Chapter 7 Earth and the Terrestrial Worlds

WHAT DO YOU THINK? Why are Venus (too HOT), Mars (too COLD) and Earth (just right!) so different in their atmospheres? Could Mars have supported life long ago? How do we know? Is life known to exist on Mars today?

In this chapter you will discover… Mercury, a Sun-scorched planet with a heavily cratered surface and a substantial iron core Venus, perpetually shrouded in thick, poisonous clouds and mostly covered by gently rolling hills Mars, a red, dusty planet that once had running water on its surface and may still have liquid water underground The role of Carbon Dioxide as a insulator for Planetary climates.

Essay Questions Describe the atmospheres of Venus, Earth, and Mars. Why are these three atmospheres so different? What does the Martian surface tell us about the planet's history, and future? Describe the composition of the Moon, and its similarities and differences with our Earth.  How did the Moon form? How do we know?

Mercury craters smooth plains cliffs

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

Mars some craters volcanoes riverbeds?

Moon craters smooth plains

Earth volcanoes craters mountains riverbeds

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

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?

Why is Earth geologically active?

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

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

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

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

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

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

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

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

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

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

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.

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

Sources of Internal Heat Gravitational potential energy of accreting planetesimals Differentiation Radioactivity

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

Cooling of Interior Convection Conduction Radiation

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

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

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

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

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

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

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.

Planetary Magnetic Fields Moving charged particles create magnetic fields. A planet’s interior can create magnetic fields if - it is electrically conducting, & - it is circulating and/or rotating. Figure 7.5 goes here.

Earth’s Magnetosphere Earth’s magnetic fields protects us from charged particles from the Sun. The charged particles can create aurorae (“Northern lights”). Figure 7.6ab NEW should go here.

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

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

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.

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.

What processes shape Earth’s surface?

Geological Processes Impact cratering Volcanism Tectonics Erosion Impacts by asteroids or comets Volcanism Eruption of molten rock onto surface Tectonics Disruption of surface by internal stresses Erosion Changes made by wind, water, or ice

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

Impact Craters Meteor Crater (Arizona) Tycho (Moon)

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.

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

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

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.

Erosion Erosion is a blanket term for weather-driven processes that break down or transport rock. Processes that cause erosion include Water/Ice movement by glaciers & rivers Atmospheric Movement by wind, storms Cyclic heating/cooling

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

Erosion by Ice Glaciers carved the Yosemite Valley.

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

Erosional Debris Erosion can create new features by depositing debris.

How does Earth’s atmosphere affect the planet ?

Effects of Atmosphere on Earth Erosion Makes the sky blue! Radiation protection Greenhouse effect

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

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

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.

Earth’s atmosphere absorbs light at most wavelengths.

Radiation Protection All X-ray light is absorbed very high in the atmosphere. Ultraviolet light is absorbed by ozone (O3) ~ 30 miles high Figure 7.13a would be good here.

The Greenhouse Effect

The Greenhouse Effect on Earth FIGURE 6-4 The Greenhouse Effect (a) Sunlight and heat from Earth’s interior warm Earth’s surface, which in turn radiates energy, mostly as infrared radiation. Much of this radiation is absorbed by atmospheric carbon dioxide and water, heating the air, which in turn raises Earth’s temperature even further. In equilibrium, Earth radiates as much energy as it receives.

Greenhouse effect: Certain molecules let sunlight through but trap escaping infrared photons. (H2O, CO2, CH4)

Greenhouse Gases Any gas that absorbs infrared Greenhouse gas: often molecules with two different types of elements (CO2, H2O, CH4) Not a greenhouse gas: molecules with one or two atoms of the same element (O2, N2)

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

Earth as a “Living” Planet What unique features on Earth are important for human life? How is human activity changing our planet? What makes a planet habitable?

What unique features of Earth are important for life? Surface liquid water Atmospheric oxygen Plate tectonics Climate stability

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

What unique features of Earth are important to human life? Surface liquid water Atmospheric oxygen Plate tectonics Climate stability PHOTOSYNTHESIS (plant life) is required to make high concentrations of O2, which also produces the protective layer of O3.

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

Continental Motion Idea of continental drift was inspired by puzzle-like fit of continents Mantle material erupts where seafloor spreads

Plate Motions

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

Tectonics & Seafloor Recycling Seafloor is recycled through a process known as subduction

Carbon Dioxide Cycle How does our atmosphere & tectonics combine to regulate temperatures? How does life play a role?

Carbon Dioxide Cycle Step 1: Evaporation/Rain Liquid water evaporates Condenses into clouds in lower atmosphere Rain falls through atmosphere forming Carbonic Acid (H2CO3)  CO2 gas is absorbed 1

Carbon Dioxide Cycle Step 2: Mineral Erosion by Acid Rain Carbonic Acid (H2CO3) in rivers erodes rocks Carbonate (CO32-) ion picked up in minerals washed to ocean Calcium easily absorbed  CO2 is carried to oceans 2

Carbon Dioxide Cycle Step 3: Tying Carbon into Rocks & Life! Calcium from rocks forms CaCO3 (Calcium Carbonate) CaCO3 = Limestone CaCO3 = Coral, Mollusk shells! 3  CO2 accumulates on seafloor

Carbon Dioxide Cycle Step 4: Tectonics & Subduction! Tectonics gradually pulls seafloor down CaCO3 broken back into CO2 & other minerals 4  CO2 now inside crust

Carbon Dioxide Cycle Step 5: Volcanic Outgassing! Eventual Volcanic Activity pushes CO2 back into atmosphere 5  CO2 now in atmosphere again!

Carbon Dioxide Cycle “Recycle” CO2 from atmosphere to crust to atmosphere over time Estimate ~25 million years or more for this to occur globally

Carbon Dioxide Cycle “Feedback Loop” Suppose evaporation stopped, during an ice age…. What would happen over time?

Carbon Dioxide Cycle Feedback Loop: Ice Age No evaporation No Rain NO CO2 gas absorbed 1

Carbon Dioxide Cycle Feedback Loop: Ice Age No evaporation No Rain NO CO2 gas absorbed But… Tectonic Activity & Volcanoes continue! Gradual CO2 concentration increase! 1 5

Carbon Dioxide Cycle Feedback Loop: Ice Age Tectonic Activity & Volcanoes continue! Gradual CO2 concentration increase! More Greenhouse Effect => warmer! Ice Melts! Cycle restored! 1 5

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.

Long-Term Climate Change CO2 from outgassing will build up if oceans are frozen, ultimately raising global temperatures again.

Carbon Dioxide Cycle “Feedback Loop” Suppose CO2 in our atmosphere traps too much heat, and we heat up? What would happen over time?

Carbon Dioxide Cycle Feedback Loop: Global Warming Liquid water evaporates FASTER MORE Rain  MORE CO2 gas is absorbed 1 5

Carbon Dioxide Cycle Feedback Loop: Global Warming Volcanoes continue at “normal” rate… Gradual CO2 concentration decrease! Less Greenhouse Effect => cooler! Cycle restored! 1 5

What unique features of Earth are important to human life? Surface liquid water Atmospheric oxygen Plate tectonics Climate stability The CO2 cycle acts like a thermostat for Earth’s temperature.

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 For example: atmospheric oxygen allows life on land Stable climate good for life Water may enable plate tectonics How many other connections between these can you think of?

How is human activity changing our planet?

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.

Global Climate Change Earth’s average temperature has increased by 0.5°C in the past 50 years. The concentration of CO2 is rising rapidly. An unchecked rise in greenhouse gases is leading to global climate change.

CO2 Concentration Global temperatures have tracked CO2 concentration for the last 500,000 years. Antarctic air bubbles indicate the current CO2 concentration is at its highest level in at least 500,000 years.

Most of the CO2 increase has happened in last 50 years! CO2 Concentration Most of the CO2 increase has happened in last 50 years!

Modeling of Climate Change Build climate models based on current/past data Models suggest recent temperature increase is consistent with human production of greenhouse gases.

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

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

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.

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.