Activity 2: The Earth’s Atmosphere & Magnetic Field

1. Module 7: Home Planet – the Earth Activity 2:The Earth’s Atmosphere & Magnetic Field

2. Summary • In this Activity, we will investigate • (a) the composition of the Earth’s atmosphere, • (b) layers & temperatures in the Earth’s atmosphere, • (c) the effects of sunlight, • (d) the greenhouse effect, • (e) the ozone layer, • (f) atmospheric circulation, and • (g) the structure of the Earth’s magnetic field.

3. (a) The composition of the Earth’s atmosphere The Earth’s atmosphere has played a pivotal part in our evolution, and as we shall see in the next Module, it toohas evolved dramatically with time. Other planets have atmospheres too, though none quitelike ours! We’ll be comparing their atmospheres to oursin later Modules. We can study an atmosphere in various ways. The easiestway to start is with its “bulk” (i.e. average) properties:

4. The bulk composition of the Earth’s atmosphere is approximately: • 78% molecular nitrogen • 20% molecular oxygen • 0.03% carbon dioxide, • and • up to 2% water vapour.

5. (b) Layers of the Earth’s Atmosphere 400 km Ionosphere 90 km Mesosphere 50 km Ozone Layer Stratosphere 18 km Tropopause 14 km Troposphere 0 km

6. Temperature variations in the Earth’s atmosphere: 400 km Ionosphere 90 km Mesosphere 50 km Ozone Layer Stratosphere 18 km Tropopause  220oK  280oK 14 km Troposphere 0 km

7. The troposphere is mainly heated by infrared radiation re-emitted by the ground, so the temperature in the troposphere decreases with altitude.

8. 400 km Ionosphere 90 km Mesosphere 50 km  260oK Ozone Layer Stratosphere 18 km Tropopause 14 km Troposphere 0 km

9. The ozone layer is in the upper stratosphere and lower mesosphere. • The ozone absorbs ultraviolet radiation from the Sun, and this process heats up the neighbouring layers • - which are therefore warmer than the upper troposphere.

10. 400 km Ionosphere  190oK 90 km Mesosphere 50 km Ozone Layer Stratosphere 18 km Tropopause 14 km Troposphere 0 km

11. The temperature gradually drops again as we go up in altitude through the mesosphere,

12. 400 km  270oK Ionosphere 90 km Mesosphere 50 km Ozone Layer Stratosphere 18 km Tropopause 14 km Troposphere 0 km

13. … until we reach the ionosphere (sometimes called the “thermosphere”) which is heated by absorbing energy from energetic X-rays from the Sun, so the temperature there can be quite high - but the density is extremely low.

14. (c) The Effects of Sunlight • The Earth’s atmosphere is profoundly affected by another member of our Solar System …. … the Sun.

15. 1.37 kJ of energy per square metre arrivesat the Earth’s orbit every second from the Sun! The solar energy incident on the daytime side of the Earthis eventually reflected or absorbed. The absorbed part heats the Earth’s atmosphere and surface. Without its atmosphere, the Earth’s surface temperature would vary more widely, and its average would be well below freezing.

16. Components of the Earth’s atmosphere and surface absorb and reflect the sun’s radiation by differing amounts: land absorbs well clouds reflect well sea absorbs the most ice reflects the most The ability of a surface to reflect light is called itsalbedo.

17. 1 0 sea land clouds polar caps The formal definition of albedo is the fraction of incident light reflected from a surface, and so has a value between 0 and 1: no light reflected all light reflected What do you expect the average albedo of the Earth to be - smaller or greater than 0.5? The Earth’s average albedo is 0.37. This makes sense - both sea and land have very low albedos. Clouds reflect well, but clouds cover only part of the Earth on average.

18. We will see in later Modules that Mercury, a planet with no cloud, has a very low albedo (0.12) - whereas Venus, which is permanently shrouded in cloud, has a very high albedo (0.76). • There are actually several different types of albedo, including: • monochromatic albedo, which is simply the ratio of incident energy to reflected energy at any particular wavelength (e.g. in the optical) • Bond albedo is the ratio of the total radiation reflected from a surface to the total light incident from the Sun averaged over all wavelengths

19. this re-radiated energy isinfrared radiation (d) The Greenhouse Effect All planets absorb energy from the Sun, but long agothey reached equilibrium - that is, the amount of energy they absorb per second is equal to the amount per second they re-radiate out into space. Incident sunlight Reflected sunlight Re-radiated energy

20. Water vapour, methane and (to a lesser extent) carbon dioxide and certain other gases in our atmosphere are good absorbers of infrared radiation, so they trap much of the re-radiated energy inside the atmosphere. Most of the re-radiated infrared radiationis trapped within the atmosphere The result is that the Earth is significantlywarmer than it would be without an atmosphere. Water vapour, carbon dioxide and methaneare examples of “greenhouse gases”. To find out why the term greenhouse is (mis)used, click here.

21. Water vapour is the main greenhouse gas. Its levelsin the Earth’s atmosphere vary from time to time, but remain roughly constant on average. The current scientific debate about the greenhouseeffect centres on the rising levels of carbon dioxide and methane in the Earth’s atmosphere, due to sources such as the burning of fossil fuels and effects such as deforestation and increased agricultural activities.

22. The average atmospheric temperature of the Earth appears to be rising somewhat - is this due to increased levels of greenhouse gases due to human activities, or other natural effects such as long-term fluctuations in the Earth’s weather? This is a continuing (and important) scientific & politicaldebate. From whatever cause, geological evidence suggests that the Earth is now as warm as it has ever been in the last 150,000 years, and the Earth’s global temperature increased by about 0.6°C in the 20th Century. Unfortunately we can’t afford to watch for the few hundred years it would take to establish a firm trend, andthe cause and effects, conclusively either way!

23. As we will see, there is one place in the Solar System where we can see the effects of a runaway greenhouse effect: Venus.

24. (e) The Ozone Layer Part of the incident sunlight striking our atmosphere ismade up of ultraviolet (UV) radiation. Incident sunlight,including UV radiation Earth’s upper atmosphere

25. The UV radiation breaks up oxygen molecules (O2), with the result that they recombine as ozone molecules (O3). Incident sunlight,including UV radiation These ozone molecules accumulatein a 30km thick layer (starting about25km above the Earth’s surface) -the Ozone Layer.

26. Ozone is a strong absorber of UV radiation, so the ozonelayer protects the oxygen in the lower atmosphere from most of the sun’s UV rays. Incident sunlight,including UV radiation The ozone layer

27. In recent times the ozone layer appears to be thinningout. For example, the ozone concentration over the Antarctic dropped by a factor of two from the 1950s to the 1980s. The prime suspects are “chlorofluorocarbons” (CFCs),released from old-style refrigerant systems & spray cans.Each chlorine atom is capable of breaking up approximately 100,000 ozone molecules.

28. The Antarctic and Arctic regions are particularly at risk, because, in the polar winters, the stratosphere in those regions becomes cold enough to form water ice and nitric acid ice particles, which act as catalysts to accelerate the production of chlorine molecules. Once summer returns to these regions, sunlight breaks (‘photodissociates’) the chlorine molecules up into chlorine atoms, which then in turn attack the ozone layer. International efforts are now taking place to reverse this trend. With prompt action, the levels of ozone in the ozone layer can be built up again.

29. (f) Atmospheric Circulation As we who live here well know, the Earth’s atmosphere is not static. Winds & storms are regular features on this and other planets.

30. As sunlight warms the surface of the Earth, it warms the layer of air directly abovethe surface. Warm air expands,becoming less dense and lighter than the airabove it. Therefore it rises, and heavier air above falls down to take its place.

31. The layer of air that had risen starts to cool down, becomingdenser again. The layer of air that is now directly above the surface warms up and rises in turn. - so the whole cycle repeats itself. These air currents are convective currents.

32. Different areas of the Earth’s surface - e.g. land & water - reach different equilibrium temperatures. At the water’s edge on a hot summer’s day, for example, warm air rises over the land and cooler air from over the ocean takes its place - providing a cooling onshorebreeze.

33. In these ways convective currents are set up in the Earth’s lower atmosphere. The Earth’s rotation twists the convective currents to establish global atmospheric circulation patterns. * In the Activities on the Jovian planets,we will compare their atmosphericcirculation to that of the Earth. * Jupiter, the Dominant Gas Giant Planet, and The Other Jovians

34. The Earth’satmosphericcirculationpatterns aretraced byits cloud patterns.

35. The Earth’s atmospheric circulation patterns are complicated by the presence of significant amounts of water vapour. Water vapour is the only gas in the Earth’s atmospherewhich can change to a liquid (in clouds) and fall to the surface (as rain). When water vapour turns to rain, the local air pressuredrops somewhat, providing local variations in the aircurrents and making the atmospheric circulation morecomplex.

36. (g) The Structure of the Earth’s Magnetic Field The Earth acts much like a bar magnet, possessing a magnetic field which deflects compasses on theEarth’s surface to point northwards. We represent the magnetic field at any point on or above the Earth’s surface by a line pointing in the direction a compasswould point.

37. rotation axis The magnetic field axisis tilted at 12° to the axisof rotation of the Earth. magnetic field axis Careful study of the magnetic structure of ancient rocks suggests that the Earth’smagnetic field has reversedits direction several timesover the Earth’s history - though exactly how this reversal mechanism works is not understood Click here to see an animation of the earths Magnetic Field

38. The magnetic pole’s northward journey It is known, however, that the magnetic poles are constantly on the move. The location of the magnetic north pole has been recorded for over 170 years and has been steadily moving north by an average 10 km per year. The global magnetic field strength has also weakened by about 10% since the 19th century. The jury is still out as to whether this means we are due for another field reversal or not.

39. To understand magnetic reversal, we first need to understand what actually drives the Earth’s magnetic field. Magnetic fields are created by moving electric charges. The Earth’s magnetic field is thought to be produced by the motion of charged particles in the convective currents of the metallic liquid outer core. liquid outer core The theory of the Earth’s self-generating magnetic field is called the “dynamo effect”, though the exact details are not fully understood.

40. For a planet to have a magnetic field, it needs a region where charged particles are moving in convective currents. The planet’s rotation is also important in helping generate its magnetic field.

41. The Earth’s magnetic field acts to protect life on Earth from cosmic ray particles coming from the Sun and from deep space. magnetic field axis Cosmic rays are mostly deflected by the Earth’s magnetic field, some spiraling around it till they reach the atmosphere over the poles.

42. When the number of cosmic rays is high, the energy they release when striking the atmosphere is seen at the polar regions as the northern and southern lights, or aurorae. Aurora are rapidly varying colourful displays that shimmer across large regions of the sky. The different colours are mainly due to excited oxygen (green and red) and nitrogen (blue) atoms and molecules in the upper atmosphere. We will discuss aurorae again in the Activity on High Energy Astronomy.

43. Summary • In this Activity, we have looked at the average properties of the Earth’s atmosphere, including its composition and structure. The effects of the Sun on our atmosphere and atmospheric circulation were also investigated, and we introduced the Earth’s magnetic field. • In the next Module, we will investigate how the Earth has evolved since its formation over 4.5 billion years ago.

44. Image Credits • NASA: View of Australia • http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_australia.jpg • NASA: Monsoon over India • http://earth.jsc.nasa.gov/lores.cgi?PHOTO=STS51F-31-0069 • NASA: View of the Mid-Pacific Ocean • http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_mid-pacific.jpg • NASA: The Northern Lights • http://www.athena.ivv.nasa.gov/curric/space/solterr/aurora.html • NASA: World Cloud Cover Pattern http://www.hq.nasa.gov/office/ese/gallery/Originals/cloud.jpg • Natural Resources Canada: Movement of Earth’s north magnetic pole • http://www.geolab.nrcan.gc.ca/geomag/images/nmppath2001.gif

45. Now return to the Module home page, and read more about the Earth’s atmosphere and magnetic field in the Textbook Readings. Hit the Esc key (escape) to return to the Module 7 Home Page

47. The Greenhouse Effect • Greenhouses maintain a higher temperature than their surroundings - which is why delicate plants are kept in them in cold winters. They achieve this due to their glass (or plastic) walls, which let light in which is largely absorbed by the plants and surfaces inside the greenhouse. These reradiate infrared radiation, which warms up the air in the greenhouse. This sounds pretty similar to the situation of the Earth andits atmosphere, which is why the term greenhouse effectis used.

48. There is an important difference though. Although the air and walls of a greenhouse do absorb infrared radiation, the main reason that a greenhouse stays warmer than its surroundings in winter is that its walls trap the warm air, preventing cooling drafts. So the Earth’s atmosphere is not exactly like a greenhouse:it has no walls. Our atmosphere is relatively warm becauseit traps re-radiated infrared radiation by absorbing most of it before it reaches space. Back to the Activity!