1	Chapter 7 Earth and the Terrestrial Worlds Chapter Outline Earth as a Planet Mercury and the Moon: Geologically Dead Mars: A Victim of Planetary Freeze-Drying Venus: A Hothouse World Earth as a Living Planet
2	What are the differences between the five terrestrial planets?
3	How can we study the interior of planets? Volcanic activity brings samples of interior material to the surface of the Earth. We can drill directly into outer layers of at least the Earth. We can use waves propagating through the body of the Earth to study the interior. We can develop mathematical models based on known physical processes.
4	Earth - Chemical Differentiation and Current Internal Structure
5	Earth’s Seismic Waves Pressure (P) waves - particles vibrate back and forth in direction in which wave propagates Longitudinal waves similar to sound waves Shear (S) waves - particles vibrate perpendicular to direction of waves' propagation Transverse waves similar to waves on string Speed through Earth (5-15 km/s) depends on material's density, compressibility, and rigidity S waves move at about half speed of P waves
6	Result of Seismic Wave Studies S waves cannot propagate through liquids as do P waves As P and S waves move downward through Earth Speed increases with increasing density of material they traverse Waves refract or reflect on reaching boundary between two distinctly different layers Picture of Earth's interior can be produce by tracking their path through body of Earth Such pictures show that Earth possesses layered structure like an onion
7	Mathematical Model Interiors Can calculate mathematical models using Theoretical arguments about physical processes governing internal structure Observed physical properties Physical properties needed Mass ( M ) and radius ( R ) Mean density, r = M / ( ⁴/₃p R³ ) Shape Rotation rate Gravitational and magnetic field strengths Surface temperature Chemical composition
8	Physical Processes for Model Interiors Since planet has stable configuration (neither contracting nor expanding), weight of matter caused by gravity pressing inward is balanced by pressure of matter deeper inside pushing out Pressure depends on density and temperature in more complicated fashion than for simple gas Flow processes for heat outward determines decrease in temperature outward Interior matter can Chemically differentiate Change from solid to liquid Deform and flow under pressure Form different types of mineral compounds Model predicts how temperature, pressure, and density vary from center to surface
9	Interior Structure from Model Studies
10	Surfaces of the Earth and the Moon
11	What physical processes produced a major reshaping of Terrestrial planet surfaces? Impact cratering: left over debris (asteroids and comets) from Solar System formation colliding with the surface Volcanism: eruption of molten rock, or lava, from a planet’s interior onto its surface Thermal-tectonic activity - heat (thermal energy) escapes from deep interior and generates convective currents in the mantle under the crust breaking it into large moving plates Erosion: solar radiation heats and expands the surface; seismic activity shakes surface; wind, ice, and water wear down geological features
12	Impact Cratering
13	Cratered Terrain
14	Maria
15	Evolution of the Moon’s Surface 3.8 - 4.7 billion years; intense impacting, but declined rapidly Cratering of lunar highlands 3.0 - 3.8 billion years; after cratering barrage ended, crust shattered by large impacts allowed molten material to fill large crater basin forming maria Present - 3.0 billion years; lunar surface dead, additional impact cratering, but at dramatically reduced rate
16	Cratered Terrain on Venus and Mars
17	What is the process by which heat flow from the Earth’s deep interior reshapes the surface? Thermal energy flow is sufficient to make sub-surface material molten. Molten sub-surface material capable of commencing convective motions to move additional thermal energy. Emergence of additional thermal energy allows surface to break up into large plates. Plates rafted along by convective currents and thereby interact with each other.
18	Convection Heat flows out of Earth’s deep interior Thermal energy is sufficient such that the upper mantle, called the asthenosphere, is capable of oozing in a plastic flow Convective flow process Hot rising material cools at base of lithosphere (crust is upper part of lithosphere Caused lithosphere to break into plates billions of years ago Plates rafted along on convective currents of asthenosphere
19	Convective Currents Carrying Plates
20	Why is Earth geologically active?
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22	Identification of Plate Boundaries
23	Mid-Atlantic Ridge Three types of plate boundary interactions Mid-ocean ridges along which new material is added to plate Sliding boundary along which two plates slide past each other Subduction zone along which one plate is forced down under an over-riding plate
24	Past, Present, and Predicted Configuration of Earth’s Continents
25	Evidence Suggests No Plate Structure on the Venusian Surface
26	Surface of Venus
27	Tectonic Forces Produce Wide Variety of Features
28	Evidence Suggests No Continuing Martian Thermal-Tectonic Activity
29	Volcanic Activity on Mars
30	Martian River Channels?
31	Martian River Channels?
32	Evidence for Water on Mars Navigation camera image was taken by the Mars Exploration Rover Opportunity on the 36th martian day, or sol, of its mission (March 1, 2004). Image shows the layered rocks of the "El Capitan" area near the rover's landing site at Meridani Planum, Mars. Visible on two of the rocks are the holes drilled by the rover, which provided scientists with a window to this part of the red planet's water-soaked past. The data indicated that the rocks are made up of types of sulfate that could have only been created by interaction between water and martian rock.
33	Regolith Regolith is material composing surface of Terrestrial planets Most regoliths are fine powdered material with consistency much like talcum powder Different composition for each planet
34	Summary of Terrestrial Surfaces
35	Longevity of Thermally Driven Activity Thermal energy content for sphere goes as volume which goes as radius cubed E_content µ ⁴/₃p R³µ R³ Rate of energy radiated to space from surface of sphere goes as surface area, which goes as radius squared E_loss µ 4 p R² µ R² Time for sphere to cool and loose all its thermal energy content goes as E_content divided by E_loss, which goes as radius cubed divided by radius squared, which equals radius T_cooling µ E_content/E_loss µ R³/R²µ R Therefore, cooling time for large radii spheres is greater than for small radii spheres
36	What factors influence both the structure and evolution of a planetary atmosphere? Planet's distance from Sun determines radiant energy input to the atmosphere. Size and mass of the planet determine chemical composition and temperature. Influences ability to retain atmosphere Chemical composition and temperature determine what chemical processes are important. Geologic and chemical evolution of surface layers influence atmospheric evolution.
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38	Escape of Atmospheric Constituents Average thermal velocity ( v_thermal ) of an atmospheric molecule depends on temperature ( T ) and mass of molecule ( m ): v_thermal µ T/m Escape velocity ( v_escape) of an atmospheric molecule depends on the mass ( M ) and radius (R ) of the planet: v_escapeµ M/R For a planet to retain a molecular component indefinitely, thermal velocity must be less than 1/10 of escape velocity: v_thermal< ¹/₁₀v_escape
39	Structure of Generic Terrestrial Atmosphere Solar x-rays are absorbed in the thermosphere Ultraviolet light is absorbed in the stratosphere Visible light reaches the ground Planets that lack ultraviolet-absorbing molecules lack a stratosphere Planets with very little gas will have only an exosphere
40	Recycling of CO₂ and the Carbonate-Silicate Cycle
41	Greenhouse Effect
42	How does Earth’s atmosphere affect the planet? Two crucial effects are protecting the surface from dangerous solar radiation—ultraviolet is absorbed by ozone and X rays are absorbed high in the atmosphere—and (2) the greenhouse effect, without which the surface temperature would be below freezing.
43	Is the greenhouse effect all bad?
44	Atmospheric Carbon Dioxide
45	The Global Warming Debate Measurements show that the Earth has indeed warmed up over the past 50 years by about 0.5^oC. Although this may sound small, it is a significant increase in such a short time period. The warming trend has continued and gotten stronger in recent years. 2. The burning of fossil fuels and other human activity is clearly increasing the amounts of greenhouse gases in the atmosphere. The current concentration of carbon dioxide in Earth’s atmosphere is significantly higher than it has been at any time during the past 400,000 years, and the concentration is rising rapidly. 3. Because we understand the basic mechanism of the greenhouse effect, there is no doubt that a continually rising concentration of greenhouse gases would eventually make our planet warm up.
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48	The Ozone Hole
49	Venus Cloud Filled Atmosphere Atmosphere is thick, hot, carbon dioxide (CO₂) gas Clouds composed of sulfuric acid (H₂SO₄) droplets High temperatures prevent sulfuric acid rain from hitting the surface.
50	Is Venus geologically active? Venus almost certainly remains geologically active today. Its surface shows evidence of major volcanic or tectonic activity in the past billion years, and it should retain nearly as much internal heat as Earth. However, geological activity on Venus differs from that on Earth in at least two key ways: lack of erosion and lack of plate tectonics.
51	Why is Venus so Hot? Venus’s extreme surface heat is a result of its thick, carbon dioxide atmosphere, which creates a very strong greenhouse effect. The reason Venus has such a thick atmosphere is its distance from the Sun: It was too close to develop liquid oceans like those on Earth, where most of the out-gassed carbon dioxide dissolved in water and became locked away in rock. Thus, the carbon dioxide remained in the atmosphere, creating the strong greenhouse effect.
52	Mars Atmospheric Structure
53	Earth’s Magnetic Field
54	Earth’s Magnetosphere
55	Why did Mars change? Mars’s atmosphere must once have been much thicker with a much stronger greenhouse effect, so change must have occurred due to loss of atmospheric gas. Much of the lost gas probably was stripped away by the solar wind, which was able to reach the atmosphere as Mars cooled and lost its magnetic field and protective magnetosphere. Water was probably also lost because ultraviolet light could break apart water molecules in the atmosphere, and the lightweight hydrogen then escaped to space.
56	Aurora in Earth’s Atmosphere
57	What unique features of Earth are important for life? Unique features of Earth on which we depend for survival are surface liquid water, made possible by Earth’s moderate temperature; 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.
58	How might human activity change our planet? Ozone depletion can leave surface life more vulnerable to dangerous solar ultraviolet radiation, and the high rate of extinctions could have unknown consequences. The human release of greenhouse gases into the atmosphere may already be causing global warming and certainly would affect the climate if it continues.
59	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 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.
60	The Big Picture Much of planetary geology can be distilled down to a few basic geological processes that depend on a handful of basic planetary properties. Every terrestrial planet was once as heavily cratered as the Moon is today, but craters have been erased on other planets to varying degrees—depending mainly on each planet’s size. Planetary atmospheres are not static. Complete atmospheric transformation over the age of the solar system appears to be the rule for large planets, not the exception. Volcanism and tectonics depend primarily on planet’s size, but erosion depends on characteristics of planetary atmospheres.