AST 105 HW #14 Solution
Week of November 30th, 2015
Note: All Problems are from The Cosmic Perspective (6ed)
Chapter 24
Review Problems
1. Describe three recent developments in the study of life on Earth that make life elsewhere seem more
plausible.
 First, we have found that life arose on Earth early in the Earth’s history, suggesting that life might
form quickly on other worlds. Second, laboratory experiments have shown that chemical
constituents thought to have been common on the young Earth readily combine to form organic
molecules. Third, we have found living organisms that can survive in conditions similar to those
found on some other worlds in our solar system.
2. How do we study the history of life on Earth? Describe the geological time scale and a few of the major
events along it.
 We study the history of life on Earth by using fossils, relics of organisms that lived long ago. The
geological time scale is a set of distinct intervals that break up the Earth's history.
 Important events along the geological time scale include:
1) Formation of the solar system and the Earth (4.5 billion years ago)
2) End of the heavy bombardment (4 billion years ago)
3) Oldest rocks on Earth formed (4 billion years ago)
4) Carbon isotope evidence for life (3.85 billion years ago)
5) First fossil microbes (3.5 billion years ago)
6) Oldest eukaryotic fossils (2.1 billion years ago)
7) Oxygen accumulates in the atmosphere (2 billion years ago to half a billion years ago)
8) Cambrian explosion (530 million years ago)
9) Plans and fungi colonize the land (480 million years ago)
10) Animals colonize the land (410 million years ago)
11) Mammals and dinosaurs appear (200 million years ago)
12) Dinosaurs prominent (200 million years ago to 65 million years ago)
13) K-T event causes dinosaur extinction (65 million years ago)
14) Mammals prominent (65 million years ago to present)
3. Summarize the evidence pointing to an early origin of life on Earth. How far back in Earth’s history did life
exist?
 Evidence pointing to an early origin of life on Earth includes 3.5-billion-year-old stromatolites, rocks
with nearly identical structure to bacterial mats found today. Evidence for an even earlier origin for
life comes from carbon isotope ratios that indicate that life may have appeared as far back as 3.85
billion years ago, although it is difficult to date these rocks.
9. Is it possible that life migrated to Earth from elsewhere? Explain.
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 Life may have migrated to Earth from elsewhere, perhaps being carried on meteorites. If life arose
on Mars or Venus, meteorites knocked off of those planets could have traveled to Earth and seeded
life here.
11. What is a habitable world? Which worlds in our solar system seem potentially habitable, and why?
 Habitable worlds are worlds that contain the basic necessities for life, including liquid water. Apart
from the Earth, the only places that seem potentially habitable are Mars and some of the large
moons of the Jovian planets. We are fairly certain that water once flowed on Mars, making it a good
candidate. The icy moons Europa, Ganymede, and Callisto all appear to have subsurface oceans and
they may have volcanic vents at the bottoms of the oceans. Titan also offers a possibility for life,
although it would have to use liquid methane in place of liquid water, or be located deep
underground where it might be warm enough for ice to melt into water.
14. What is the “rare Earth hypothesis”? Summarize the arguments on both sides regarding the validity of
this hypothesis.
 The rare Earth hypothesis is the idea that Earth's hospitality is the result of rare planetary luck. The
arguments in favor of this hypothesis are that there may be a fairly narrow ring at about our solar
system's distance from the center of the galaxy where habitable planets might have enough heavy
elements to form but not be sterilized by nearby supernovae too frequently. They also argue that
without a Jupiter-like planet, impact rates may remain high longer, posing a danger to life on
terrestrial planets. Finally, they argue that plate tectonics, which keeps Earth's climate stable, may
be rare since Venus does not have evidence of such activity. Counterarguments to these points are
that it does not seem that a large abundance of heavy elements is necessary to form planets and
that atmospheres may protect organisms on the surfaces from the radiation of supernovae. It is also
pointed out that Jovian planets appear to be common in our studies of extrasolar planets, so the
probability of a Jupiter-like planet in a system seems reasonably good. Finally, plate tectonics may
not occur on Venus because of its runaway greenhouse. Since the runaway greenhouse is caused by
Venus's proximity to the Sun, it is possible that any Earth-like planet in the habitable zone could
have plate tectonics.
16. What is the Drake equation? Define each of its factors, and describe the current state of understanding
about the potential values of each factor.
 The Drake equation gives a simple way to calculate the number of civilizations capable of interstellar
communication that are currently sharing the Milky Way with us, at least in principle.
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Figure 1: Copied from http://www.noeticscience.co.uk/wp-content/uploads/2013/04/drake-equation.jpg
 Note: You can write R* as N/T, where N is the number of the stars in our galaxy and T is the age of
our universe. Aside, our galaxy is about 13.21 billion years old and have 100 billion stars in it (So
what is R*?)
Fantasy or Science Fiction?
For each of the following futuristic scenarios, decide whether it is plausible or implausible according to our
present understanding of science. Explain clearly; not all these have definitive answers, so your explanation is
more important than your chosen answer.
19. The first human explorers on Mars discover the ruins of an ancient civilization, including remnants of tall
buildings and temples.
 The surface of Mars have been thoroughly studied by the scientists. If such civilization ever existed
on Mars, we would have found it by now.
20. The first human explorers on Mars drill a hole into a Martian volcano to collect a sample of soil from
deep underground. Upon analysis of the soil, they discover that it holds living microbes resembling
terrestrial bacteria but with a different biochemistry.
 This is plausible. If there is life on mars, it could have a different biochemistry from life on Earth.
23. A century from now, after completing a careful study of planets around stars within 100 light-years of
Earth, astronomers discover that the most diverse life exists on a planet orbiting a young star that formed
just 100 million years ago.
 This could not happen according to our present understanding of the origin of planets and life,
because during the first 100 million years of a star system’s history, we expect the planet to be
pelted by many large impacts. If life could arise so quickly at all, it would almost certainly be
extinguished.
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Process of Science
39. Extraordinary Claims. As discussed in the chapter, both the Viking results and the study of a Martian
meteorite led some scientists to think we had found evidence of life on Mars, even while most scientists
disagreed. Those who disagree often point to Carl Sagan’s dictum that “extraordinary claims require
extraordinary evidence.” Briefly discuss one of these cases, and decide whether you think it should require
extraordinary evidence before being accepted. What follow-up evidence might lead scientists to re-evaluate
their positions?
 The study of the meteorite and the Viking results are very interesting, but they are simply not
conclusive. Each of these evidences can be explained by chemical processes that does not involve
living organisms. If scientists can somehow rule out such alternative chemical processes, the claim of
the past life on Mars would be strengthened.
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