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Chapter 10: Planetary Atmospheres Earth and the Other Terrestrial Worlds Slide 2 Comparing Terrestrial Atmospheres Slide 3 What is an Atmosphere? A layer of gas which surrounds a world is called an atmosphere. they are usually very thin compared to planet radiusthey are usually very thin compared to planet radius Pressure is created by atomic & molecular collisions in an atmosphere. heating a gas in a confined space increases pressureheating a gas in a confined space increases pressure number of collisions increasenumber of collisions increase unit of measure: 1 bar = 14.7 lbs/inch 2 = Earths atmospheric pressure at sea levelunit of measure: 1 bar = 14.7 lbs/inch 2 = Earths atmospheric pressure at sea level Pressure balances gravity in an atmosphere. Slide 4 2014 Pearson Education, Inc. Earth's Atmosphere About 10 kilometers thick Consists mostly of molecular nitrogen (N 2 ) and oxygen (O 2 ). Slide 5 2014 Pearson Education, Inc. Where does an atmosphere end? Small amounts of gas are present even above 300 kilometers. Slide 6 Effects of an Atmosphere on a Planet greenhouse effect makes the planetary surface warmer than it would be otherwisemakes the planetary surface warmer than it would be otherwise scattering and absorption of light absorb high-energy radiation from the Sunabsorb high-energy radiation from the Sun scattering of optical light brightens the daytime skyscattering of optical light brightens the daytime sky creates pressure can allow water to exist as a liquid (at the right temperature)can allow water to exist as a liquid (at the right temperature) creates wind and weather promotes erosion of the planetary surfacepromotes erosion of the planetary surface creates auroras interaction with the Solar wind when magnetic fields are presentinteraction with the Solar wind when magnetic fields are present Slide 7 The Greenhouse Effect Visible Sunlight passes through a planets atmosphere. Some of this light is absorbed by the planets surface. Planet re-emits this energy (heat) as infrared (IR) light. planets temperature lower than Sunplanets temperature lower than Sun IR light is trapped by the atmosphere. its return to space is slowedits return to space is slowed This causes the overall surface temperature to be higher than if there were no atmosphere at all. Slide 8 Greenhouse Gases Key to Greenhouse Effectgases which absorb IR light effectively: water [H 2 O]water [H 2 O] carbon dioxide [CO 2 ]carbon dioxide [CO 2 ] methane [CH 4 ]methane [CH 4 ] These are molecules which rotate and vibrate easily. they re-emit IR light in a random directionthey re-emit IR light in a random direction The more greenhouse gases which are present, the greater the amount of surface warming. Slide 9 Planetary Energy Balance Solar energy received by a planet must balance the energy it returns to space planet can either reflect or emit the energy as radiationplanet can either reflect or emit the energy as radiation this is necessary for the planet to have a stable temperaturethis is necessary for the planet to have a stable temperature Slide 10 What Determines a Planets Surface Temperature? Greenhouse Effect cannot change incoming Sunlight, so it cannot change the total energy returned to space. it increases the energy (heat) in lower atmosphereit increases the energy (heat) in lower atmosphere it works like a blanketit works like a blanket In the absence of the Greenhouse Effect, what would determine a planets surface temperature? the planet's distance from the Sunthe planet's distance from the Sun the planets overall reflectivitythe planets overall reflectivity the higher the albedo, the less light absorbed, planet coolerthe higher the albedo, the less light absorbed, planet cooler Earths average temperature would be 17 C (1 F) without the Greenhouse EffectEarths average temperature would be 17 C (1 F) without the Greenhouse Effect Slide 11 Greenhouse Effect on the Planets Greenhouse Effect warms Venus, Earth, & Mars on Venus: it is very strongon Venus: it is very strong on Earth: it is moderateon Earth: it is moderate on Mars: it is weakon Mars: it is weak avg. temp. on Venus & Earth would be freezing without itavg. temp. on Venus & Earth would be freezing without it Slide 12 Structure of Earths Atmosphere pressure & density of atmosphere decrease with altitude temperature varies back and forth with altitude these temperature variations define the major atmospheric layersthese temperature variations define the major atmospheric layers exosphere low density; fades into space thermosphere temp begins to rise at the top stratosphere rise and fall of temp troposphere layer closest to surface temp drops with altitude Slide 13 Atmospheres Interact with Light X rays ionize atoms & moleculesionize atoms & molecules dissociate moleculesdissociate molecules absorbed by almost all gasesabsorbed by almost all gases Ultraviolet (UV) dissociate some moleculesdissociate some molecules absorbed well by O 3 & H 2 Oabsorbed well by O 3 & H 2 O Visible (V) passes right through gasespasses right through gases some photons are scatteredsome photons are scattered Infrared (IR) absorbed by greenhouse gasesabsorbed by greenhouse gases Slide 14 Reasons for Atmospheric Structure Light interactions are responsible for the structure we see. Troposphere absorbs IR photons from the surfaceabsorbs IR photons from the surface temperature drops with altitudetemperature drops with altitude hot air rises and high gas density causes storms (convection)hot air rises and high gas density causes storms (convection) Stratosphere lies above the greenhouse gases (no IR absorption)lies above the greenhouse gases (no IR absorption) absorbs heat via Solar UV photons which dissociate ozone (O 3 )absorbs heat via Solar UV photons which dissociate ozone (O 3 ) UV penetrates only top layer; hotter air is above colder airUV penetrates only top layer; hotter air is above colder air no convection or weather; the atmosphere is stratified no convection or weather; the atmosphere is stratified Thermosphere absorbs heat via Solar X-rays which ionizes all gasesabsorbs heat via Solar X-rays which ionizes all gases contains ionosphere, which reflects back human radio signalscontains ionosphere, which reflects back human radio signals Exosphere hottest layer; gas extremely rarified; provides noticeable drag on satelliteshottest layer; gas extremely rarified; provides noticeable drag on satellites Slide 15 Structure of Terrestrial Planet Atmospheres Mars, Venus, Earth all have warm tropospheres (and greenhouse gases) have warm thermospheres which absorb Solar X rays Only Earth has a warm stratosphere an UV-absorbing gas (O 3 ) All three planets have warmer surface temps due to greenhouse effect Slide 16 Magnetospheres The Sun ejects a stream of charged particles, called the solar wind. it is mostly electrons, protons, and Helium nucleiit is mostly electrons, protons, and Helium nuclei Earths magnetic field attracts and diverts these charged particles to its magnetic poles. the particles spiral along magnetic field lines and emit lightthe particles spiral along magnetic field lines and emit light this causes the aurora (aka northern & southern lights)this causes the aurora (aka northern & southern lights) this protective bubble is called the magnetospherethis protective bubble is called the magnetosphere Other terrestrial worlds have no strong magnetic fields solar wind particles impact the exospheres of Venus & Marssolar wind particles impact the exospheres of Venus & Mars solar wind particles impact the surfaces of Mercury & Moonsolar wind particles impact the surfaces of Mercury & Moon Slide 17 Earths Magnetosphere Slide 18 What are Weather and Climate? These are Earths global wind patterns or circulation local weather systems move along with themlocal weather systems move along with them weather moves from W to E at mid-latitudes in N hemisphereweather moves from W to E at mid-latitudes in N hemisphere Two factors cause these patterns atmospheric heatingatmospheric heating planetary rotationplanetary rotation weather short-term changes in wind, clouds, temperature, and pressure in an atmosphere at a given location climate long-term average of the weather at a given location Slide 19 Global Wind Patterns air heated more at equator warm air rises at equator; heads for poleswarm air rises at equator; heads for poles cold air moves towards equator along the surfacecold air moves towards equator along the surface two circulation cells are created in each hemisphere cells do not go directly from pole to equator; air circulation is diverted by Coriolis effect moving objects veer right on a surface rotating counterclockwise moving objects veer left on a surface rotating clockwise Slide 20 Global Wind Patterns On Earth, the Coriolis effect breaks each circulation cell into three separate cells winds move either W to E or E to Wwinds move either W to E or E to W Coriolis effect not strong on Mars & Venus Mars is too small Venus rotates too slowly In thick Venusian atmosphere, the pole- to-equator circulation cells distribute heat efficiently surface temperature is uniform all over the planet Slide 21 Clouds, Rain and Snow Clouds strongly affect the surface conditions of a planet they increase its albedo, thus reflecting away more sunlightthey increase its albedo, thus reflecting away more sunlight they provide rain and snow, which causes erosionthey provide rain and snow, which causes erosion Formation of rain and snow: Slide 22 Four Major Factors which affect Long-term Climate Change Slide 23 Gain/Loss Processes of Atmospheric Gas Unlike the Jovian planets, the terrestrials were too small to capture significant gas from the Solar nebula. what gas they did capture was H & He, and it escapedwhat gas they did capture was H & He, and it escaped present-day atmospheres must have formed at a later timepresent-day atmospheres must have formed at a later time Sources of atmospheric gas: outgassing release of gas trapped in interior rock by volcanismoutgassing release of gas trapped in interior rock by volcanism evaporation/sublimation surface liquids or ices turn to gas when heatedevaporation/sublimation surface liquids or ices turn to gas when heated bombardment micrometeorites, Solar wind particles, or high- energy photons blast atoms/molecules out of surface rockbombardment micrometeorites, Solar wind particles, or high- energy photons blast atoms/molecules out of surface rock occurs only if the planet has no substantial atmosphere already Slide 24 Gain/Loss Processes of Atmospheric Gas Ways to lose atmospheric gas: condensation gas turns into liquids or ices on the surface when cooledcondensation gas turns into liquids or ices on the surface when cooled chemical reactions gas is bound into surface rocks or liquidschemical reactions gas is bound into surface rocks or liquids stripping gas is knocked out of the upper atmosphere by Solar wind particlesstripping gas is knocked out of the upper atmosphere by Solar wind particles impacts a comet/asteroid collision with a planet can blast atmospheric gas into spaceimpacts a comet/asteroid collision with a planet can blast atmospheric gas into space thermal escape lightweight gas molecules are lost to space when they achieve escape velocitythermal escape lightweight gas molecules are lost to space when they achieve escape velocity gas is lost forever! Slide 25 Gain/Loss Processes of Atmospheric Gas Slide 26 Origin of the Terrestrial Atmospheres Venus, Earth, & Mars received their atmospheres through outgassing. most common gases: H 2 O, CO 2, N 2, H 2 S, SO 2most common gases: H 2 O, CO 2, N 2, H 2 S, SO 2 Chemical reactions caused CO 2 on Earth to dissolve in oceans and go into carbonate rocks (like limestone.) this occurred because H 2 O could exist in liquid statethis occurred because H 2 O could exist in liquid state N 2 was left as the dominant gas; O 2 was exhaled by plant lifeN 2 was left as the dominant gas; O 2 was exhaled by plant life as the dominant gas on Venus, CO 2 caused strong greenhouse effectas the dominant gas on Venus, CO 2 caused strong greenhouse effect Mars lost much of its atmosphere through impacts less massive planet, lower escape velocityless massive planet, lower escape velocity Slide 27 Origin of the Terrestrial Atmospheres Lack of magnetospheres on Venus & Mars made stripping by the Solar wind significant. further loss of atmosphere on Marsfurther loss of atmosphere on Mars dissociation of H 2 O, H 2 thermally escapes on Venusdissociation of H 2 O, H 2 thermally escapes on Venus Gas and liquid/ice exchange occurs through condensation and evaporation/sublimation: on Earth with H 2 Oon Earth with H 2 O on Mars with CO 2on Mars with CO 2 Since Mercury & the Moon have no substantial atmosphere, fast particles and high-energy photons reach their surfaces bombardment creates a rarified exospherebombardment creates a rarified exosphere Slide 28 2014 Pearson Education, Inc. Exospheres of the Moon and Mercury Sensitive measurements show that the Moon and Mercury have extremely thin atmospheres. Gas comes from impacts that eject surface atoms. Slide 29 Martian Weather Today Seasons on Mars are more extreme than on EarthSeasons on Mars are more extreme than on Earth Mars orbit is more ellipticalMars orbit is more elliptical CO 2 condenses & sublimes at opposite polesCO 2 condenses & sublimes at opposite poles changes in atmospheric pressure drive pole-to-pole windschanges in atmospheric pressure drive pole-to-pole winds sometimes cause huge dust stormssometimes cause huge dust storms Slide 30 Martian Weather: N Polar Ice Cap & Dust Storm Slide 31 2014 Pearson Education, Inc. Changing Axis Tilt Calculations suggest Mars's axis tilt ranges from 0 to 60. Such extreme variations can cause climate changes. Alternating layers of ice and dust in polar regions reflect these climate changes. Slide 32 Climate History of Mars More than 3 billion years ago, Mars must have had a thick CO 2 atmosphere and a strong greenhouse effect.More than 3 billion years ago, Mars must have had a thick CO 2 atmosphere and a strong greenhouse effect. the so-called warm and wet periodthe so-called warm and wet period Eventually CO 2 was lost to space.Eventually CO 2 was lost to space. some gas was lost to impactssome gas was lost to impacts cooling interior meant loss of magnetic fieldcooling interior meant loss of magnetic field Solar wind stripping removed gasSolar wind stripping removed gas Greenhouse effect weakened until Mars froze. Slide 33 Venusian Weather Today Venus has no seasons to speak of.Venus has no seasons to speak of. rotation axis is nearly 90 to the ecliptic planerotation axis is nearly 90 to the ecliptic plane Venus has little wind at its surfaceVenus has little wind at its surface rotates very slowly, so there is no Coriolis effectrotates very slowly, so there is no Coriolis effect The surface temperature stays constant all over Venus.The surface temperature stays constant all over Venus. thick atmosphere distributes heat via two large circulation cellsthick atmosphere distributes heat via two large circulation cells There is no rain on the surface. it is too hot and Venus has almost no H 2 Oit is too hot and Venus has almost no H 2 O Venusian clouds contain sulfuric acid! implies recent volcanic outgassing?implies recent volcanic outgassing? Slide 34 Climate History of Venus Venus should have outgassed as much H 2 O as Earth. Early on, when the Sun was dimmer, Venus may have had oceans of waterEarly on, when the Sun was dimmer, Venus may have had oceans of water Venus proximity to the Sun caused all H 2 O to evaporate. H 2 O caused runaway greenhouse effectH 2 O caused runaway greenhouse effect surface heated to extreme temperaturesurface heated to extreme temperature UV photons from Sun dissociate H 2 O; H 2 escapes, O is strippedUV photons from Sun dissociate H 2 O; H 2 escapes, O is stripped Slide 35 The Uniqueness of Earths Atmosphere Outgassing from volcanoes on Venus, Earth, & Mars released the same gasses: primarily water (H 2 O), Carbon dioxide (CO 2 ), and Nitrogen (N 2 )primarily water (H 2 O), Carbon dioxide (CO 2 ), and Nitrogen (N 2 ) So why did Earths atmosphere end up so different? why did Earth retain most of its H 2 O enough to form oceans?why did Earth retain most of its H 2 O enough to form oceans? why does Earth have so little CO 2 in its atmosphere, when it should have outgassed just as much CO 2 as Venus?why does Earth have so little CO 2 in its atmosphere, when it should have outgassed just as much CO 2 as Venus? why does Earth have so much more Oxygen (O 2 ) than Venus & Mars?why does Earth have so much more Oxygen (O 2 ) than Venus & Mars? why does Earth have an ultraviolet-absorbing stratosphere?why does Earth have an ultraviolet-absorbing stratosphere? Slide 36 The Uniqueness of Earths Atmosphere Earths H 2 O condensed because of the temperature. oceans formed, in which the CO 2 gas dissolvedoceans formed, in which the CO 2 gas dissolved chemical reactions bound the C of CO 2 into rocks like limestonechemical reactions bound the C of CO 2 into rocks like limestone low level of atmospheric CO 2 causes moderate greenhouse effectlow level of atmospheric CO 2 causes moderate greenhouse effect temperatures on Earth remain where H 2 O can be a liquid CO 2 temperatures on Earth remain where H 2 O can be a liquid CO 2 There was once liquid H 2 O on Mars and maybe Venus. before CO 2 could dissolve out, temperatures fell/rose so thatbefore CO 2 could dissolve out, temperatures fell/rose so that oceans boiled away on Venus and froze out on Marsoceans boiled away on Venus and froze out on Mars Earths O 2 was not outgassed by volcanoes. O 2 is a highly reactive chemicalO 2 is a highly reactive chemical it would disappear in a few million years if not replenishedit would disappear in a few million years if not replenished no geologic process creates O 2no geologic process creates O 2 Slide 37 The Uniqueness of Earths Atmosphere Earths O 2 was created through the evolution of life. plants & microorganism release O 2 via photosynthesisplants & microorganism release O 2 via photosynthesis they convert CO 2 into O 2they convert CO 2 into O 2 In the upper atmosphere, O 2 in converted into ozone (O 3 ). via chemical processed involving Solar ultraviolet lightvia chemical processed involving Solar ultraviolet light O 3 absorbs Solar UV photons which heats the stratosphereO 3 absorbs Solar UV photons which heats the stratosphere Venus & Mars lack plant & microbial life. so they have no O 2 in their atmospheres and no stratospheresso they have no O 2 in their atmospheres and no stratospheres Slide 38 The CO 2 Cycle Slide 39 The CO 2 Cycle is a Feedback Mechanism which Regulates Earths Climate Slide 40 Stability of Earths Climate Plate tectonics causes the relative stability of Earths climate. plate tectonics makes the CO 2 cycle workplate tectonics makes the CO 2 cycle work it takes about 40,000 years for the CO 2 cycle to restore balanceit takes about 40,000 years for the CO 2 cycle to restore balance There have been temporary episodes of extreme cooling and heating in Earths history. these ice ages & hothouse period have their own feedback mechanismsthese ice ages & hothouse period have their own feedback mechanisms Slide 41 Studying Past Life We know the history of life on Earth by studying fossil records. fossils are more difficult to find as we look back to earlier epochsfossils are more difficult to find as we look back to earlier epochs more organisms which lacked skeletons leave fewer fossils erosion erases much old fossil evidence subduction destroys fossils carries deep beneath Earths surface we have found fossils of large & small animals, plants, microorganismswe have found fossils of large & small animals, plants, microorganisms the fossil record goes back 3.5 billion yearsthe fossil record goes back 3.5 billion years deficit of 13 C, a sign of life, in rocks as old as 3.85 billion years Slide 42 Origin of Life All known organisms: build proteins from same subset of amino acidsbuild proteins from same subset of amino acids use ATP to store energy in cellsuse ATP to store energy in cells use DNA molecules to transmit genesuse DNA molecules to transmit genes All organisms share same genetic codesequence of chemical bases Organisms have similar genes. Indicates that all living organisms share a common ancestor. Life on Earth is: divided into three major groupings plants & animals are just two tiny branches Slide 43 Origin of Life We have no direct evidence of when or how life began. We have a plausible scenario of how chemistry begat biology: chemicals found on Earth, sparked by lighting, can form complex organic molecules naturallychemicals found on Earth, sparked by lighting, can form complex organic molecules naturally RNA can form, and if some of it becomes self-replicating, it can lead toRNA can form, and if some of it becomes self-replicating, it can lead to DNA and full, self-replicating organisms.DNA and full, self-replicating organisms. Alternatively, this process could have begun on Mars or Venus. life could have been transported to Earth via meteorites organic molecules have been found in meteorites Slide 44 Evolution of Life The oceans were full of single-celled life 3.5 billion years ago. conditions on land were too inhospitable due to lack of O 3 in atmosphereconditions on land were too inhospitable due to lack of O 3 in atmosphere Organisms DNA is reproduced, but can change due to copying errors or external factors a change in the base sequence of an organisms DNA is called a mutationa change in the base sequence of an organisms DNA is called a mutation Some mutations are lethal, others make the cell better able to survive. those organisms which are better at adapting to their environment thrivethose organisms which are better at adapting to their environment thrive this process is called natural selection, first proposed by Charles Darwinthis process is called natural selection, first proposed by Charles Darwin Natural selection helps some species to dominate, and creates entirely new species from older ones. life on Earth rapidly diversifiedlife on Earth rapidly diversified Some 2 billion years ago, life was still confined to the oceans. Slide 45 The Rise of Oxygen Single-celled organisms called cyanobacteria appear to have created O 2 via photosynthesis as early as 3.5 billion years ago. the first O 2 produced was absorbed into rocks via chemical reactionsthe first O 2 produced was absorbed into rocks via chemical reactions it was not until about 2 billion years ago than the rocks were saturated with O 2 and so it began to accumulate in the atmosphereit was not until about 2 billion years ago than the rocks were saturated with O 2 and so it began to accumulate in the atmosphere fossil record indicates current levels of O 2 were reached 200 million yrs agofossil record indicates current levels of O 2 were reached 200 million yrs ago The build-up of O 2 in the atmosphere permitted: the evolution of oxygen-dependent animalsthe evolution of oxygen-dependent animals the formation of O 3 in the stratosphere, making it safe for life to move out onto dry landthe formation of O 3 in the stratosphere, making it safe for life to move out onto dry land Plants first appeared on land some 475 million years ago. Animals soon followed Slide 46 The Rise of Humans Some 540 million years ago, most of the life on Earth was still one-celled and tiny. For the next 40 million years, until 500 million years ago: there was a dramatic increase in the number of species, especially animalsthere was a dramatic increase in the number of species, especially animals animals diversified into all the basic body plans which we find todayanimals diversified into all the basic body plans which we find today this incredible diversification of species is called the Cambrian explosionthis incredible diversification of species is called the Cambrian explosion The first humans appeared a few million years ago. humans civilization is 10,000 years old, industrial society 200 years oldhumans civilization is 10,000 years old, industrial society 200 years old Although latecomers to the scene, humans are the most successful species to survive on Earth Human population has grown exponentially How will this affect our host the Earth? Slide 47 Mass Extinctions Evolution of species normally occurs gradually. one species goes extinct per centuryone species goes extinct per century Evolution can receive a jolt during a mass extinction. historically, this has been caused by an impacthistorically, this has been caused by an impact even species not directly killed by the impact will soon go extinct due to lack of food and changes in the ecosystemeven species not directly killed by the impact will soon go extinct due to lack of food and changes in the ecosystem species at the top of the food chain are most susceptiblespecies at the top of the food chain are most susceptible Human activity in recent times have driven many more species to extinction. could we be undergoing an episode of mass extinction now?could we be undergoing an episode of mass extinction now? what will the consequences be for humans?what will the consequences be for humans? Slide 48 Ozone Depletion O 3 in the stratosphere shields Earths surface from Solar UV A depletion of O 3 was observed ozone hole over Antarcticaozone hole over Antarctica discovered in mid-1980sdiscovered in mid-1980s appears in Antarctic springappears in Antarctic spring The apparent cause of the ozone hole is a man-made chemical, CFC used as a refrigerant, CFC is inert and rises to the stratosphere Solar UV photons break down CFCs and create Cl Chlorine gas Cl serves as a catalyst in destroying O 3 reaction goes faster at low temperatures like over Antarctica Mars is an example of no ozone, UV photons sterilize the surface. increased UV on Earth would increase cancer rates and genetic mutations 19791998 Slide 49 2014 Pearson Education, Inc. Long-Term Climate Change Changes in Earth's axis tilt might lead to ice ages. Widespread ice tends to lower global temperatures by increasing Earth's reflectivity. CO 2 from outgassing will build up if oceans are frozen, ultimately raising global temperatures again. Slide 50 2014 Pearson Education, Inc. How is human activity changing our planet? Slide 51 2014 Pearson Education, Inc. Dangers of Human Activity Human-made CFCs in the atmosphere destroy ozone, reducing protection from ultraviolet radiation. Human activity is driving many species to extinction. Human use of fossil fuels produces greenhouse gases that can cause global warming. Slide 52 2014 Pearson Education, Inc. Global Warming Earth's average temperature has increased by 0.5C in past 50 years. The concentration of CO 2 is rising rapidly. An unchecked rise in greenhouse gases will eventually lead to global warming. Slide 53 2014 Pearson Education, Inc. CO 2 Concentration Global temperatures have tracked CO 2 concentration for last 500,000 years. Current CO 2 concentration is the highest it's been in at least 500,000 years. Slide 54 2014 Pearson Education, Inc. CO 2 Concentration Most of the CO 2 increase has happened in last 50 years! Slide 55 2014 Pearson Education, Inc. Modeling of Climate Change Complex models of global warming suggest that recent temperature increase is consistent with human production of greenhouse gases. Slide 56 2014 Pearson Education, Inc. Consequences of Global Warming Storms more numerous and intense Rising ocean levels; melting glaciers Uncertain effects on food production, availability of fresh water Potential for social unrest Slide 57 What have we learned? Describe the general atmospheric properties of each of the five terrestrial worlds.Describe the general atmospheric properties of each of the five terrestrial worlds. Moon and Mercury: essentially airless with very little atmospheric gas. Venus: thick CO 2 atmosphere, with high surface temperature and pressure. Mars: thin CO 2 atmosphere, usually below freezing and pressure too low for liquid water. Earth: nitrogen/oxygen atmosphere with pleasant surface temperature and pressure.Moon and Mercury: essentially airless with very little atmospheric gas. Venus: thick CO 2 atmosphere, with high surface temperature and pressure. Mars: thin CO 2 atmosphere, usually below freezing and pressure too low for liquid water. Earth: nitrogen/oxygen atmosphere with pleasant surface temperature and pressure. What is atmospheric pressure?What is atmospheric pressure? The result of countless collisions between atoms and molecules in a gas. Measured in bars (1 bar = Earths pressure at sea level.) Summarize the effects of atmospheres.Summarize the effects of atmospheres. Atmospheres absorb and scatter light, create pressure, warm the surface and distribute heat, create weather, and interact with the Solar wind to make auroras.Atmospheres absorb and scatter light, create pressure, warm the surface and distribute heat, create weather, and interact with the Solar wind to make auroras. Slide 58 What have we learned? What is the greenhouse effect?What is the greenhouse effect? Planetary warming caused by the absorption of infrared light from a planets surface by greenhouse gases such as carbon dioxide, methane, and water vapor.Planetary warming caused by the absorption of infrared light from a planets surface by greenhouse gases such as carbon dioxide, methane, and water vapor. How would planets be different without the greenhouse effect?How would planets be different without the greenhouse effect? They would be colder, with temperatures determined only by distance from the Sun and reflectivity.They would be colder, with temperatures determined only by distance from the Sun and reflectivity. Compare the greenhouse effect on Venus, Earth, & Mars.Compare the greenhouse effect on Venus, Earth, & Mars. All three planets are warmed by the greenhouse effect, but it is weak on Mars, moderate on Earth, and very strong on Venus.All three planets are warmed by the greenhouse effect, but it is weak on Mars, moderate on Earth, and very strong on Venus. Slide 59 What have we learned? Describe the basic structure of Earths atmosphere.Describe the basic structure of Earths atmosphere. Pressure and density decrease rapidly with altitude. Temperature drops with altitude in the troposphere, rises with altitude in the lower part of the stratosphere, and rises again in the thermosphere and exosphere.Pressure and density decrease rapidly with altitude. Temperature drops with altitude in the troposphere, rises with altitude in the lower part of the stratosphere, and rises again in the thermosphere and exosphere. How do interactions with light explain atmospheric structure?How do interactions with light explain atmospheric structure? Solar X rays heat and ionize gas in the thermosphere. Solar ultraviolet is absorbed by molecules such as ozone, heating the stratosphere. Visible light warms the surface (and colors the sky), which radiates infrared light that warms the troposphere.Solar X rays heat and ionize gas in the thermosphere. Solar ultraviolet is absorbed by molecules such as ozone, heating the stratosphere. Visible light warms the surface (and colors the sky), which radiates infrared light that warms the troposphere. Contrast the atmospheric structures of Venus, Earth, and Mars.Contrast the atmospheric structures of Venus, Earth, and Mars. Venus and Mars lack and ultraviolet-absorbing stratosphere.Venus and Mars lack and ultraviolet-absorbing stratosphere. Slide 60 What have we learned? What is a magnetosphere?What is a magnetosphere? Created by a global magnetic field, it acts like a protective bubble surrounding the planet that diverts charged particles from the Solar wind, channeling some to the magnetic poles where they can lead to auroras.Created by a global magnetic field, it acts like a protective bubble surrounding the planet that diverts charged particles from the Solar wind, channeling some to the magnetic poles where they can lead to auroras. What is the difference between weather and climate?What is the difference between weather and climate? Weather refers to short-term changes in wind, clouds, temperature, and pressure. Climate is the long-term average of weather.Weather refers to short-term changes in wind, clouds, temperature, and pressure. Climate is the long-term average of weather. What creates global wind patterns?What creates global wind patterns? Atmospheric heating at the equator creates two huge equator-to-pole circulation cells. If the Coriolis effect is strong enough, these large cells may split into smaller cells. This split occurs on Earth, but not on Venus (because of slow rotation) or Mars (because of small size).Atmospheric heating at the equator creates two huge equator-to-pole circulation cells. If the Coriolis effect is strong enough, these large cells may split into smaller cells. This split occurs on Earth, but not on Venus (because of slow rotation) or Mars (because of small size). Slide 61 What have we learned? What causes rain or snow to fall?What causes rain or snow to fall? Convection carries evaporated (or sublimated) water vapor to high, cold altitudes, where it condenses into droplets or ice flakes, forming clouds. When the droplets or ice flakes get large enough, convection cannot hold them aloft and they fall as rain, snow, or hail.Convection carries evaporated (or sublimated) water vapor to high, cold altitudes, where it condenses into droplets or ice flakes, forming clouds. When the droplets or ice flakes get large enough, convection cannot hold them aloft and they fall as rain, snow, or hail. Describe four factors that can cause long-term climate change.Describe four factors that can cause long-term climate change. The gradual brightening of the Sun over the history of the Solar System. Changes in a planets axis tilt. Changes in a planets reflectivity. Changes in a planets abundance of greenhouse gases.The gradual brightening of the Sun over the history of the Solar System. Changes in a planets axis tilt. Changes in a planets reflectivity. Changes in a planets abundance of greenhouse gases. Slide 62 What have we learned? Describe the processes by which an atmosphere can gain and lose gas.Describe the processes by which an atmosphere can gain and lose gas. Gains come from outgassing, evaporation/sublimation, or bombardment, but the latter only if theres very little atmosphere. Gases can be lost by condensation, chemical reactions with surface materials, stripping from the upper atmosphere by small particles or photons, being blasted away by impacts, or by achieving thermal escape velocity.Gains come from outgassing, evaporation/sublimation, or bombardment, but the latter only if theres very little atmosphere. Gases can be lost by condensation, chemical reactions with surface materials, stripping from the upper atmosphere by small particles or photons, being blasted away by impacts, or by achieving thermal escape velocity. Why are the atmospheres of the Moon & Mercury all exosphere?Why are the atmospheres of the Moon & Mercury all exosphere? They have no current source for outgassing and they are too small and warm to hold any atmosphere they may have had in the past. They have small amounts of gas above their surfaces only because of bombardment by Solar wind particles.They have no current source for outgassing and they are too small and warm to hold any atmosphere they may have had in the past. They have small amounts of gas above their surfaces only because of bombardment by Solar wind particles. Slide 63 What have we learned? Describe major, seasonal features of Martian weather today.Describe major, seasonal features of Martian weather today. Seasonal changes in temperature cause carbon dioxide to alternately condense and sublime at the polls, driving pole-to-pole winds and sometimes creating huge dust storms.Seasonal changes in temperature cause carbon dioxide to alternately condense and sublime at the polls, driving pole-to-pole winds and sometimes creating huge dust storms. Why did Mars early warm and wet period come to an end?Why did Mars early warm and wet period come to an end? Mars once had a thick carbon dioxide atmosphere and strong greenhouse effect. Most of the CO 2 was eventually lost to space, probably because the cooling interior could no longer create a strong magnetic field to protect the atmosphere from the Solar wind. As CO 2 to was lost, the greenhouse effect weakened until the planet froze.Mars once had a thick carbon dioxide atmosphere and strong greenhouse effect. Most of the CO 2 was eventually lost to space, probably because the cooling interior could no longer create a strong magnetic field to protect the atmosphere from the Solar wind. As CO 2 to was lost, the greenhouse effect weakened until the planet froze. Slide 64 What have we learned? Why is Venus so hot?Why is Venus so hot? At its distance from the Sun, any liquid water was destined to evaporate, alternately driving a runaway greenhouse effect that dried up the planet and heated it to its extreme temperature.At its distance from the Sun, any liquid water was destined to evaporate, alternately driving a runaway greenhouse effect that dried up the planet and heated it to its extreme temperature. Could Venus ever have had oceans?Could Venus ever have had oceans? Venus probably outgassed plenty of water vapor. Early in the Solar Systems history, when the Sun was dimmer, it is possible that this water vapor could have condensed to make rain and oceans, though we cannot be sure.Venus probably outgassed plenty of water vapor. Early in the Solar Systems history, when the Sun was dimmer, it is possible that this water vapor could have condensed to make rain and oceans, though we cannot be sure. After studying Mars and Venus, why does Earths climate seem surprising?After studying Mars and Venus, why does Earths climate seem surprising? Mars and Venus both underwent dramatic and permanent climate change early in their histories. Earth has somehow maintained a relatively stable climate, even as the Sun has warmed with time.Mars and Venus both underwent dramatic and permanent climate change early in their histories. Earth has somehow maintained a relatively stable climate, even as the Sun has warmed with time.