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.