Chapter 7 Reading Notes: Olivia Ward

Earth and the Terrestrial Worlds

7.1 Earth as a Planet
Why is Earth geologically active?

• Earth is geologically active: its surface is continually being reshaped by volcanic eruptions, earthquakes, erosion
• Most geological activity is from deep within the planet
• Interior Structure (layered interiors):
  • Core: The highest-density material, consisting primarily of metals such as nickel and iron
  • Mantle: Rocky material of moderate density- mostly minerals that contain silicon, oxygen, and other elements
  • Crust: The lowest-density rock, such as granite and basalt
• Earth's metallic core consists of two regions: a solid inner core and a molten outer core.
• Earth's outer layer consists of relatively cool and rigid rock called the lithosphere (crust and part of the upper mantle) that "floats" on warmer, softer rock beneath.
• Differentiation and Internal Heat
  
  • Differentiation: Denser materials pulled to the bottom, less dense materials driven to the top
    
    • Layered interiors of terrestrial planets show that they underwent differentiation.
  • Terrestrial worlds were hot inside for two major reasons:
    
    • They gained heat from the process of formation.
    • Planetesimals collided at high speed with forming planets, depositing large amounts of energy that turned into heat.
    •  The metal and rock that make up terrestrial planets include small amounts of radioactive elements. As radioactive material decays, it releases heat.
  • None of the terrestrial worlds are still hot enough to have liquid interiors, but they differ in the heat they have retained.
    
    • Size is the most important factor is planetary cooling. A large planet says hotter inside longer than a small one.
    •  A thick lithosphere inhibits volcanic and tectonic activity.
    •  Earth (largest of terrestrial planets) remains hot and has a thin lithosphere. Venus is similar.
    •  Mercury and the Moon have thick lithospheres and no geological activity.
    •  Mars (intermediate in size), has cooled but probably retains some internal heat.
• The Magnetic Field
  
  • Interior heat is responsible for Earth's magnetic field.
    
    • Internal heat causes the liquid metal to rise and fall (convection). Earth's rotation twists and distorts the convection pattern.
  • The magnetic field shields us by creating magnetosphere: a kind of protective bubble that surrounds our planet and deflects most of the charged particles from the Sun.
    
    • The few particles that make it through the magnetosphere collide with atoms and molecules at the poles, creating the aurora lights.

What processes shape Earth's surface?

•  All surface features can be explained by four major geological processes.
  
  • Impact Cratering
    • The excavation of bowl-shaped impact craters by asteroids or comets crashing into a planet's surface (external cause).
    •  Impacting objects typically hit the surface at speeds between 40,000 and 250,000 kilometers per hour. The impact releases enough energy to vaporize solid rock and blast out a crater.
    • Craters are typically 10 times wider and 10-20% deeper than the object that created them.
    • Craters can be erased by time: The Earth has had at least the same amount of impacts as the Moon but geological activity like volcanic eruptions and erosion have erased them from the surface.
  • Volcanism
    • The eruption of molten rock (lava) from a planet's interior onto its surface.
    • Molten rock tends to rise for three main reasons:
      
      • Molten rock is generally less dense than solid rock. Lower-density materials tend to rise when surrounded by higher-density materials.
      • Most of Earth's interior is not molten. The solid rock surrounding the molten rock can squeeze it, driving it upward under pressure.
      • Molten rock often contains trapped gases that expand as it rises.
    •  Volcanism explains the existence of our atmosphere and oceans.
      
      • Water and ice were brought to Earth by planetesimals. Water and gases became trapped beneath the surface. Volcanic eruptions released some of the gas into the atmosphere (outgassing).
      • Outgassed water vapor rained won to form oceans.
  • Tectonics
    • The disruption of a planet's surface by internal stresses.
    • Tectonic activity is a direct or indirect result of mantle convection.
    • Earth: the underlying mantle convection fractured the planet's lithosphere into plates (plate tectonics).
    • Tectonic activity usually goes hand in hand with volcanism.
  • Erosion
    • The wearing down or building up of geological features by wind, water, ice, and other phenomena of planetary weather.
    • The shaping of valleys by glaciers, the carving of canyons by rivers, the shifting of sand dunes by wind
    • Over long periods of time, erosion has piled sediments into layers on ocean floors (sedimentary rock).
    • Erosion plays a more important role on Earth than any other terrestrial world because of strong winds (rapid rotation) and water (outgassing by volcanism)

How does Earth's atmosphere affect the planet?

• Surface Protection
  
  • The atmosphere protects us from the Sun's ultraviolet and X-ray radiation.
    
    • X-ray photons ionize the atoms or molecules they strike which can damage living tissue. X rays are absorbed high in the atmosphere, leaving none to reach the ground.
    • Ozone protects us from ultraviolet light. It is primarily in a middle layer of Earth's atmosphere (stratosphere). It absorbs most of the ultraviolet radiation.
  • The Sun emits most of its radiation in the form of visible light which easily passes through the atmosphere and reaches the surface.
    
    • This allows us to see (visible light) and provides energy for photosynthesis.
    • Visible light heats the ground (primary source of heat for Earth's surface).
  • Few visible light photons are randomly scattered around the sky. This is why the sky is bright and we cannot see stars during the day.
• The Greenhouse Effect
  
  • Our atmosphere traps additional heat through the greenhouse effect.
  • The greenhouse effect occurs when the atmosphere temporarily traps some of the infrared light that the ground emits, slowing its return to space.
  • The greenhouse effect only occurs when the atmosphere contains gases that can absorb infrared light (greenhouse gases).

7.2 The Moon and Mercury: Geologically Dead
Was there every geological activity on the Moon or Mercury?

• Geological Features of the Moon
  
  • Some regions are heavily cratered while others are smoother and darker (lunar maria formed by floods of molten lava).
  • During bombardment, craters covered the Moon's entire surface. The largest impacts fractured the Moon's lithosphere but the Moon's interior had already cooled since its formation. Mantle material melted (3 to 4 billion years ago) and  lava flooded the craters due to heat released by the decay of radioactive elements.
  • Today, the Moon is desolate and nearly unchanging.
  • Ice deposits in craters near the Moon's north pole
• Geological Features of Mercury
  • Impact craters are visible almost everywhere, indicating an ancient surface.
  • Mercury's craters are less crowded together than those of the Moon, suggesting that molten lava covered up some of the craters that formed on Mercury during heavy bombardment.
  • As much volcanism as the Moon, and its volcanoes may have died out a billion years later than the Moon's
  • Caloris basin (huge crater impact) spans more than half of Mercury's radius.
    
    • It has few craters within it. It must have formed at a time when the heavy bombardment was already subsiding.
  • Many tremendous cliffs (vertical faces of up to 3+ km high and hundreds of km across)- evidence of past tectonics.
  • The Moon was once larger and contained more iron

7.3 Mars: A Victim of Planetary Freeze-Drying
What geological features tell us that water once flowed on Mars?

• No liquid water exists on the surface of Mars today.
  • Mars is so cold that any liquid water would immediately freeze.
  • Even when it does rise about freezing temperature, the air pressure is so low that liquid water would quickly evaporate.
  • Mars must have had some warm and wet periods in the distant past.
• The Geology of Mars
  
  • Dramatic difference in terrain
    • Much of the southern hemisphere has high elevation and many large craters. The northern plains have few impacts and tend to be below average surface level.
    • The southern hemisphere is much older than the northern plains.
  • Towering volcanoes
    
    • Olympus Mons is the tallest known volcano in the solar system.
    • Olympus Mons and several other large volcanoes are concentrated near Tharsis Bulge.
  • Tectonic features
    
    • The most prominent tectonic feature is a deep system of valleys called Valles Marineris.
• Ancient Water Flows
  
  • Impacts, volcanism, and tectonics explain most of the major geological features of Mars. Water erosion did cause some features.
  • Channels caused by water erosion were formed long ago (channels are 2-3 billion years old). There are even impact craters that lie on top of the channels.
  • Mars had rain and surface water
• Martian Water Today
  
  • Significant amounts of water still remain frozen at the polar caps and in the top meter of the surface soil.
  • Water ice probably lies deeper underground.

Why did Mars change?

• Before 3 billion years ago: Mars had wetter and possibly warmer periods with rainfall.
• Mars underwent major and permanent climate change. It was once able to sustain liquid water but is now a frozen wasteland.
• Mars once had a denser atmosphere. It somehow lost most of its carbon dioxide gas. This weakened the greenhouse effect until it froze over.
• Mars lost water vapor due to solar winds. The atmsphere also lacks ultraviolet-absorbing gases. Water molecules would have been easily broken apart by ultraviolet photons.

7.4 Venus: A Hothouse World
Is Venus geologically active?

• Geological Feature of Venus
  
  • Features similar to Earth's: occasional impact craters, volcanoes, and a lithosphere that has been contorted by tectonic forces.
  • A large and circular coronae (made by hot, rising plumesof mantle rock)
  • Venus remains geologically active today, since it should still retain as much internal heat as Earth.
  • The surface is geologically young (few impact craters).
  • Lack of erosion (minor process) can be traced to two facts:
    
    • Venus is far too hot for any type of rain or snow on its surface.
    • Venus has virtually no wind or weather because of its slow rotation.
• The Absence of Plate Tectonics
  
  • Venus shows no evidence of Earth-like plate tectonics.
  • The surface is about the same age everywhere (750 million years old).
  • Most scientists suspect that Venus's lithosphere resists fracturing into plates because it is thicker and stronger than Earth's lithosphere.

Why is Venus so hot?

• Atmospheric Composition
  
  • Venus's atmosphere has huge amounts of carbon dioxide, but virtually no water (causing high temperature).
  • Ultraviolet light from the Sun breaks apart water molecules in Venus's atmosphere. The hydrogen atoms escape to space.
  • Venus's lack of magnetic field leaves its atmosphere vulnerable to solar winds.
• The Runaway Greenhouse Effect
  • Venus is about 30% closer to the Sun than Earth.
  • Greater intensity of sunlight made it just warm enough that oceans either never formed or soon evaporated, leaving Venus with a tick atmosphere full of greenhouse gases.Why is Venus so hot?
   

7.5 Earth as a Living Planet
What unique features of Earth are important for life?

•  Four unique features particularly important to life on Earth
  
  • Surface liquid water: Earth is the only planet on which temperature and pressure conditions allow surface water to be stable as a liquid.
  • Atmospheric oxygen: Earth is the only planet with significant oxygen in its atmosphere and an ozone layer.
  • Plate tectonics: Earth is the only planet with a surface shaped largely by this distinctive type of tectonics.
  • Climate stability: Earth differs from the other terrestrial worlds with significant atmospheres (Venus and Mars) in having a climate that has remained relatively stable throughout its history.
• Our Unique Oceans and Atmosphere
  
  • Abundant liquid water and atmospheric oxygen
  • Oxygen makes up 21% of Earth's atmosphere.
• Plate Tectonics
  
  • Plate tectonics and climate stability
  • Plates move at speeds of only a few cm per year: creating new crust and recycling old crust back into the mantle.
  • Subduction occurs where seafloor plates run into continental plates.
• Climate Stability
  • Earth's climate is not perfectly stable (ice ages and warm periods). Regardless, temperatures have been at where some liquid water could exist.

How is human activity changing our planet?

•  The global average temperature has risen in the past hundred years.
• A rise in atmospheric CO2 concentration (result of fossil fuel burning and other human activity)
  
  • The concentration is higher than at any time in the past million years.
• Global warming

What makes a planet habitable?

• Earth is habitable due to its large size and distance from the Sun.
• Internal heat is kept due to the size
• Plate tectonics helps to regulate our climate through the carbon dioxide cycle.