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Chapter 7

Astronomy 1021 Chapter Notes - Chapter 7: Himalayas, Valles Marineris, Lunar Mare

Course Code
ASTR 1021
Jan Cami

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7.1 Earth as a Planet
Volcanoes and earthquakes are processes acting to reshape Earth’s surface. Other
processes: far greater change can occur on the rare occasions when an asteroid or a comet
slams into Earth. More gradual processes: the Colorado River causes only small changes
in the landscape from year to year, but its unrelenting flow over the past few million
years carved the Grand Canyon. The Rocky Mountains were twice as tall, but they have
been cut down in size through erosion by wind. Entire continents move slowly. The great
differences in terrestrial planets’ present day appearances must be the result of changes
that have occurred through time.
Mercury and the Moon: scars from heavy bombardment. Venus: covered by a thick,
cloudy atmosphere, but a surface dotted with volcanoes. Mars: shaped by running water,
but now dry. Earth: diverse surface with evidence of life.
Why is Earth Geologically active?
Earth is geologically active, meaning that its surface is continually being reshaped by
volcanic eruptions, earthquakes, erosion, and other geological processes.
Interior Structure:
Earth: the most direct data come from seismic waves, vibrations that travel both
through the interior and along the surface after an earthquake. Same with Moon
and Mars. Less direct methods for other worlds: comparing a world’s overall
average density of its surface rock tells us how much more dense it must be
inside, measurements of a world’s gravity from spacecraft can tell us how mass is
distributed inside it, studies of magnetic fields tell us about the interior layers in
which these fields are generated, and volcanic rocks can tell us about interior
3 layers by density:
Core: highest-density material, consisting primarily of metals such as
nickel ad iron, resides in a central core.
Mantle: Rocky material of moderate density mostly minerals that
contain silicon, oxygen, and other elements forms a thick mantle that
surrounds the core.
Crust: The lowest-density rock, such as granite and basalt forms a thin
crust, essentially representing the world’s outer skin
Earth’s metallic core actually consists of two distinct regions: a solid inner core
and a molten outer core.
Solidity arises from electrical bonds between its atoms and molecules. They ca
break and re-form when subjected to heat or sustained stress, which means that
even solid rock can slowly deform and flow over millions and billions of years.
Rock becomes softer and easier to deform when it is warmer.
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Earth’s outer layer consists of relatively cool and rigid rock, called the
lithosphere, softer rock beneath.
Differentiation and Internal Heat:
Gravity pulls the denser water to the bottom, driving the less dense oil to the top.
This process is called differentiation, because it results in layers made of
different materials. The worlds must once have been hot enough inside for their
interior rock and metal to melt.
As a planet formed through accretion, the gravitational potential energy of
incoming planetesimals was ultimately converted into thermal energy in the
planet`s interior. Second, the rock and metal that built the terrestrial worlds
contained radioactive isotopes of elements such as uranium, potassium, and
thorium. As these radioactive materials decay, they release heat directly into the
planetary interiors, in essence converting some of the mass-energy of radioactive
nuclei to thermal energy. Larger worlds tend to stay hot longer than smaller
Internal Heat and Geological Activity:
Temperature increases with depth inside a planet. If the interior is hot enough, the
mantle can undergo convection, in which hot material gradually contracts and
falls. It would take 100 million years for a piece of rock to be carried from the
base of the mantle to the top.
Size is also the primary factor in the strength of mantle convection and
lithospheric thickness. As a planet`s internal cools, the rigid lithosphere grows
thicker and convection occurs only deeper inside the planet. A thick lithosphere
inhibits volcanic and tectonic activity, because any molten rock is too deeply
buried to erupt to the surface and the strong lithosphere resists distortion by
tectonic stresses. If the interior cools enough, convection may stop entirely,
leaving the planet geologically dead.
Earth and Venus remain quite hot inside, so they have thin lithospheres and
substantial geological activity. Mercury and the Moon have lost so much heat that
they now have very thick lithospheres.
The Magnetic Field:
Interior heat is also responsible for Earth`s global magnetic field. Charged
particles move with the molten metal in Earth`s liquid outer core. Internal heat
causes the liquid metal to rise and fall (convection), while Earth`s rotation twists
and distorts the convection pattern. The result is that electrons in the molten metal
move within the outer core in much the same way they move in an electromagnet,
generating Earth`s magnetic field.
The magnetic field creates a magnetosphere that acts like a protective bubble
surrounding our planet, shielding Earth`s surface from energetic charged particles
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