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PHYS 100 - ASTRONOMY
CHAPTER 7
GUIDEPOSTS
Earth: The Standard of Comparative Planetology
SUMMARY - GUIDEPOSTS
7-1 A TRAVEL GUIDE TO THE TERRESTRIAL PLANETS
How does Earth fit among the terrestrial planets?
Earth is the standard of comparative planetology in the study of the terrestrial planets because we know it
best and because it contains all of the phenomena found on the other terrestrial planets.
The terrestrial planets include Earth, the moon, Mercury, Venus, and Mars. Earth’s moon is included
because it is a complex world and makes a striking comparison with Earth.
The terrestrial worlds differ mainly in size, but they all have low-density crusts, mantles of dense rock, and
metallic cores.
Comparative planetology warns you to expect that cratered surfaces are old, that heat flowing out of a
planet drives geological activity, and that the nature of a planet’s atmosphere depends on the size of the
planet and its temperature.
7-2 THE EARLY HISTORY OF EARTH
How has Earth changed since it formed?
Earth formed rapidly from the solar nebula and was hot enough to be molten.
Earth has passed through four stages as it has evolved: (1) differentiation; (2) cratering; (3) flooding by lava
and water; and (4) slow surface evolution.
Earth is peculiar in that it has large amounts of liquid water on its surface, and that water drives strong
erosion that alters the surface geology.
Earth is also peculiar in that it is the only known home for life.
7-3 THE SOLID EARTH
What is the inside of Earth like?
Seismic waves generated by earthquakes can be detected by seismographs all over the world and can reveal
Earth’s internal structure.
Pressure (P) waves can travel through a liquid, but shear (S) waves cannot. Observations show that S waves
cannot pass through Earth’s core, and that is evidence that the core is liquid. Heat fl owing outward from
the interior combined with models reveal that the core is very hot and composed of iron and nickel.
Although Earth’s crust is brittle and breaks under stress, the mantle is plastic and can deform and flow
under pressure.
Earth’s magnetic field is generated by the dynamo effect in the liquid, convecting, rotating, conducting
core. The magnetic field shields Earth from the solar wind by producing a bow shock and a magnetosphere
around the planet. Radiation belts called the Van Allen belts and auroras are also produced by the field.
Earth is dominated by plate tectonics that breaks the crust into moving sections. Plate tectonics is driven by
heat flowing upward from the interior.
Tectonic plates are made of low-density, brittle rock that floats on the hotter plastic upper layers of the
mantle. Rift valleys can be produced where plates begin pulling away from each other.
New crust is formed along midocean rises where molten rock solidifies to form basalt. Crust is destroyed
where it sinks into the mantle along subduction zones. Volcanism and earthquakes are common along the
edge of the plates.
The motion of a plate across a hot spot can produce a chain of volcanic islands such as the HawaiianEmperor island chain. Hot-spot volcanism is not related to subduction zones.
The continents are drifting slowly on the plastic mantle, and their arrangement changes with time. Where
they collide, they can form folded mountain ranges.
Most geological features on Earth, such as mountain ranges and the Grand Canyon, have been formed
recently. The first billion years of Earth’s geology are almost entirely erased by plate tectonics and erosion.
20-4 EARTH’S ATMOSPHERE
How has Earth’s atmosphere formed and evolved?
Because Earth formed hot, it never had a primeval atmosphere rich in hydrogen and helium that was later
replaced by a secondary atmosphere baked out of the interior.
Because Earth formed in a molten state, its first atmosphere was probably mostly carbon dioxide, nitrogen,
and water vapor. Most of the carbon dioxide was dissolved in seawater, and plant life has added oxygen.
Ultraviolet photons can break up water molecules in a planet’s atmosphere, but as soon as Earth had
enough oxygen, an ozone layer could form high in Earth’s atmosphere. The ozone absorbs ultraviolet
photons and protects water molecules.
The albedo of a planet is the fraction of sunlight hitting it that it reflects into space. Small changes in the
albedo of Earth caused by changes in clouds and atmospheric currents can have a dramatic effect on
climate.
The greenhouse effect can warm a planet if gases such as carbon dioxide in the atmosphere are transparent
to light but opaque to infrared. The natural greenhouse effect warms Earth and makes it comfortable for
life, but greenhouse gases added by industrial civilization are responsible for global warming.
The ozone layer high in Earth’s atmosphere protects the surface from ultraviolet radiation, but certain
chemicals called chlorofluorocarbons released in industrial processes attack the ozone layer and thin it.
This is allowing more harmful ultraviolet radiation to reach Earth’s surface.