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Comparing Terrestrial Atmospheres

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1 Chapter 10: Planetary Atmospheres Earth and the Other Terrestrial Worlds

2 Comparing Terrestrial Atmospheres

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 radius Pressure is created by atomic & molecular collisions in an atmosphere. heating a gas in a confined space increases pressure number of collisions increase unit of measure: 1 bar = 14.7 lbs/inch2 = Earth’s atmospheric pressure at sea level Pressure balances gravity in an atmosphere.

4 Earth's Atmosphere About 10 kilometers thick
Consists mostly of molecular nitrogen (N2) and oxygen (O2). © 2014 Pearson Education, Inc.

5 Where does an atmosphere end?
Small amounts of gas are present even above 300 kilometers. © 2014 Pearson Education, Inc.

6 Effects of an Atmosphere on a Planet
greenhouse effect makes the planetary surface warmer than it would be otherwise scattering and absorption of light absorb high-energy radiation from the Sun scattering of optical light brightens the daytime sky creates pressure can allow water to exist as a liquid (at the right temperature) creates wind and weather promotes erosion of the planetary surface creates auroras interaction with the Solar wind when magnetic fields are present

7 The Greenhouse Effect Visible Sunlight passes through a planet’s atmosphere. Some of this light is absorbed by the planet’s surface. Planet re-emits this energy (heat) as infrared (IR) light. planet’s temperature lower than Sun IR light is “trapped” by the atmosphere. its return to space is slowed This causes the overall surface temperature to be higher than if there were no atmosphere at all.

8 Greenhouse Gases Key to Greenhouse Effect…gases which absorb IR light effectively: water [H2O] carbon dioxide [CO2] methane [CH4] These are molecules which rotate and vibrate easily. they re-emit IR light in a random direction The more greenhouse gases which are present, the greater the amount of surface warming.

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 radiation this is necessary for the planet to have a stable temperature

10 What Determines a Planet’s 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 atmosphere it works like a blanket In the absence of the Greenhouse Effect, what would determine a planet’s surface temperature? the planet's distance from the Sun the planet’s overall reflectivity the higher the albedo, the less light absorbed, planet cooler Earth’s average temperature would be –17º C (–1º F) without the Greenhouse Effect

11 Greenhouse Effect on the Planets
Greenhouse Effect warms Venus, Earth, & Mars on Venus: it is very strong on Earth: it is moderate on Mars: it is weak avg. temp. on Venus & Earth would be freezing without it

12 Structure of Earth’s Atmosphere
pressure & density of atmosphere decrease with altitude temperature varies “back and forth” with altitude these 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

13 Atmospheres Interact with Light
X rays ionize atoms & molecules dissociate molecules absorbed by almost all gases Ultraviolet (UV) dissociate some molecules absorbed well by O3 & H2O Visible (V) passes right through gases some photons are scattered Infrared (IR) absorbed by greenhouse gases

14 Reasons for Atmospheric Structure
Light interactions are responsible for the structure we see. Troposphere absorbs IR photons from the surface temperature drops with altitude hot air rises and high gas density causes storms (convection) Stratosphere lies above the greenhouse gases (no IR absorption) absorbs heat via Solar UV photons which dissociate ozone (O3) UV penetrates only top layer; hotter air is above colder air no convection or weather; the atmosphere is stratified Thermosphere absorbs heat via Solar X-rays which ionizes all gases contains ionosphere, which reflects back human radio signals Exosphere hottest layer; gas extremely rarified; provides noticeable drag on satellites

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 (O3) All three planets have warmer surface temps due to greenhouse effect

16 Magnetospheres The Sun ejects a stream of charged particles, called the solar wind. it is mostly electrons, protons, and Helium nuclei Earth’s magnetic field attracts and diverts these charged particles to its magnetic poles. the particles spiral along magnetic field lines and emit light this causes the aurora (aka northern & southern lights) this protective “bubble” is called the magnetosphere Other terrestrial worlds have no strong magnetic fields solar wind particles impact the exospheres of Venus & Mars solar wind particles impact the surfaces of Mercury & Moon

17 Earth’s Magnetosphere

18 What are Weather and Climate?
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 These are Earth’s global wind patterns or circulation local weather systems move along with them weather moves from W to E at mid-latitudes in N hemisphere Two factors cause these patterns atmospheric heating planetary rotation

19 Global Wind Patterns air heated more at equator
warm air rises at equator; heads for poles cold 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

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 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

21 Clouds, Rain and Snow Clouds strongly affect the surface conditions of a planet they increase its albedo, thus reflecting away more sunlight they provide rain and snow, which causes erosion Formation of rain and snow:

22 Four Major Factors which affect Long-term Climate Change

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 escaped present-day atmospheres must have formed at a later time Sources of atmospheric gas: outgassing – release of gas trapped in interior rock by volcanism evaporation/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 rock occurs only if the planet has no substantial atmosphere already

24 Gain/Loss Processes of Atmospheric Gas
Ways to lose atmospheric gas: condensation – gas turns into liquids or ices on the surface when cooled chemical reactions – gas is bound into surface rocks or liquids stripping – 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 space thermal escape – lightweight gas molecules are lost to space when they achieve escape velocity gas is lost forever!

25 Gain/Loss Processes of Atmospheric Gas

26 Origin of the Terrestrial Atmospheres
Venus, Earth, & Mars received their atmospheres through outgassing. most common gases: H2O, CO2, N2, H2S, SO2 Chemical reactions caused CO2 on Earth to dissolve in oceans and go into carbonate rocks (like limestone.) this occurred because H2O could exist in liquid state N2 was left as the dominant gas; O2 was exhaled by plant life as the dominant gas on Venus, CO2 caused strong greenhouse effect Mars lost much of its atmosphere through impacts less massive planet, lower escape velocity

27 Origin of the Terrestrial Atmospheres
Lack of magnetospheres on Venus & Mars made stripping by the Solar wind significant. further loss of atmosphere on Mars dissociation of H2O, H2 thermally escapes on Venus Gas and liquid/ice exchange occurs through condensation and evaporation/sublimation: on Earth with H2O on Mars with CO2 Since Mercury & the Moon have no substantial atmosphere, fast particles and high-energy photons reach their surfaces bombardment creates a rarified exosphere

28 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. © 2014 Pearson Education, Inc.

29 Martian Weather Today Seasons on Mars are more extreme than on Earth
Mars’ orbit is more elliptical CO2 condenses & sublimes at opposite poles changes in atmospheric pressure drive pole-to-pole winds sometimes cause huge dust storms

30 Martian Weather: N Polar Ice Cap & Dust Storm

31 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. Be sure to point out the landslide caught in progress here! © 2014 Pearson Education, Inc.

32 Climate History of Mars
More than 3 billion years ago, Mars must have had a thick CO2 atmosphere and a strong greenhouse effect. the so-called “warm and wet period” Eventually CO2 was lost to space. some gas was lost to impacts cooling interior meant loss of magnetic field Solar wind stripping removed gas Greenhouse effect weakened until Mars froze.

33 Venusian Weather Today
Venus has no seasons to speak of. rotation axis is nearly 90º to the ecliptic plane Venus has little wind at its surface rotates very slowly, so there is no Coriolis effect The surface temperature stays constant all over Venus. thick atmosphere distributes heat via two large circulation cells There is no rain on the surface. it is too hot and Venus has almost no H2O Venusian clouds contain sulfuric acid! implies recent volcanic outgassing?

34 Climate History of Venus
Venus should have outgassed as much H2O as Earth. Early on, when the Sun was dimmer, Venus may have had oceans of water Venus’ proximity to the Sun caused all H2O to evaporate. H2O caused runaway greenhouse effect surface heated to extreme temperature UV photons from Sun dissociate H2O; H2 escapes, O is stripped

35 The Uniqueness of Earth’s Atmosphere
Outgassing from volcanoes on Venus, Earth, & Mars released the same gasses: primarily water (H2O), Carbon dioxide (CO2), and Nitrogen (N2) So why did Earth’s atmosphere end up so different? why did Earth retain most of its H2O – enough to form oceans? why does Earth have so little CO2 in its atmosphere, when it should have outgassed just as much CO2 as Venus? why does Earth have so much more Oxygen (O2) than Venus & Mars? why does Earth have an ultraviolet-absorbing stratosphere?

36 The Uniqueness of Earth’s Atmosphere
Earth’s H2O condensed because of the temperature. oceans formed, in which the CO2 gas dissolved chemical reactions bound the C of CO2 into rocks like limestone low level of atmospheric CO2 causes moderate greenhouse effect temperatures on Earth remain where H2O can be a liquid CO2 There was once liquid H2O on Mars and maybe Venus. before CO2 could dissolve out, temperatures fell/rose so that oceans boiled away on Venus and froze out on Mars Earth’s O2 was not outgassed by volcanoes. O2 is a highly reactive chemical it would disappear in a few million years if not replenished no geologic process creates O2

37 The Uniqueness of Earth’s Atmosphere
Earth’s O2 was created through the evolution of life. plants & microorganism release O2 via photosynthesis they convert CO2 into O2 In the upper atmosphere, O2 in converted into ozone (O3). via chemical processed involving Solar ultraviolet light O3 absorbs Solar UV photons which heats the stratosphere Venus & Mars lack plant & microbial life. so they have no O2 in their atmospheres and no stratospheres

38 The CO2 Cycle

39 The CO2 Cycle is a Feedback Mechanism which Regulates Earth’s Climate

40 Stability of Earth’s Climate
Plate tectonics causes the relative stability of Earth’s climate. plate tectonics makes the CO2 cycle work it takes about 40,000 years for the CO2 cycle to restore balance There have been temporary episodes of extreme cooling and heating in Earth’s history. these ice ages & hothouse period have their own feedback mechanisms

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 epochs more organisms which lacked skeletons leave fewer fossils erosion erases much old fossil evidence subduction destroys fossils carries deep beneath Earth’s surface we have found fossils of large & small animals, plants, microorganisms the fossil record goes back 3.5 billion years deficit of 13C, a sign of life, in rocks as old as 3.85 billion years

42 Origin of Life All known organisms:
build proteins from same subset of amino acids use ATP to store energy in cells use DNA molecules to transmit genes All organisms share same genetic code…sequence 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

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 naturally RNA can form, and if some of it becomes self-replicating, it can lead to… 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

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 O3 in atmosphere Organism’s DNA is reproduced, but can change due to copying errors or external factors a change in the base sequence of an organism’s 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… thrive this 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 diversified Some 2 billion years ago, life was still confined to the oceans.

45 The Rise of Oxygen Single-celled organisms called cyanobacteria appear to have created O2 via photosynthesis as early as 3.5 billion years ago. the first O2 produced was absorbed into rocks via chemical reactions it was not until about 2 billion years ago than the rocks were saturated with O2 and so it began to accumulate in the atmosphere fossil record indicates current levels of O2 were reached 200 million yrs ago The build-up of O2 in the atmosphere permitted: the evolution of oxygen-dependent animals the formation of O3 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…

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 animals animals diversified into all the basic body plans which we find today this 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 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?

47 Mass Extinctions Evolution of species normally occurs gradually.
one species goes extinct per century Evolution can receive a jolt during a mass extinction. historically, 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 ecosystem species 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? what will the consequences be for humans?

48 Ozone Depletion O3 in the stratosphere shields Earth’s surface from Solar UV A depletion of O3 was observed ozone hole over Antarctica discovered in mid-1980’s appears in Antarctic spring 1979 1998 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 O3 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

49 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. CO2 from outgassing will build up if oceans are frozen, ultimately raising global temperatures again. © 2014 Pearson Education, Inc.

50 How is human activity changing our planet?
© 2014 Pearson Education, Inc.

51 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. © 2014 Pearson Education, Inc.

52 Global Warming Earth's average temperature has increased by 0.5°C in past 50 years. The concentration of CO2 is rising rapidly. An unchecked rise in greenhouse gases will eventually lead to global warming. © 2014 Pearson Education, Inc.

53 CO2 Concentration Global temperatures have tracked CO2 concentration for last 500,000 years. Current CO2 concentration is the highest it's been in at least 500,000 years. © 2014 Pearson Education, Inc.

54 CO2 Concentration Most of the CO2 increase has happened in last 50 years! © 2014 Pearson Education, Inc.

55 Modeling of Climate Change
Complex models of global warming suggest that recent temperature increase is consistent with human production of greenhouse gases. © 2014 Pearson Education, Inc.

56 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 © 2014 Pearson Education, Inc.

57 What have we learned? Describe the general atmospheric properties of each of the five terrestrial worlds. Moon and Mercury: essentially airless with very little atmospheric gas. Venus: thick CO2 atmosphere, with high surface temperature and pressure. Mars: thin CO2 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? The result of countless collisions between atoms and molecules in a gas. Measured in bars (1 bar = Earth’s pressure at sea level.) 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.

58 What have we learned? What is the greenhouse effect?
Planetary warming caused by the absorption of infrared light from a planet’s surface by greenhouse gases such as carbon dioxide, methane, and water vapor. How would planets be different without the greenhouse effect? They would be colder, with temperatures determined only by distance from the Sun and reflectivity. 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.

59 What have we learned? Describe the basic structure of Earth’s 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. 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. Contrast the atmospheric structures of Venus, Earth, and Mars. Venus and Mars lack and ultraviolet-absorbing stratosphere.

60 What have we learned? 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. 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. 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).

61 What have we learned? 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. 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 planet’s axis tilt. Changes in a planet’s reflectivity. Changes in a planet’s abundance of greenhouse gases.

62 What have we learned? 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 there’s 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”? 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.

63 What have we learned? 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. 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 CO2 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 CO2 to was lost, the greenhouse effect weakened until the planet froze.

64 What have we learned? 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. Could Venus ever have had oceans? Venus probably outgassed plenty of water vapor. Early in the Solar System’s 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 Earth’s 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.


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