenhouse effect is the atmosphere’s process of is the atmosphere’s process of “trapping” heat emanating from the ground“trapping” heat emanating from the ground (originally (originally captured from the sun).captured from the sun).– The earth get heated up by sunlightThe earth get heated up by sunlight– The warmed earth radiates back in the infrared (i.e. via The warmed earth radiates back in the infrared (i.e. via

Wien’s Law)Wien’s Law)– The atmosphere absorbs this infrared radiation becomes The atmosphere absorbs this infrared radiation becomes

another source of heat (in addition to the sun)another source of heat (in addition to the sun)Common greenhouse gases: carbon dioxide, water, methaneCommon greenhouse gases: carbon dioxide, water, methane

– The natural amount of greenhouse gases allows the earth The natural amount of greenhouse gases allows the earth to be warm enough for water to remain liquidto be warm enough for water to remain liquid

– The increase of greenhouse gases such as carbon The increase of greenhouse gases such as carbon dioxide may increase the temperature of our planet to too dioxide may increase the temperature of our planet to too high of levelshigh of levels

Greenhouse

Greenhouse2

Wien


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Earth’s AtmosphereEarth’s Atmosphere

Without the Greenhouse effect ….Without the Greenhouse effect ….If no greenhouse gases are present in the If no greenhouse gases are present in the atmosphere the temperature of a planet would be atmosphere the temperature of a planet would be cooler and would depend on 2 things:cooler and would depend on 2 things:– The planet’s distance from the SunThe planet’s distance from the Sun– The planet’s overall reflectivity or The planet’s overall reflectivity or albedoalbedo

The global, non-greenhouse temperatures of The global, non-greenhouse temperatures of terrestrial worlds are given in Table 10.2terrestrial worlds are given in Table 10.2– The Earth would be too cold from liquid water to existThe Earth would be too cold from liquid water to exist

See Mathematical Insight 10.1See Mathematical Insight 10.1

NoGreenhouseTable 10-2


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Troposphere (0-10 km)Troposphere (0-10 km)– Livable zone, cloud formationLivable zone, cloud formation

Stratosphere (10-80 km)Stratosphere (10-80 km)– Lower stratosphere contains the jet streamLower stratosphere contains the jet stream– Mid-stratosphere contains ozone layerMid-stratosphere contains ozone layer

Thermosphere (80-110 km)Thermosphere (80-110 km)– AM radio wave bounces off upper part of AM radio wave bounces off upper part of

thermospherethermosphere

Exosphere (above 110 km)Exosphere (above 110 km)– This is where atmosphere fades into spaceThis is where atmosphere fades into space


Atmospheric StructureAtmospheric Structure Profile


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Interactions between light and atmospheric gasesInteractions between light and atmospheric gases

Different types of light do different things to particles of Different types of light do different things to particles of gas:gas:– X-raysX-rays have enough energy to ionize almost any atom or have enough energy to ionize almost any atom or

moleculemolecule

– UVUV can break apart some molecules such as water (H can break apart some molecules such as water (H22O) and O) and

ozone (Oozone (O33))

– VisibleVisible light passes through the atmosphere without being light passes through the atmosphere without being absorbed but some are absorbed but some are scatteredscattered

– InfraredInfrared light can be absorbed by greenhouse gases because light can be absorbed by greenhouse gases because they are easily rotated or vibratedthey are easily rotated or vibrated


Atmospheric StructureAtmospheric Structure interactions

EM spec


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On the Earth we see these effectsOn the Earth we see these effects

The The TroposphereTroposphere is warmed by absorption of infrared is warmed by absorption of infrared light light coming from the groundcoming from the ground by greenhouse gases by greenhouse gases– Promotes convection in our atmosphere because of temperature Promotes convection in our atmosphere because of temperature

gradientgradient

TheThe Stratosphere Stratosphere only absorbs incoming sunlight only absorbs incoming sunlight– The ozone layer absorbs UV photons before they reach the The ozone layer absorbs UV photons before they reach the

bottom of the stratospherebottom of the stratosphere– This makes the stratosphere warmer with increasing altitude until This makes the stratosphere warmer with increasing altitude until

we reach about 50 km.we reach about 50 km.– Convection is therefore not present and air is stagnant there.Convection is therefore not present and air is stagnant there.



Profile


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The The ThermosphereThermosphere absorbs X-rays coming from space absorbs X-rays coming from space making this region warmer as a function of altitude and is making this region warmer as a function of altitude and is very hot in the upper part of the thermospherevery hot in the upper part of the thermosphere– In this upper part of the thermosphere some of the gas gets In this upper part of the thermosphere some of the gas gets

ionized becomes the ionized becomes the ionosphereionosphere, which bounces radio signals , which bounces radio signals back to the groundback to the ground

The The Exosphere Exosphere contains so few molecules and atoms contains so few molecules and atoms that collisions are rare and lightweight molecules can that collisions are rare and lightweight molecules can escape the Earthescape the Earth



Profile


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A comparison of profile from Venus, Earth, MarsA comparison of profile from Venus, Earth, Mars

All three planets have a warm Troposphere via the All three planets have a warm Troposphere via the greenhouse effect and are warmer at the surface than greenhouse effect and are warmer at the surface than they would be without a greenhouse effectthey would be without a greenhouse effect

All three planets have a warm Exosphere where X-rays All three planets have a warm Exosphere where X-rays are being absorbed.are being absorbed.

The Earth has an extra “bump” The Earth has an extra “bump” on the temperature profileon the temperature profile in the Stratosphere where the absorption of UV photons in the Stratosphere where the absorption of UV photons is taking place in the ozone layer.is taking place in the ozone layer.



Profile

comparison


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Earth’s MagnetosphereEarth’s MagnetosphereThe interaction of the solar wind and the magnetic The interaction of the solar wind and the magnetic field of the Earth forms a field of the Earth forms a magnetospheremagnetosphere around around our planetour planet– Not present on Mars or Venus because of lack of (strong) Not present on Mars or Venus because of lack of (strong)

magnetic fieldmagnetic field

In 1958 the first US satellite was launched In 1958 the first US satellite was launched ((Explorer IExplorer I) carrying a Geiger counter built) carrying a Geiger counter built by by James Van AllenJames Van Allen– There were a large number of charged particles just above There were a large number of charged particles just above

the ionospherethe ionosphere

TheThe Van Allen belts Van Allen belts are doughnut-shaped regions are doughnut-shaped regions composed of charged particles (protons & composed of charged particles (protons & electrons) emitted by the Sun & captured by the electrons) emitted by the Sun & captured by the magnetic field of the Earth.magnetic field of the Earth.

Earth Mag

magnetosphere

magnetosphere2


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Earth’s MagnetosphereEarth’s Magnetosphere

AurorasAuroras result from disturbances in the Earth’s result from disturbances in the Earth’s magnetic field that cause some of the particles to magnetic field that cause some of the particles to follow the magnetic field lines down to the follow the magnetic field lines down to the atmosphere, where their collisions with atoms of atmosphere, where their collisions with atoms of the air cause it to glow.the air cause it to glow.Usually the colors you see are due to energetic Usually the colors you see are due to energetic electron striking hydrogen, nitrogen, and oxygen electron striking hydrogen, nitrogen, and oxygen atoms.atoms.Aurora borealisAurora borealis (northern lights) and (northern lights) and aurora aurora australisaustralis (southern lights) can be seen (southern lights) can be seen occasionally far away from the occasionally far away from the corresponding corresponding magnetic pole magnetic pole

magnetosphere

magnetosphere2

aurora

Aurora from the Ground


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Aurora From Space


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Weather and ClimateWeather and Climate

A planet which has an atmosphere can have A planet which has an atmosphere can have weatherweather, the ever changing combination of winds, , the ever changing combination of winds, clouds, temperature, and pressureclouds, temperature, and pressureClimateClimate,, is the long-term average of weather.is the long-term average of weather.

What creates wind and weather?What creates wind and weather?On the planetary scale winds blow in distinctive On the planetary scale winds blow in distinctive patterns called patterns called global wind patterns.global wind patterns.Near the equator winds blow to the west, at the mid-Near the equator winds blow to the west, at the mid-latitudes they blow to the east, and then at the poles latitudes they blow to the east, and then at the poles they blow to the westthey blow to the westWhy? Atmospheric heating and planetary rotationWhy? Atmospheric heating and planetary rotation

Global wind


2020



Global circulationGlobal circulationThe excess heat from the equator causes air to rise The excess heat from the equator causes air to rise and travel towards the poles where in turn they cool and travel towards the poles where in turn they cool down and flow back to the equator.down and flow back to the equator.If the Earth did not rotate, one would get two If the Earth did not rotate, one would get two circulation cellscirculation cellsOn Venus, the thick atmosphere allows these cells On Venus, the thick atmosphere allows these cells transport a great deal of thermal energy to poles transport a great deal of thermal energy to poles making the poles and equator nearly equal in making the poles and equator nearly equal in temperaturetemperatureOn Mars, the thin atmosphere allows little transport so On Mars, the thin atmosphere allows little transport so that the poles are much colder than the equatorthat the poles are much colder than the equator

circulation


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Coriolis EffectCoriolis EffectThis is the apparent deflection of objects This is the apparent deflection of objects from a straight line on a rotating surfacefrom a straight line on a rotating surface

Its main effect on Earth is to cause Its main effect on Earth is to cause circulation patterns of currents of air or circulation patterns of currents of air or waterwater

It also splits the 2 zones of It also splits the 2 zones of circulation cellscirculation cells into 6 zones (i.e. Hadley Cells, 3 for each into 6 zones (i.e. Hadley Cells, 3 for each hemisphere)hemisphere)

Hadley cellReal Earth

Coriolis2

Coriolis

LoHi

Lo earthN and S

LO Earth


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Weather and ClimateWeather and ClimateClouds and PrecipitationClouds and Precipitation

Water or other gases can Water or other gases can precipitateprecipitate out of the out of the atmosphere (rain, hail, snow)atmosphere (rain, hail, snow)Clouds can reflect sunlight back into space and reduce the Clouds can reflect sunlight back into space and reduce the amount of sunlight that hits the groundamount of sunlight that hits the groundClouds tend to composed of minority components of the Clouds tend to composed of minority components of the atmosphereatmosphereThe Hadley cells have an effect on precipitationThe Hadley cells have an effect on precipitation– The equator produces stronger convection and therefore more The equator produces stronger convection and therefore more

clouds and rainclouds and rain– At the north or south extreme of the first Hadley zone (latitude 20At the north or south extreme of the first Hadley zone (latitude 20°°

to 30to 30°° North or South) the moisture has been tapped out and those North or South) the moisture has been tapped out and those region are generally dry.region are generally dry.

watercycle


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Weather and ClimateWeather and ClimateFour factors leading to Long term climate changeFour factors leading to Long term climate change

Solar BrighteningSolar Brightening– The sun has gradually gotten brighter over timeThe sun has gradually gotten brighter over time

Changes in axial tiltChanges in axial tilt– The Earth’s tilt has ranged from 22The Earth’s tilt has ranged from 22°° to 25 to 25° (currently 23.5°)° (currently 23.5°)– More extreme tilts can warm up polar regions, while modest tilts More extreme tilts can warm up polar regions, while modest tilts

tend to keep the poles coolertend to keep the poles cooler

Changes in reflectivityChanges in reflectivity– More reflectivity leads to less sunlight which can lead to More reflectivity leads to less sunlight which can lead to

planetwide coolingplanetwide cooling– Changes can be caused by the release of particles in the air, Changes can be caused by the release of particles in the air,

naturally or artificially.naturally or artificially.

Changes in Greenhouse Gas AbundanceChanges in Greenhouse Gas Abundance– In general, the more greenhouse gases in the atmosphere the In general, the more greenhouse gases in the atmosphere the

warmer the planet gets (and vice versa).warmer the planet gets (and vice versa).

four reasons

tilt


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The terrestrial planets started out with no The terrestrial planets started out with no atmosphere. How does a planet gain or lose atmosphere. How does a planet gain or lose atmospheric gas?atmospheric gas?

Gaining gasGaining gas

Volcanic outgassingVolcanic outgassing– Gases from volcanoes can put water, carbon dioxide, nitrogen, Gases from volcanoes can put water, carbon dioxide, nitrogen,

and sulfur-bearing compounds into the airand sulfur-bearing compounds into the air

Evaporation of surface liquids or sublimationEvaporation of surface liquids or sublimation

MicrometeoritesMicrometeorites– These can knock individual atoms or molecule free from the These can knock individual atoms or molecule free from the

surfacesurface– Works best on an nearly airless world.Works best on an nearly airless world.

Gainatmo


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Weather and ClimateWeather and ClimateLosing gasLosing gas

CondensationCondensation– Gases can condense out of the atmosphereGases can condense out of the atmosphere

Chemical ReactionsChemical Reactions– e.g. Oxidatione.g. Oxidation

Moderate to giant impactsModerate to giant impacts– These can blast material off world, works best on smaller worldsThese can blast material off world, works best on smaller worlds

Ionization of atoms in atmosphere by solar windIonization of atoms in atmosphere by solar wind– Common if planet lacks magnetosphereCommon if planet lacks magnetosphere

Thermal escape (see next page)Thermal escape (see next page)The first two are reversible while the latter 3 are permanent The first two are reversible while the latter 3 are permanent

changeschanges

Gainatmo

lossatmo


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Losing gas - Thermal escapeLosing gas - Thermal escape

The relative importance of the escape of gas The relative importance of the escape of gas into space depends on 3 factorsinto space depends on 3 factors– The planet’s escape velocityThe planet’s escape velocity– The atmospheric temperatureThe atmospheric temperature– The mass of the gas particleThe mass of the gas particle

The terrestrial planets are too small and warm The terrestrial planets are too small and warm to retain H and He. Venus, Earth and Mars to retain H and He. Venus, Earth and Mars have retained some to the heavier gases.have retained some to the heavier gases.

See Mathematical insight 10.2See Mathematical insight 10.2

Thermalvelocity

lossatmo


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Moon and MercuryMoon and Mercury

These two worlds are so small and warm that These two worlds are so small and warm that they have virtually no atmospherethey have virtually no atmosphere

The atmosphere currently produced comes from The atmosphere currently produced comes from surface ejection via micrometeorite and solar surface ejection via micrometeorite and solar wind impacts which gives both worlds a thin but wind impacts which gives both worlds a thin but extensive exosphere (of sodium atoms)extensive exosphere (of sodium atoms)

Craters in perpetual shadow (near the poles) Craters in perpetual shadow (near the poles) have been found to have icehave been found to have ice

Moonexosphere


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MarsMars

Mars has a thin atmosphere which is about 0.7% Mars has a thin atmosphere which is about 0.7% that of the Earth’s surface, mostly COthat of the Earth’s surface, mostly CO22 and no and no oxygen and ozoneoxygen and ozoneThe global average temperature is about -50The global average temperature is about -50°C°CSeasons are similar to Earth (due to similar axial Seasons are similar to Earth (due to similar axial tilt) but are twice as long and tilt) but are twice as long and doesdoes depend on depend on Mars’s distance from the SunMars’s distance from the SunThe Southern hemisphere has more extreme The Southern hemisphere has more extreme seasons than the Northern hemisphereseasons than the Northern hemisphere– Much of the COMuch of the CO2 2 from the polar caps sublimes away in from the polar caps sublimes away in

local summer and can transfer between poleslocal summer and can transfer between poles

MarsSeason

Marspoles


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MarsMars

The pole to pole winds change with alternating The pole to pole winds change with alternating seasons and can cause global dust stormsseasons and can cause global dust storms

When the dust resettles it can major changes in When the dust resettles it can major changes in reflectivityreflectivity– This fooled 19This fooled 19thth and early 20 and early 20thth century observers in century observers in

thinking that vegetation was growingthinking that vegetation was growing

Sometimes Sometimes Dust Devils can be see swirling on can be see swirling on MarsMars

The Martian sky is nearly black, except when a The Martian sky is nearly black, except when a lot of dust is in the air, then it can look pinkish-redlot of dust is in the air, then it can look pinkish-red

MarsSeason

MarsDuststorm

Dustdevil

PinkRedsky


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MarsMars

The axial tilt has changed severely on Mars over The axial tilt has changed severely on Mars over the past billion yearsthe past billion yearsThe axial tilt can range from near 0 to 60The axial tilt can range from near 0 to 60° on a ° on a scale of hundreds of thousands of year to millions scale of hundreds of thousands of year to millions of yearsof years– Unlike the Earth, Mars has no great satellite to stabilize Unlike the Earth, Mars has no great satellite to stabilize

to axial tiltto axial tilt– Also Jupiter tidal influence is greater than it is for EarthAlso Jupiter tidal influence is greater than it is for Earth

The great axial tilt allows the poles to completely The great axial tilt allows the poles to completely sublime into the atmosphere, thereby increasing sublime into the atmosphere, thereby increasing the greenhouse effect on Marsthe greenhouse effect on Mars

Layers of iceAnd dust


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MarsMarsWhy did Mars Change?Why did Mars Change?

Mars’ likely was wetter and possibly warmer some 3 Mars’ likely was wetter and possibly warmer some 3 billion years agobillion years ago– Probably had a thicker atmosphere to allow rainfallProbably had a thicker atmosphere to allow rainfall– Outgassing of water and COOutgassing of water and CO22 from volcanoes would have from volcanoes would have

created a sufficient greenhouse effectcreated a sufficient greenhouse effect– A denser atmosphere would allow liquid water to flow on A denser atmosphere would allow liquid water to flow on

surfacesurface– COCO22

in the atmosphere dissolved in water and into rocks, in the atmosphere dissolved in water and into rocks, causing the atmospheric temperature to drop, freezing all causing the atmospheric temperature to drop, freezing all the water, though much of the water and COthe water, though much of the water and CO22 was lost into was lost into spacespace

– Oxygen formed from water which remained, COOxygen formed from water which remained, CO2 2 rusted the rusted the surface giving Mars the ruddy appearance of todaysurface giving Mars the ruddy appearance of today Mars

TodayandYesterday


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VenusVenusVenus’s atmosphere is composed Venus’s atmosphere is composed of 96% of 96% carbon dioxide (COcarbon dioxide (CO22),), 3.5% nitrogen (N 3.5% nitrogen (N22), and ), and small amounts of water (Hsmall amounts of water (H22O), sulfuric acid O), sulfuric acid (H(H22SOSO44), and hydrochloric acid (HCl).), and hydrochloric acid (HCl).The upper atmosphere is very windy; wind The upper atmosphere is very windy; wind speeds reach 350 km/hr (225 mi/hr).speeds reach 350 km/hr (225 mi/hr).As one descends to Venus’s surface, the wind As one descends to Venus’s surface, the wind speed decreases to almost zero (< 6 km/hr).speed decreases to almost zero (< 6 km/hr).The atmospheric pressure on Venus’s surface The atmospheric pressure on Venus’s surface is about 90 times that found at the Earth’s is about 90 times that found at the Earth’s surface.surface.

Atmosphereprofile

Cloudy Venus

Atmosphereprofile2


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VenusVenus

Venus’s clouds form a layer between altitudes of 50 Venus’s clouds form a layer between altitudes of 50 and 70 km down to 30 km. and 70 km down to 30 km. Surface temperature of Venus has been measured Surface temperature of Venus has been measured at about 460°C (860°F) and is nearly uniform at about 460°C (860°F) and is nearly uniform throughout the planet (equator to pole/day and night)throughout the planet (equator to pole/day and night)Since Venus rotates very slowly, the Coriolis effect is Since Venus rotates very slowly, the Coriolis effect is negligible and the circulation patterns in Venus’s negligible and the circulation patterns in Venus’s atmosphere is quite different than the Earth’s.atmosphere is quite different than the Earth’s.The entire atmosphere moves westward faster than The entire atmosphere moves westward faster than Venus rotates westwardVenus rotates westwardBig circulation patterns go from equator to pole as Big circulation patterns go from equator to pole as “cooler” air from below gets heated as it approaches “cooler” air from below gets heated as it approaches the equator and rises into the upper atmosphere.the equator and rises into the upper atmosphere. Venus

Hadley cell

Coriolis

AtmosphereProfile2

Atmo-tempdiurnal


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VenusVenusHypothesis for Venus/Earth DifferencesHypothesis for Venus/Earth Differences

About twice as much sunlight hits Venus as it does About twice as much sunlight hits Venus as it does Earth. Earth. However, due to the larger abundance of highly However, due to the larger abundance of highly reflective clouds of Venus today, the Earth actually reflective clouds of Venus today, the Earth actually gets more sunlight to the surface than Venus.gets more sunlight to the surface than Venus.– Then why is it hotter on Venus than Earth?Then why is it hotter on Venus than Earth?

The Venusian atmosphere is 200,000 denser than The Venusian atmosphere is 200,000 denser than Earth’s and this and Venus closer distance from the Earth’s and this and Venus closer distance from the Sun caused a Sun caused a runaway greenhouse effectrunaway greenhouse effect– Best imagined if you were to Move the Earth to Best imagined if you were to Move the Earth to

Venus’s distance from SunVenus’s distance from Sun– How did Venus’s atmosphere get so thick?How did Venus’s atmosphere get so thick?

GH effect

Moving EarthTo Venus


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VenusVenus

The Earth and Venus should have outgassed a The Earth and Venus should have outgassed a similar amount of Hsimilar amount of H22O and COO and CO22..

– On Earth the temperatures allowed the HOn Earth the temperatures allowed the H22O to rain and O to rain and

form the oceans and the COform the oceans and the CO22 dissolved in the water dissolved in the water

and made and made carbonate rockscarbonate rocks

– On Venus, the lack of water kept the COOn Venus, the lack of water kept the CO22 in the in the

atmosphereatmosphere– The water on Venus probably ended up in the The water on Venus probably ended up in the

atmosphere where it was broken apart by the UV atmosphere where it was broken apart by the UV photons and lost forever as the hydrogen escaped and photons and lost forever as the hydrogen escaped and the oxygen chemically combinedthe oxygen chemically combined


3636

Understanding Earth’s unique atmosphere (as Understanding Earth’s unique atmosphere (as compared to Venus and Mars) comes down to compared to Venus and Mars) comes down to understanding 4 questions:understanding 4 questions:1.1. Why did Earth retain most of its outgassed water?Why did Earth retain most of its outgassed water?

2.2. Why does Earth have so little COWhy does Earth have so little CO22??

3.3. Why is Earth’s atm. composed of NWhy is Earth’s atm. composed of N22 and O and O22??

4.4. Why does Earth have an UV absorbing Why does Earth have an UV absorbing atmosphere?atmosphere?

The first of these has been answered, so we’ll The first of these has been answered, so we’ll concentrate on the last two concentrate on the last two


Earth Unique Atmosphere


3737

Nitrogen is the third most common Nitrogen is the third most common outgassed product, so if water and COoutgassed product, so if water and CO22 are removed from the atmosphere then are removed from the atmosphere then nitrogen would be left over.nitrogen would be left over.Oxygen was formed from the life on the Oxygen was formed from the life on the Earth via photosynthesisEarth via photosynthesisThe oxygen that was formed on the The oxygen that was formed on the ground was converted to ozone in the ground was converted to ozone in the stratosphere.stratosphere.


Earth Unique Atmosphere

Oxygenozone


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Climate StabilityClimate Stability– The Earth’s climate has remained stable The Earth’s climate has remained stable

enough to keep water liquid on the surfaceenough to keep water liquid on the surface– The Earth’s temperature has remained in The Earth’s temperature has remained in

much the same range despite the fact the much the same range despite the fact the Sun has brightened substantially in the last 4 Sun has brightened substantially in the last 4 billion years (by about 30%).billion years (by about 30%).


Earth as a Living Planet


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Climate StabilityClimate Stability– The self-regulation of the Earth temperature is done The self-regulation of the Earth temperature is done

by the COby the CO22 cycle: cycle:Atmospheric COAtmospheric CO22 dissolves in rainwater, creating a mild dissolves in rainwater, creating a mild acidacidThis mild acid rain breaks down minerals in the rocks and This mild acid rain breaks down minerals in the rocks and send the material to the oceanssend the material to the oceansThese minerals combine with dissolved COThese minerals combine with dissolved CO22 making making carbonate rocks such as limestonecarbonate rocks such as limestonePlate tectonics carries the carbonate rocks to subduction Plate tectonics carries the carbonate rocks to subduction zones to the mantlezones to the mantleAs these rocks are pushed deeper into the mantle some of As these rocks are pushed deeper into the mantle some of the subducted rock melts and releases COthe subducted rock melts and releases CO2 2 which outgases which outgases back into the atmosphereback into the atmosphere

– So COSo CO2 2 acts like a thermostatacts like a thermostat


Earth as a Living Planet

CO2

cycle


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Changes made by Human Activity: Global WarmingChanges made by Human Activity: Global Warming– Global Warming seems to be a reality because of three Global Warming seems to be a reality because of three

facts:facts:By burning fossil fuels we are increasing the amount of By burning fossil fuels we are increasing the amount of greenhouse gasesgreenhouse gasesWe understand the greenhouse effect enough to know that this We understand the greenhouse effect enough to know that this increase make our planet warm up at some timeincrease make our planet warm up at some timeSophisticated models have matched the climate data quite well Sophisticated models have matched the climate data quite well and may be able to accurately predict to future increase in global and may be able to accurately predict to future increase in global temperaturestemperatures

– Consequences include the raising a water levels due to Consequences include the raising a water levels due to slight warmer water temperature and the melting of inland slight warmer water temperature and the melting of inland ice.ice.


Earth as a Living Planet AveargeGlobal T

CO2levels

Greenlandice


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The EndThe End

© sierra college astronomy department 1 atmospheres of the terrestrial planets

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