Chapter 7 Earth and the Terrestrial Worlds

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, when 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? Short answer: the Earth is big enough to still have a hot interior. So what do we know about the interior of the Earth and why is it hot? So let’s see what that interior is and why it’s hot.

Internal Structure, by density Remove lithosphere label & arrow, and dotted circle

Internal structure, by rock strength The “lithosphere” is the cool rigid rock that forms a planet’s outer layer: the crust and some of the mantle. The lithosphere floats on the lower layers.

The thickness of the lithosphere controls many geological processes Remove lithosphere label & arrow, and dotted circle

Internal Structure of the Terrestrial Planets

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

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 Layers ordered by density Highest density on the bottom Gravity sorts materials by density. Differentiation converts gravitational potential energy to heat.

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.

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.

Heat Drives Geological Activity Convection: hot rock rises, cool rock falls. 1 cycle takes 100 million years on Earth.

A large planet… Is still warm inside Has a convecting mantle Has a thinner, weaker lithosphere Has molten rock nearer the surface which makes it more geologically active

Comparing the Planets Which planets have the most and least geological activity?

Planetary magnetic fields Moving charged particles create magnetic fields. So can a planet’s interior, if the core it electrically conducting, convecting, and 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.

How do we know what is inside the Earth? We can drill deep inside the Earth. We can use optical fibers to see deep inside the Earth. X-ray machines allow us to view the inside of the Earth. Seismic waves generated by earthquakes probe the Earth’s interior.

How do we know what is inside the Earth? We can drill deep inside the Earth. We can use optical fibers to see deep inside the Earth. X-ray machines allow us to view the inside of the Earth. Seismic waves generated by earthquakes probe the Earth’s interior. Specifically, different kinds of seismic waves propagate at different speeds and directions through the planetary interior depending on the properties of the internal layers.

What processes shape Earth’s surface? Impact cratering Volcanism Tectonics Erosion

Impact Cratering Figure 3e-13.20 but of course it ain’t on the cd. (huh, and figure 10.7 isn’t either.) This figure is Mars.

Impact Cratering Moon must be hit as often as Earth. Where are Earth’s craters? Erased by volcanic activity and erosion. The more craters, the older the surface

Volcanism Molten rock rises when it is: Less dense than its surroundings. Squeezed by its surroundings. Pushed by expanding trapped gas (water vapor, CO2, N2, H2S, SO2)

Volcanism Erases other geological features Provided gas for our atmosphere Provided water for our oceans

Why doesn’t Mars have as much volcanic activity as Earth? It’s too far from the Sun, so it cooled off faster. It’s smaller than the Earth, so it cooled off faster. It might, we just haven’t seen them erupt yet.

Why doesn’t Mars have as much volcanic activity as Earth? It’s too far from the Sun, so it cooled off faster. It’s smaller than the Earth, so it cooled off faster. It might, we just haven’t seen them erupt yet.

Tectonics and Plate Tectonics Tectonics: any surface reshaping from forces on the lithosphere Plate tectonics: pieces of lithosphere moving around Only Earth has plate tectonics. Internal Heat Required!

Erosion Wearing down or building up of geological features by wind, water and ice (weather) Important on Earth - why?

How does Earth’s atmosphere affect Earth? Erosion (already mentioned) Protection from radiation Changes the surface temperature: greenhouse effect Makes the sky blue!

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

The Greenhouse Effect

A Greenhouse Gas Any gas that absorbs infrared Greenhouse gas: molecules with 2 different types of elements (CO2, H2O, CH4) NOT a greenhouse gas: molecules with single or 2 atoms of the same element (O2, N2) The Earth is much warmer because of the greenhouse effect than it would be without an atmosphere…but so is Venus. It is only bad if it is a “runaway” effect.

Mercury and the Moon: Geologically Dead Our Goals for Learning Was there ever geological activity on the Moon or Mercury?

Moon Figure 7.15, make a line pointing to the Mare Humorum. Some volcanic activity 3 billion years ago must have flooded lunar craters, creating lunar maria. The Moon is now geologically dead.

Mercury Plenty of craters - including a huge ‘basin’ Figure 7.17a Plenty of craters - including a huge ‘basin’ Smooth plains from volcanism (recent or long ago???)

Did Mercury shrink? Figure 7.18 Steep long cliffs formed when the core cooled, shrinking the planet by ~20 km. Mercury is now geologically dead.

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?

Mars vs. Earth 50% Earth’s radius, 10% Earth’s mass 1.5 A.U from the Sun Axis tilt about the same as Earth. Similar rotation period. Orbit is more elliptical than Earth’s: seasons more extreme in the south than the north. Thin CO2 atmosphere: little greenhouse So main difference is … Mars is SMALLER!

Surface of Mars appears to have ancient river beds

Eroded crater Condition of craters indicates surface history

Closeup of eroded crater

Need Figure 7.20.

Volcanoes…as recent as 180 million years ago… Meteoritic evidence, radiometric dating Volcanoes…as recent as 180 million years ago…

Past tectonic activity…

Low-lying regions may once have had oceans

Low-lying regions may once have had oceans

Opportunity Spirit Inset shows hypothetical ancient ‘water line’ for Gusev Crater where Spirit landed

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?

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

Why did Mars change? Enlarge labels?

Venus: A Hothouse World Our Goals for Learning Is Venus geologically active? Why is Venus so hot?

Is Venus geologically active? Figure 7.29 bc and maybe d.. Radar images show lots of volcanic & tectonic features, as expected for a large terrestrial planet

Why is Venus so hot? Greenhouse effect. But why is it so prominent on Venus? Answer: Thick CO2 atmosphere of Venus locks heat in

(and some in plant life and in the ocean) Where is Earth’s CO2? Rocks - like limestone (and some in plant life and in the ocean) Why did this happen on Earth and not on Venus? Venus lacks oceans to dissolve the carbon dioxide and lock it away in rock on the seafloor And why is that? ‘strip tease’ of bullet-by-bullet unveiling is warranted on this slide

We can understand Venus’ history by thinking about what would happen if Earth were moved to Venus’ orbit. How would the initial heating affect the oceans and the greenhouse effect?

A runaway greenhouse effect would then occur. Eventually, water molecules would break down & escape to space, just as apparently happened on Venus

Earth as a Living Planet Our Goals for Learning What unique features on Earth are important for human life? How might human activity change 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? Earth’s distance from the Sun and moderate greenhouse effect make liquid water possible 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 PHOTOSYNTHESIS (plant life) is required to make high concentrations of O2, which 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.

The Carbon Dioxide Cycle

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 the Earth’s temperature.

These unique features are intertwined: • plate tectonics creates 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

Earth’s ice ages end as oceans freeze over and volcanoes release CO2 into the atmosphere

Human activity is increasing the concentration of greenhouse gases in the atmosphere, which may strengthen the greenhouse effect and lead to global warming.

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 & retain water and atmosphere.

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

What have we learned? • What unique features of Earth are important for life? Unique features of Earth on which we depend for survival are (1) surface liquid water, made possible by Earth’s moderate temperature; (2) atmospheric oxygen, a product of photosynthetic life; (3) plate tectonics, driven by internal heat; and (4) climate stability, a result of the carbon dioxide cycle, which in turn requires plate tectonics.

What have we learned? • What makes a planet habitable? We can trace Earth’s habitability to its relatively large size and its distance from the Sun. Its size keeps the internal heat that allowed volcanic outgassing to lead to our oceans and atmosphere, and also drives the plate tectonics that helps to regulate our climate through the carbon dioxide cycle. Its distance from the Sun is neither too close nor too far, thereby allowing liquid water to exist on Earth’s surface.