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Chapter 3&4

Astronomy 2021A/B Chapter Notes - Chapter 3&4: Phanerozoic, Lunar Mare, Late Heavy Bombardment

16 pages61 viewsFall 2013

Department
Astronomy
Course Code
Astronomy 2021A/B
Professor
Prof
Chapter
3&4

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CHAPTER 3 THE UNIVERSAL CONTEXT OF LIFE
3.1 The Universe and Life
What major lessons does modern astronomy teach us about our place in the universe?
Newton delivered the final, shattering blow to the Aristotelian conception that Earth must by necessity be unique.
The heavens could no longer be considered a separate realm made from different material (the ether or quintessence) and operating under different laws from Earth.
Three ideas are especially important in framing the universal context for everything else we will study:
-The universe is vase and old. Its vastness implies an enormous number of worlds on which life might possibly have arisen, and its old age means there has been plenty of time for life to
being and evolve.
-The elements of life are widespread. Observation shows that the basic chemical elements that make up Earth and life are present throughout the universe.
-The same physical laws that operate on Earth operate throughout the universe. Every experiment and observation made to date has given additional support to Newton's conclusion that
the laws of nature are the same everywhere.
Together, these ideas reinforce the primary lesson of the Copernican revolution: we are not the center of the universe.
3.3 The Nature of the Worlds
How do other worlds in our solar system compare to Earth?
Today, we know that no other world in our solar system is much like Earth, at least on the surface.
No other world has surface oceans of liquid water, an atmosphere rich in oxygen, or a climate so hospitable to life.
The four inner planets (Mercury, Venus, Earth, and Mars) are much smaller than the four outer planets (Jupiter, Saturn, Uranus, and Neptune). These size differences reflect basic
differences in planetary character.
The four inner planets are made almost entirely of metal and rock, which makes their average densities several times that of water. They have solid surfaces and their atmospheres (if any)
are quite thin com- pared to the planets themselves.
Because Earth is a member of this group, we refer generally to these rocky worlds as terrestrial planets (terrestrial means “Earth-like”).
Note that the terrestrial planets have few moons; Earth is the only one with a large moon, while Mercury and Venus have no moons and Mars has two very small moons.
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Jupiter, Saturn, Uranus, and Neptune are quite different
in character and composition from the terrestrial planets.
Because Jupiter is the largest member of this group, we
refer generally to these worlds as jovian planets (jovian
means “Jupiter-like”).
Rather than metal and rock, the jovian planets are made
largely of hydrogen, helium, and hydrogen compounds
such as water (H2O), methane (CH4), and ammonia
(NH3); this composition gives them much lower average
densities than the terrestrial planets.
The jovian planets of our solar system have at least 165
moons among them, with Jupiter alone having more than
60 known moons. All the jovian planets also have rings
made up of countless small particles orbiting them,
though only Saturns rings are easily visible from Earth.
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Because hydrogen compounds are generally gases under earthly conditions (except for water, which can be either solid, liquid, or gas on Earth), the jovian planets are often called “gas
giants.”
However, the pressure throughout most of their interiors is so high that these “gases” are not actually in the gas phase; instead, they may be compressed into liquid or into other high-
density phases, which makes them behave quite differently than they do on Earth.
Moreover, while the jovian planets contain metal and rock deep in their cores, the high pressure means that even these cores are unlikely to resemble the solid surfaces of the terrestrial
worlds. There would be no place to “land” on the jovian planets; if you plunged into one of them, you would continue your descent until you were crushed by the growing pressure.
Pluto, Eris, and other objects large enough for their own gravity to have made them round are now considered dwarf planets.
The rest of the small bodies have traditionally been categorized in two groups: asteroids made mostly of metal and rock and comets made mostly of rock and ice.
Most asteroids orbit in the region called the asteroid belt, which lies between the orbits of Mars and Jupiter.
We now know that comets come from the distant reaches of the solar system, and they grow tails only when they come close enough to the Sun for the Sun’s heat to convert some of their
ice into gas.
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