1. Chapter 7 Earth and the Terrestrial Worlds

2. Mercury craters smooth plains cliffs

3. Venus volcanoes few craters Radar view of a twin- peaked volcano

4. Mars some craters volcanoes riverbeds?

5. Moon craters smooth plains

6. Earth volcanoes craters mountains riverbeds

7. Why have the planets turned out so differently, even though they formed at the same time from the same materials?

8. 9.1 Earth as a Planet Our goals for learning: Why is Earth geologically active? What processes shape Earth’s surface? How does Earth’s atmosphere affect the planet?

9. Why is Earth geologically active?

10. Earth’s Interior Core: Highest density; nickel and iron Mantle: Moderate density; silicon, oxygen, etc. Crust: Lowest density; granite, basalt, etc.

11. Terrestrial Planet Interiors Applying what we have learned about Earth’s interior to other planets tells us what their interiors are probably like.

12. Why do water and oil separate? A.Water molecules repel oil molecules electrically. B.Water is denser than oil, so oil floats on water. C.Oil is more slippery than water, so it slides to the surface of the water. D.Oil molecules are bigger than the spaces between water molecules.

13. Why do water and oil separate? A.Water molecules repel oil molecules electrically. B.Water is denser than oil, so oil floats on water. C.Oil is more slippery than water, so it slides to the surface of the water. D.Oil molecules are bigger than the spaces between water molecules.

14. Differentiation Gravity pulls high-density material to center Lower-density material rises to surface Material ends up separated by density

15. Thought Question What is necessary for differentiation to occur in a planet? A.It must have metal and rock in it. B.It must be a mix of materials of different density. C.Material inside must be able to flow. D.All of the above. E.b and c.

16. Thought Question What is necessary for differentiation to occur in a planet? A.It must have metal and rock in it. B.It must be a mix of materials of different density. C.Material inside must be able to flow. D.All of the above. E.b and c.

17. Lithosphere A planet’s outer layer of cool, rigid rock is called the lithosphere. It “floats” on the warmer, softer rock that lies beneath.

18. Thought Question Do rocks s-t-r-e-t-c-h? A.No—rock is rigid and cannot deform without breaking. B.Yes—but only if it is molten rock. C.Yes—rock under strain may slowly deform.

19. Thought Question Do rocks s-t-r-e-t-c-h? A.No—rock is rigid and cannot deform without breaking. B.Yes—but only if it is molten rock. C.Yes—rock under strain may slowly deform.

20. Strength of Rock Rock stretches when pulled slowly but breaks when pulled rapidly. The gravity of a large world pulls slowly on its rocky content, shaping the world into a sphere.

21. Heat Drives Geological Activity Convection: hot rock rises, cool rock falls. One convection cycle takes 100 million years on Earth.

22. Sources of Internal Heat 1.Gravitational potential energy of accreting planetesimals 2.Differentiation 3.Radioactivity

23. Heating of Interior over Time Accretion and differentiation when planets were young Radioactive decay is most important heat source today

24. Cooling of Interior Convection transports heat as hot material rises and cool material falls Conduction transfers heat from hot material to cool material Radiation sends energy into space

25. Thought Question What cools off faster? A.A grande-size cup of Starbucks coffee B.A teaspoon of cappuccino in the same cup

26. Thought Question What cools off faster? A.A grande-size cup of Starbucks coffee B.A teaspoon of cappuccino in the same cup

27. Thought Question What cools off faster? A.A big terrestrial planet B.A tiny terrestrial planet

28. Thought Question What cools off faster? A.A big terrestrial planet B.A tiny terrestrial planet

29. Role of Size Smaller worlds cool off faster and harden earlier. Moon and Mercury are now geologically “dead.”

30. Surface Area to Volume Ratio Heat content depends on volume. Loss of heat through radiation depends on surface area. Time to cool depends on surface area divided by volume: Larger objects have a smaller ratio and cool more slowly.

31. Planetary Magnetic Fields Moving charged particles create magnetic fields. A planet’s interior can create magnetic fields if its core is electrically conducting, convecting, and rotating.

32. Earth’s Magnetosphere Earth’s magnetic fields protects us from charged particles from the Sun. The charged particles can create aurorae (“Northern lights”).

33. Thought Question If the planet core is cold, do you expect it to have magnetic fields? A.Yes, refrigerator magnets are cold, and they have magnetic fields. B.No, planetary magnetic fields are generated by moving charges around, and if the core is cold, nothing is moving.

34. Thought Question If the planet core is cold, do you expect it to have magnetic fields? A.Yes, refrigerator magnets are cold, and they have magnetic fields. B.No, planetary magnetic fields are generated by moving charges around, and if the core is cold, nothing is moving.

35. Special Topic: How do we know what’s inside a planet? P waves push matter back and forth. S waves shake matter side to side.

36. Special Topic: How do we know what’s inside a planet? P waves go through Earth’s core, but S waves do not. We conclude that Earth’s core must have a liquid outer layer.

37. End of Lecture 10 Mar 2008

38. What processes shape Earth’s surface…

39. and might do the same on the other terrestrial planets?

40. Geological Processes Impact cratering —Impacts by asteroids or comets Volcanism —Eruption of molten rock onto surface Tectonics —Disruption of a planet’s surface by internal stresses Erosion —Surface changes made by wind, water, or ice

41. Impact Cratering The Production of a Crater

42. Impact Cratering Most cratering happened soon after the solar system formed. The Production of a Crater

43. Impact Cratering Most cratering happened soon after the solar system formed. Craters are about 10 times wider than objects that made them. The Production of a Crater

44. Impact Cratering Most cratering happened soon after the solar system formed. Craters are about 10 times wider than objects that made them. Small craters greatly outnumber large ones. The Production of a Crater

45. Impact Craters Meteor Crater (Arizona) Tycho (Moon)

46. Impact Craters Meteor Crater (Arizona) ~50,000 years old ~50,000 years old Tycho (Moon)

47. Impact Craters Meteor Crater (Arizona) ~50,000 years old ~50,000 years old Tycho (Moon) ~108 million years old ~108 million years old

48. Impact Craters Meteor Crater (Arizona) ~50,000 years old ~50,000 years old ~4,000 feet across ~4,000 feet across Tycho (Moon) ~108 million years old ~108 million years old

49. Impact Craters Meteor Crater (Arizona) ~50,000 years old ~50,000 years old ~4,000 feet across ~4,000 feet across Tycho (Moon) ~108 million years old ~108 million years old ~50 miles across ~50 miles across

50. Impact Craters Meteor Crater (Arizona) ~50,000 years old ~50,000 years old ~4,000 feet across ~4,000 feet across ~550 feet deep ~550 feet deep Tycho (Moon) ~108 million years old ~108 million years old ~50 miles across ~50 miles across

51. Impact Craters Meteor Crater (Arizona) ~50,000 years old ~50,000 years old ~4,000 feet across ~4,000 feet across ~550 feet deep ~550 feet deep Tycho (Moon) ~108 million years old ~108 million years old ~50 miles across ~50 miles across ~3 miles deep ~3 miles deep

52. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. Volcanic Eruptions and Lava Flows

53. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. To have volcanism, you need: Volcanic Eruptions and Lava Flows

54. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. To have volcanism, you need: –a molten interior Volcanic Eruptions and Lava Flows

55. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. To have volcanism, you need: –a molten interior –a thin lithosphere Volcanic Eruptions and Lava Flows

56. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. To have volcanism, you need: –a molten interior –a thin lithosphere You don’t expect volcanism on small planets Volcanic Eruptions and Lava Flows

57. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. Volcanic Eruptions and Lava Flows

58. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. Molten rock is called lava after it reaches the surface. Volcanic Eruptions and Lava Flows

59. Volcanism Volcanism happens when molten rock (magma) finds a path through lithosphere to the surface. Molten rock is called lava after it reaches the surface. There are several different kinds of lava Volcanic Eruptions and Lava Flows

60. Lava and Volcanoes Runny lava makes flat lava plains.

61. Lava and Volcanoes Runny lava makes flat lava plains. Slightly thicker lava makes broad shield volcanoes.

62. Lava and Volcanoes Runny lava makes flat lava plains. Slightly thicker lava makes broad shield volcanoes. Thickest lava makes steep stratovolcanoes.

63. Outgassing Volcanism also releases gases from Earth’s interior into the atmosphere

64. Outgassing Volcanism also releases gases from Earth’s interior into the atmosphere This is one way atmospheres are obtained

65. Outgassing Volcanism also releases gases from Earth’s interior into the atmosphere This is one way atmospheres are obtained There are other ways as well

66. Ways to add gas to an atmosphere

67. Ways to take gas out of an atmosphere

68. Tectonics Convection of the mantle creates stresses in the crust called tectonic forces. Tectonics and Convection of the Mantle

69. Tectonics Convection of the mantle creates stresses in the crust called tectonic forces. Compression forces make mountain ranges.

70. Tectonics Convection of the mantle creates stresses in the crust called tectonic forces. Compression forces make mountain ranges. A valley can form where the crust is pulled apart.

71. Tectonics Convection of the mantle creates stresses in the crust called tectonic forces. Compression forces make mountain ranges. A valley can form where the crust is pulled apart. There’s a special kind of tectonics on Earth

72. Plate Tectonics on Earth Earth’s continents slide around on separate plates of crust. Plate Tectonics on Earth

73. Earth’s continents slide around on separate plates of crust. This is important for life here on Earth Plate Tectonics on Earth

74. Earth’s continents slide around on separate plates of crust. This is important for life here on Earth We’ll get to that later Plate Tectonics on Earth

75. Erosion Erosion is a blanket term for weather-driven processes that break down or transport rock.

76. Erosion Erosion is a blanket term for weather-driven processes that break down or transport rock. Processes that cause erosion include —Glaciers —Rivers —Wind

77. Erosion by Water The Colorado River continues to carve the Grand Canyon.

78. Erosion by Ice Glaciers carved the Yosemite Valley.

79. Erosion by Wind Wind wears away rock and builds up sand dunes.

80. Erosional Debris Erosion can create new features by depositing debris.

81. Erosional Debris Erosion can create new features by depositing debris. Extensive erosion happens on Earth because of our atmosphere

82. Effects of Atmosphere on Earth Erosion

83. Effects of Atmosphere on Earth Erosion Earth’s atmosphere does other things:

84. Effects of Atmosphere on Earth Erosion Earth’s atmosphere does other things: Radiation protection

85. Effects of Atmosphere on Earth Erosion Earth’s atmosphere does other things: Radiation protection Greenhouse effect

86. Effects of Atmosphere on Earth Erosion Earth’s atmosphere does other things: Radiation protection Greenhouse effect Makes the sky blue!

87. Radiation Protection All X-ray light is absorbed very high in the atmosphere.

88. Radiation Protection All X-ray light is absorbed very high in the atmosphere. Ultraviolet light is absorbed by ozone (O 3 ).

89. Earth’s atmosphere absorbs light at most wavelengths.

90. The Greenhouse Effect

91. Greenhouse effect: Certain molecules let sunlight through but trap escaping infrared photons. (H 2 O, CO 2, CH 4 ) The Green House Effect

92. The Greenhouse Effect Which Molecules are Greenhouse Gases?

93. A Greenhouse Gas Any gas that absorbs infrared Greenhouse gas: molecules with two different types of elements (CO 2, H 2 O, CH 4 ) Not a greenhouse gas: molecules with one or two atoms of the same element (O 2, N 2 )

94. Greenhouse Effect: Bad? The Earth is much warmer because of the greenhouse effect than it would be without an atmosphere…but so is Venus.

95. Thought Question Why is the sky blue? A.The sky reflects light from the oceans. B.Oxygen atoms are blue. C.Nitrogen atoms are blue. D.Air molecules scatter blue light more than red light. E.Air molecules absorb red light.

96. Thought Question Why is the sky blue? A.The sky reflects light from the oceans. B.Oxygen atoms are blue. C.Nitrogen atoms are blue. D.Air molecules scatter blue light more than red light. E.Air molecules absorb red light.

97. Why the sky is blue Atmosphere scatters blue light from the Sun, making it appear to come from different directions. Sunsets are red because less of the red light from the Sun is scattered.

98. End of Lecture 12 Mar 2008

99. What have we learned? Why is Earth geologically active? —Earth retains plenty of internal heat because it is large for a terrestrial planet.

100. What have we learned? Why is Earth geologically active? —Earth retains plenty of internal heat because it is large for a terrestrial planet. —That heat drives geological activity, keeping the core molten and driving geological activity.

101. What have we learned? Why is Earth geologically active? —Earth retains plenty of internal heat because it is large for a terrestrial planet. —That heat drives geological activity, keeping the core molten and driving geological activity. —The circulation of molten metal in the core generates Earth’s magnetic field.

102. What have we learned? What geological processes shape Earth’s surface?

103. What have we learned? What geological processes shape Earth’s surface? —Impact cratering, volcanism, tectonics, and erosion

104. What have we learned? What geological processes shape Earth’s surface? —Impact cratering, volcanism, tectonics, and erosion How does Earth’s atmosphere affect the planet?

105. What have we learned? What geological processes shape Earth’s surface? —Impact cratering, volcanism, tectonics, and erosion How does Earth’s atmosphere affect the planet? —Erosion

106. What have we learned? What geological processes shape Earth’s surface? —Impact cratering, volcanism, tectonics, and erosion How does Earth’s atmosphere affect the planet? —Erosion —Protection from radiation

107. What have we learned? What geological processes shape Earth’s surface? —Impact cratering, volcanism, tectonics, and erosion How does Earth’s atmosphere affect the planet? —Erosion —Protection from radiation —Greenhouse effect

108. What have we learned? What geological processes shape Earth’s surface? —Impact cratering, volcanism, tectonics, and erosion How does Earth’s atmosphere affect the planet? —Erosion —Protection from radiation —Greenhouse effect Now let’s look at the other terrestrials

109. 7.2 Mercury and the Moon: Geologically Dead Our goals for learning: Was there ever geological activity on the Moon or Mercury?

111. Moon Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria.

112. Moon Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria. The Moon is now geologically dead.

113. Cratering of Mercury Mercury has a mixture of heavily cratered and smooth regions like the Moon.

114. Cratering of Mercury Mercury has a mixture of heavily cratered and smooth regions like the Moon. And like the Moon, the smooth regions are likely ancient lava flows.

115. Cratering of Mercury The Caloris basin is the largest impact crater on Mercury Region opposite the Caloris Basin is jumbled from seismic energy of impact

116. Tectonics on Mercury Long cliffs indicate that Mercury shrank early in its history

117. Tectonics on Mercury Long cliffs indicate that Mercury shrank early in its history…as the core cooled

118. Tectonics on Mercury Long cliffs indicate that Mercury shrank early in its history…as the core cooled…because the cooler core was smaller Why does this make sense?

119. Tectonics on Mercury Long cliffs indicate that Mercury shrank early in its history…as the core cooled…because the cooler core was smaller…and dragged the surface down as it shrank

120. What have we learned? Was there ever geological activity on the Moon or Mercury? —Early cratering on Moon and Mercury is still present, indicating that geological activity ceased long ago.

121. What have we learned? Was there ever geological activity on the Moon or Mercury? —Early cratering on Moon and Mercury is still present, indicating that geological activity ceased long ago. —Lunar maria resulted from early volcanism.

122. What have we learned? Was there ever geological activity on the Moon or Mercury? —Early cratering on Moon and Mercury is still present, indicating that geological activity ceased long ago. —Lunar maria resulted from early volcanism. —Tectonic features on Mercury indicate early shrinkage.

123. 7.3 Mars: A Victim of Planetary Freeze-drying Our goals for learning: What geological features tell us that water once flowed on Mars? Why did Mars change?

124. Mars versus Earth 50% Earth’s radius, 10% Earth’s mass

125. Mars versus Earth 50% Earth’s radius, 10% Earth’s mass 1.5 AU from the Sun

126. Mars versus Earth 50% Earth’s radius, 10% Earth’s mass 1.5 AU from the Sun Axis tilt about the same as Earth

127. Mars versus Earth 50% Earth’s radius, 10% Earth’s mass 1.5 AU from the Sun Axis tilt about the same as Earth Similar rotation period

128. Mars versus Earth 50% Earth’s radius, 10% Earth’s mass 1.5 AU from the Sun Axis tilt about the same as Earth Similar rotation period Thin CO 2 atmosphere: little greenhouse

129. Mars versus Earth 50% Earth’s radius, 10% Earth’s mass 1.5 AU from the Sun Axis tilt about the same as Earth Similar rotation period Thin CO 2 atmosphere: little greenhouse Main difference: Mars is SMALLER

130. Seasons on Mars Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit.

131. Seasons on Mars Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit. Strong pole-to-pole winds occur when CO 2 freezes out of the atmosphere at the winter pole (pressure ↓) and sublimates at the summer pole (pressure ↑)

132. Seasons on Mars Seasons on Mars are more extreme in the southern hemisphere because of its elliptical orbit. Strong pole-to-pole winds occur when CO 2 freezes out of the atmosphere at the winter pole (pressure ↓) and sublimates at the summer pole (pressure ↑) The winds are caused by the pressure differences

133. Storms on Mars Those seasonal winds on Mars can drive huge dust storms.

134. The surface of Mars has signs of geological activity and of past liquid water

135. The surface of Mars appears to have ancient riverbeds.

136. The condition of craters indicates surface history. Eroded crater

137. Close-up of eroded crater

139. Volcanoes…as recent as 180 million years ago…

140. Past tectonic activity…

141. Low-lying regions may once have had oceans.

143. Opportunity Spirit

144. Today, most water lies frozen underground (blue regions) Some scientists believe accumulated snowpack melts carve gullies even today.

145. 2004 Opportunity Rover provided strong evidence for abundant liquid water on Mars in the distant past. How could Mars have been warmer and wetter in the past?

146. Why did Mars change?

147. Climate Change on Mars Mars has not had widespread surface water for 3 billion years.

148. Climate Change on Mars Mars has not had widespread surface water for 3 billion years. The greenhouse effect probably kept the surface warmer before that.

149. Climate Change on Mars Mars has not had widespread surface water for 3 billion years. The greenhouse effect probably kept the surface warmer before that. Which means it must have had thicker atmosphere

150. Climate Change on Mars Mars has not had widespread surface water for 3 billion years. The greenhouse effect probably kept the surface warmer before that. Which means it must have had thicker atmosphere So what happened?

151. Climate Change on Mars Magnetic field may have preserved early Martian atmosphere

152. Climate Change on Mars Magnetic field may have preserved early Martian atmosphere…How do magnetic fields do that?

153. Climate Change on Mars Magnetic field may have preserved early Martian atmosphere Solar wind may have stripped atmosphere after the interior cooled and the field decreased

154. What have we learned? What geological features tell us water once flowed on Mars? —Dry riverbeds, eroded craters, and rock-strewn floodplains all show that water once flowed on Mars. —Mars today has ice, underground water ice, and perhaps pockets of underground liquid water. Why did Mars change? —Mars’ atmosphere must have once been much thicker for its greenhouse effect to allow liquid water on the surface. —Somehow Mars lost most of its atmosphere, perhaps because of a declining magnetic field.

155. 7.4 Venus: A Hothouse World Our goals for learning: Is Venus geologically active? Why is Venus so hot?

156. Is Venus geologically active?

157. Cratering on Venus Impact craters, but fewer than Moon, Mercury, Mars

158. Volcanoes on Venus Many volcanoes, including both shield volcanoes and stratovolcanoes

159. Tectonics on Venus Fractured and contorted surface indicates tectonic stresses

160. Erosion on Venus Photos of rocks taken by lander show little erosion

161. Does Venus have plate tectonics? Most of Earth’s major geological features can be attributed to plate tectonics, which gradually remakes Earth’s surface. Venus does not appear to have plate tectonics, but from crater counts its entire surface seems to have been “repaved” ~750 million years ago. No one really understands the geology of Venus

162. Why is Venus so hot?

163. The greenhouse effect on Venus keeps its surface temperature at 470°C (880°F). But why is the greenhouse effect on Venus so much stronger than on Earth?

164. Atmosphere of Venus Venus has a very thick carbon dioxide atmosphere with a surface pressure 90 times that of Earth.

165. Greenhouse Effect on Venus Thick carbon dioxide atmosphere produces an extremely strong greenhouse effect. Earth escapes this fate because most of its carbon and water are in rocks and oceans.

166. Atmosphere of Venus Reflective clouds contain droplets of sulfuric acid. The upper atmosphere has fast winds that remain unexplained. Though Venus should have as much water as Earth, there is very little water there

167. Runaway Greenhouse Effect The runaway greenhouse effect would account for why Venus has so little water. Once the water was in the atmosphere, solar UV split it Greater heat, more evaporation More evaporation, stronger greenhouse effect

168. End of Lecture 24 Mar 2008

169. What have we learned? Is Venus geologically active? —Its surface shows evidence of major volcanism and tectonics during the last billion years. —There is no evidence for erosion or plate tectonics. Why is Venus so hot? —The runaway greenhouse effect made Venus too hot for liquid oceans. —All carbon dioxide remains in the atmosphere, leading to a huge greenhouse effect.

170. 7.5 Earth as a Living Planet Our goals for learning: What unique features on Earth are important for human life? How is human activity changing our planet? What makes a planet habitable?

171. What unique features of Earth are important for life? 1.Surface liquid water 2.Atmospheric oxygen 3.Plate tectonics 4.Climate stability

172. What unique features of Earth are important to human life? 1.Surface liquid water 2.Atmospheric oxygen 3.Plate tectonics 4.Climate stability Earth’s distance from the Sun and moderate greenhouse effect make liquid water possible.

173. What unique features of Earth are important to human life? 1.Surface liquid water 2.Atmospheric oxygen 3.Plate tectonics 4.Climate stability PHOTOSYNTHESIS (plant life) is required to make high concentrations of O 2, which produces the protective layer of O 3.

174. What unique features of Earth are important to human life? 1.Surface liquid water 2.Atmospheric oxygen 3.Plate tectonics 4.Climate stability Plate tectonics are an important step in the carbon dioxide cycle.

175. Continental Motion Motion of continents can be measured with GPS

176. Continental Motion Idea of continental drift was inspired by puzzle- like fit of continents

177. Continental Motion Idea of continental drift was inspired by puzzle- like fit of continents The continents have moved apart because of the upwelling of magma from the upper mantle along mid-ocean ridges

178. Continental Motion Idea of continental drift was inspired by puzzle- like fit of continents The continents have moved apart because of the upwelling of magma from the upper mantle along mid-ocean ridges This also recycles the seafloor and provides the basis for the carbon dioxide cycle

179. Seafloor Recycling Seafloor is recycled through a process known as subduction

180. Seafloor Recycling Seafloor is recycled through a process known as subduction Subduction also causes volcanoes and earthquakes

181. Plate Tectonics The convection drives plate tectonics, acting like a conveyor belt to move the plates of the Earth’s crust around

182. Plate Motions Measurements of plate motions tell us past and future layout of continents

183. Carbon Dioxide Cycle 1.Atmospheric CO 2 dissolves in rainwater. 2.Rain erodes minerals that flow into the ocean. 3.Minerals combine with carbon to make rocks on ocean floor.

184. Carbon Dioxide Cycle 4.Subduction carries carbonate rocks down into the mantle. 5.Rock melts in mantle and outgasses CO 2 back into atmosphere through volcanoes.

185. What unique features of Earth are important to human life? 1.Surface liquid water 2.Atmospheric oxygen 3.Plate tectonics 4.Climate stability The CO 2 cycle acts like a thermostat for Earth’s temperature.

186. These unique features are intertwined: Plate tectonics create climate stability Climate stability allows liquid water Liquid water is necessary for life Life is necessary for atmospheric oxygen

187. How is human activity changing our planet?

188. Dangers of Human Activity Human-made CFCs in the atmosphere destroy ozone, reducing protection from UV radiation. Human activity is driving many other species to extinction. Human use of fossil fuels produces greenhouse gases that can cause global warming.

189. Global Warming Earth’s average temperature has increased by 0.5°C in the past 50 years. The concentration of CO 2 is rising rapidly. An unchecked rise in greenhouse gases will eventually lead to global warming.

190. CO 2 Concentration Global temperatures have tracked CO 2 concentration for the last 500,000 years. Antarctic air bubbles indicate the current CO 2 concentration is at its highest level in at least 500,000 years.

191. CO 2 Concentration Most of the CO 2 increase above previous peaks has happened in last 50 years

192. Modeling of Climate Change Complex models of global warming suggest that the recent temperature increase is indeed consistent with human production of greenhouse gases.

193. Modeling of Climate Change Complex models of global warming suggest that the recent temperature increase is indeed consistent with human production of greenhouse gases. And the increase is continuing, causing “calving” of vast ice sheets in Antarctica.

194. Modeling of Climate Change Complex models of global warming suggest that the recent temperature increase is indeed consistent with human production of greenhouse gases. And the increase is continuing, causing “calving” of vast ice sheets in Antarctica. If we aren’t careful, it’s quite possible that while Earth will still be habitable, it will be a very different place to inhabit.

195. Modeling of Climate Change Complex models of global warming suggest that the recent temperature increase is indeed consistent with human production of greenhouse gases. And the increase is continuing, causing “calving” of vast ice sheets in Antarctica. If we aren’t careful, it’s quite possible that while Earth will still be habitable, it will be a very different place to inhabit. Which brings up the question: What is a “habitable planet”?

196. What makes a planet habitable? Located at an optimal distance from the Sun for liquid water to exist

197. What makes a planet habitable? Large enough for geological activity to release and retain water and atmosphere

198. Planetary Destiny Earth is habitable because it is large enough to remain geologically active, and it is at the right distance from the Sun so oceans could form.

199. What have we learned? What unique features of Earth are important for life? —Surface liquid water —Atmospheric oxygen —Plate tectonics —Climate stability

200. What have we learned? How is human activity changing our planet? —Human activity is releasing carbon dioxide into Earth’s atmosphere, increasing the greenhouse effect and producing global warming. What makes a planet habitable? —Earth’s distance from the Sun allows for liquid water on Earth’s surface. —Earth’s size allows it to retain an atmosphere and enough internal heat to drive geological activity.