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Chapter 6

AY 101 Chapter Notes - Chapter 6: Radiometric Dating, Planetesimal

Course Code
AY 101
Raymond White

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The Big Picture
This chapter introduced the major features of our solar system and the current scientific theory of its
formation. As you continue your study of the solar system, keep in mind the following “big picture”
Our solar system is not a random collection of objects moving in random directions. Rather, it is highly
organized, with clear patterns of motion and common traits among families of objects.
We explain the origin of our solar system with the nebular theory, which holds that the solar system
formed from the gravitational collapse of an interstellar gas cloud.
Most of the general features of the solar system were determined by processes that occurred very early
in the solar system’s history.
Our solar system is old-born about 4.5 billion years ago-but it is still only about one-third as old as our
14-billion-year-old universe.
Summary of Key Concepts
What does the solar system look like?
The planets are tiny compared to the distances between them. Our solar system consists of the Sun, the
planets and their moons, and vast numbers of asteroids and comets. Each world has its own unique
character, but there are many clear patterns among the worlds.
What features of our solar system provide clues to how it formed?
The Sun, planets, and large moons generally rotate and orbit in a very organized way.
The planets divide clearly into two groups: terrestrial and jovian.
The solar system contains vast numbers of asteroids and comets.
There are some notable exceptions to these general patterns.
What is the nebular theory?
The nebular theory holds that the solar system formed from the gravitational collapse of a great cloud
of gas and dust, which itself was the product of recycling of gas through many generations of stars
within our galaxy.
What caused the orderly patterns of motion?
As the solar nebula collapsed under gravity, natural processes caused it to heat up, spin faster, and
flatten out as it shrank. The orderly motions we observe today all came from the orderly motion of this
spinning disk.
Why are there two major types of planets?
The inner regions of the solar nebula were relatively hot, so only metal and rock could condense into
tiny, solid grains; these grains accreted into larger planetesimals that ultimately merged to make the
terrestrial planets. Beyond the frost line, cooler temperatures also allowed more abundant hydrogen
compounds to condense into ice, building ice-rich planetesimals; some of these grew large enough for
their gravity to draw in hydrogen and helium gas, forming the jovian planets.
Where did asteroids and comets come from?
Asteroids are the rocky leftover planetesimals of the inner solar system, and comets are the ice-rich
leftover planetesimals of the outer solar system. These objects still occasionally collide with planets or
moons, but the vast majority of impacts occurred during the heavy bombardment in the solar system’s
first few hundred million years.
How do we explain the “exceptions to the rules”?
Most of the exceptions probably arose from collisions or close encounters with leftover planetesimals.
Our Moon is most likely the result of a giant impact between a Mars-size planetesimal and the young
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