AY 101 Chapter Notes - Chapter 19: Drake Equation, Interstellar Travel
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The Big Picture
Throughout our view of astronomy, we have taken the “big picture” view of trying to understand how we
ﬁt into the universe. Here, at last, we have returned to Earth and we have examined the role of our own
generation in the big picture of human history. Tens of thousands of past human generations have walked
this Earth. Ours is the ﬁrst generation with the technology to study the far reaches of our universe, to
search for life elsewhere, and to travel beyond our home planet. It is up to us to decide whether we will
use this technology to advance our species or to destroy it.
Imagine for a moment the grand view, a gaze across the centuries and millennia from this moment
forward. Picture our descendants living among the stars, having created or joined a great galactic
civilization. They will have the privilege of experiencing ideas, worlds, and discoveries far beyond our
wildest imagination. Perhaps, in their history lessons, they will learn of our generation-the generation that
history placed at the turning point and that managed to steer its way past the dangers of self-destruction
and onto the path to the stars.
Summary of Key Concepts
When did life arise on Earth?
•Fossil evidence puts the origin of life at least 3.5 billion years ago, and carbon isotope evidence pushes
this date to more than 3.85 billion years ago. Life therefore arose within a few hundred million years
after Earth’s birth, and possibly in a much shorter time.
How did life arise on Earth?
•Genetic evidence suggests that all life on Earth evolved from a common ancestor, which may have
resembled microbes that live today in hot water near undersea volcanic vents. We do not know how this
ﬁrst organism arose, but laboratory experiments suggest that it may have been the result of natural
chemical processes on the early Earth. Once life arose, it rapidly diversiﬁed and evolved through
What are the necessities of life?
•Life on Earth thrives in a wide range of environments and in general seems to require only three things:
a source of nutrients, a source of energy, and a liquid that is most likely water.
Could there be life on Mars?
•Mars once had conditions that may have been conducive to an origin of life. If life arose, it might still
survive in pockets of liquid water underground.
Could there be life on Europa or other jovian moons?
•Europa probably has a subsurface ocean of liquid water and may have undersea volcanoes on its ocean
ﬂoor. If so, it has conditions much like those in which early life on Earth may have thrived, making it a
good candidate for life. Ganymede and Callisto might have oceans as well. Titan has other liquids on its
surface, though it is too cold for liquid water. Perhaps life can survive in these other liquids, or perhaps
Titan has liquid water deep underground. Enceladus also shows evidence of subsurface liquids, offering
yet another possibility for life.
What are the requirements for surface habitability?
•To have a habitable surface, a planet must reside in its star’s habitable zone, it must have volcanism to
create an atmosphere and plate tectonics to help maintain a stable climate, and it must have a magnetic
ﬁeld to protect its atmosphere from the stellar wind. For a world in the habitable zone, it is likely that all
the requirements will be met if the planet is similar in size to Earth.
Is life rare or common?
•We don’t know, and reasonable arguments can be made on both sides of this question. However, it is
likely that we will soon have the technology to answer the question, though it will require large
telescopes capable of detecting spectral signatures of life on distant planets.
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