Transcript Chapter Nineteen

In this chapter, you will discover…
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what qualities scientists believe a world must have in order
to support life
why many scientists are open to the possibility that
primitive life exists elsewhere in the solar system
how scientists estimate the number of planets orbiting
other stars that could support complex life
how scientists search for life beyond our own solar
system—and the results of those searches
how we are trying to communicate with advanced
extraterrestrial civilizations
Viking Mars Lander
Astronomer and renowned science popularizer Carl Sagan poses by a model of
the Viking lander. This image was taken in Death Valley, California, where the
background creates the feel of a Martian landscape. Sagan was instrumental in
choosing some of the experiments flown on the Viking spacecraft.
Chemical Building Blocks
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The chemical building blocks of life exist throughout the Milky
Way Galaxy.
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Organic molecules and water have been discovered in interstellar
clouds, in some meteorites, in comets, and in newly forming star
and planet systems.
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Five elements can bond covalently (that is, by sharing electrons)
with three or more elements: boron (B), carbon (C), nitrogen (N),
silicon (Si), and phosphorus (P).
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Bonding by donating or receiving electrons (called ionic bonding)
creates bonds that are unsuitable for the formation of the
complex, flexible, rapidly changing molecules that life requires.
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Carbon is unique among the five elements that can make at least
three covalent bonds in that its bonds are flexible yet strong.
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As counter-examples, silicon-silicon bonds come apart under the
slightest disturbance; silicon-oxygen bonds create gels and
liquids that are very hard to alter.
Creating Complex Molecules
(a) Atoms (denoted by capital letters) that can bond strongly to only two other
atoms can make linear chains, as depicted here. However, when such atoms
bond to atoms that can only make one bond, denoted here as Y and Z, the
chain stops. In no case can atoms X, Y, and Z combine to create nonlinear
chains other than loops. (b) When an atom, like carbon (C), can share
electrons with more than two other atoms (in carbon’s case, with four atoms),
then the complex, nonlinear chains essential for life can form. Chains of
carbon atoms form the backbone of organic molecules. For example, glucose,
with carbon, oxygen (O), and hydrogen (H), is a nonlinear molecule that
serves as a nutrient for many life-forms; it is a sugar. The lines indicate bonds
between atoms.
Non-Carbon Organic Molecules?
When any element other than carbon that can make three or more covalent
bonds combines, it makes compounds that are either too soft, too hard, too
reactive, or too inert to be useful in supporting life. Consider silicon. (a) The
silicon-oxygen pair that creates the backbone of silicone is too inert to allow such
molecules to react rapidly and thereby serve as organic molecules. Furthermore,
these bonds produce gel or liquid compounds, as shown. (b) When the
backbone is silicon-oxygen-oxygen, the bonds are rigid, as in this quartz rock.
Carbonaceous Chondrite
Carbonaceous chondrites are meteorites that date back to the
formation of the solar system. This sample is a piece of the Allende
meteorite, a large carbonaceous chondrite that fell in Mexico in 1969.
Miller-Urey Experiment Updated
Modern versions of this classic
experiment prove that numerous organic
compounds important to life can be
synthesized from gases that were present
in Earth’s primordial atmosphere. This
experiment supports the hypothesis that
life on Earth arose as a result of ordinary
chemical reactions.
Miller-Urey Experiment Updated
This photograph shows Harold Urey observing his
experiment as it was underway.
Zones for Habitable Planets
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If a planet’s environment is otherwise hostile—such as having
too much radiation, being too windy, or being too seismically
active—life may either not be able to spread or may quickly
become extinct.
If the star is too massive, it will explode before advanced life
on any of its planets has a chance to evolve very far. If the star
is too small, the planet would have to be very close to it to be
warm enough for life, but then tidal forces from the star would
lock the planet in synchronous rotation, making most of its
surface either too hot (daytime side) or too cold (nighttime
side) for life to flourish.
Even a world in synchronous rotation with a suitable
temperature on the star-facing side is very unlikely to support
life because water in its atmosphere will drift to the night side,
permanently freeze, and the star-lit side will eventually
become arid.
Zone for Habitable
Planets
This figure summarizes the
locations in the Galaxy and in
orbit around stars where
habitable planets might be
found. Earth, of course, is in
such a location.
Hyperthermophiles
These microscopic
thermophiles (heat-loving
organisms) live in water
that is between 80°C and
100°C (85°F–140°F).
Hyperthermophiles
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Geologists and biologists have discovered life on Earth
in some incredibly challenging environments, such as on
the ocean floor, in a lake far under the Antarctic ice pack,
deep inside our planet’s crust, and even in hot
geothermal vents.
It therefore seems reasonable to believe that life could
have originated off Earth under similarly challenging
conditions.
Scientists consider at least four places in our solar
system as possible habitats for life past or present. They
are Jupiter’s moons Europa, Ganymede, and Callisto,
and the planet Mars.
Hyperthermophiles
Tube worms (light-green tubes) with hemoglobin-rich red plumes.
They reside around black smokers—vents in the ocean bottom that
are in the same temperature range as the hot springs shown in the
previous figure. These vents are over 3 km (2 mi) under water.
SETI
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Astronomers are using radio telescopes to search for
signals from other self-aware life in the Galaxy. This effort
is called the search for extraterrestrial intelligence, or
SETI.
SETI is primarily done at frequencies where radio waves
pass most easily through the interstellar medium. So far,
these searches have not detected any life outside Earth.
Water Hole
The so-called water hole is a range of radio wavelengths from about
3 to 30 centimeters that happens to have relatively little cosmic
noise. Some scientists suggest that this noise-free region would be
well-suited for interstellar communication.
Radio Telescope Used for SETI
The Arecibo observatory’s radio telescope, with a diameter of 305 m (1000 ft) is the
largest single-aperture telescope in the world. It is located in Arecibo, Puerto Rico.
In the past decade, it was used in an all-sky survey, along with an antenna located
in the Mojave Desert in California to search for extraterrestrial intelligence. In 1996,
an antenna in Canberra, Australia, joined the network.
Drake Equation:
Used to calculate civilizations which are likely to exist in the Milky Way
N = R* fp ne f l fi fc L
R* = rate at which Sunlike stars form in the Galaxy
fp = fraction of Sunlike stars that have planets
ne = number of planets per solar-type star system suitable for life
fl = fraction of those habitable planets on which life actually arises
fi = fraction of those life-forms that evolve into intelligent species
fc = fraction of those species that develop adequate technology and then
choose to send messages out into space
L = lifetime of that technologically advanced civilization
Human Memorabilia in Space
Humans have beamed radio signals into space, hoping that the
message will someday be intercepted by an alien civilization. This is a
visual version of the signal sent in 1974 from the Arecibo radio
telescope toward the globular cluster M13. The Pioneer and Voyager
spacecraft, now in interstellar space, also carry messages from Earth.
Human Memorabilia in Space
The plaques on Pioneer 10 and Pioneer 11 provide information about
where we are, what we look like, and some of the science we know.
Human Memorabilia in Space
Images and sounds sent on Voyager 1 and Voyager 2 were stored on
phonographic records, long before DVDs were even a twinkle in an engineer’s
eye. There are also instructions for playing the record, which contains information
about our biology, our technology, and our knowledge base. Each record also
contains the sounds of children’s voices. It is remotely possible that another race
might someday discover the spacecraft.
Summary of Key Ideas
Astrobiology and SETI
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The chemical building blocks of life exist throughout the
Milky Way Galaxy.
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Organic molecules and water have been discovered in
interstellar clouds, in some meteorites, in comets, and in
newly forming star and planet systems.
Astronomers are using radio telescopes to search for
signals from other self-aware life in the Galaxy. This
effort is called the search for extraterrestrial intelligence,
or SETI. SETI is primarily done at frequencies where
radio waves pass most easily through the interstellar
medium. So far, these searches have not detected any
life outside Earth.
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Drake Equation and Space Memorabilia
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The Drake equation is used to estimate the
number of technologically advanced civilizations
in the Galaxy whose radio transmissions we
might discover. Estimates of this number vary
from 1 to millions.
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Everyday radio and television transmissions
from Earth, along with intentional broadcasts into
space, may be detected by other life-forms.
Key Terms
astrobiology
Drake equation
habitable zone
organic molecule
SETI
water hole