Life on Other Worlds Guidepost Outline The Physical Basis of

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Life on Other Worlds

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Guidepost
This program is either unnecessary or critical,
depending on our point of view. If we believe
that astronomy is the study of the physical
universe above the clouds, then this chapter does
not belong here. But if we believe that astronomy
is the study of our position in the universe, not
only our physical position but also our role as
living beings in the origin and evolution of the
universe, then everything else in this book is
just preparation for this chapter. Astronomy is
the only science that truly acts as a mirror. In
studying the universe up there, we learn what we
are down here. Astronomy is not really about
stars, galaxies, and planets it is about us.
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Outline
I. The Nature of Life A. The Physical Basis of
Life B. Information Storage and Duplication C.
Modifying the Information II. The Origin of
Life A. The Origin of Life on Earth B. Geologic
Time C. Life in Our Solar System D. Life in
Other Planetary Systems III. Communication with
Distant Civilizations A. Travel Between the
Stars B. Radio Communication C. How Many
Inhabited Worlds?
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The Physical Basis of Life
All life forms on Earth, from viruses to complex
mammals (including humans) are based on carbon
chemistry.
Carbon-based DNA and RNA molecule strands are the
basic carriers of genetic information in all life
forms on Earth.
This complex mammal contains about 30 AU of DNA.
The Tobacco Mosaic Virus contains a single strand
of RNA, about 0.1 mm long
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Information Storage and Duplication
All information guiding all processes of life are
stored in long spiral molecules of DNA
(Deoxyribonucleic Acid)
Basic building blocks are four Amino acids
Adenine, Cytosine, Guanine, and Thymine
Information is encoded in the order in which
those amino acids are integrated in the DNA
molecule.
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Processes of Life in the Cell
Information stored in the DNA in the nucleus is
copied over to RNA (ribonucleic acid) strands,
which acts as a messenger to govern the chemical
processes in the cell.
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Duplication and Division
In the course of cell division, the DNA strands
in the nucleus (chromosomes) are duplicated by
splitting the double-helix strand up and
replacing the open bonds with the corresponding
amino acids
Process must be sufficiently accurate, but also
capable of occasional minor mistakes to allow for
evolution.
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The Origin of Life on Earth

• Life develops into more complex forms through
  gradual evolution, spanning many thousands of
  generations.

• Life began in the sea as single-celled
  creatures.

• Those as well as early multi-celled creatures
  had no hard parts to leave fossils.

Earliest, microscopic fossils date back 4
billion years.
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The Origin of Life on Earth (2)
0.5 billion years ago, in the Cambrian Period,
the diversity and complexity of life on Earth
dramatically increased ?Cambrian Explosion
Best-known fossils from the Cambrian period
Trilobites.
All known fossils from the Cambrian period are
from sea creatures.
No traces of life on land until 400 million
years ago.
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The Miller Experiment
Miller Experiment in 1952 Simulating conditions
on Earth when life began 4 billion years ago
Experiment produced some of the fundamental
building blocks of life amino acids, fatty
acids, and urea.
Water (oceans), primitive atmosphere gases
(hydrogen, ammonia, methane), and energy from
electric discharges (lightning).
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The Origins of Life on Earth (3)

• Miller experiment shows that basic building
  blocks of life form naturally.
• Amino acids and other organic compounds
  naturally tend to link up to form more complex
  structures.

• Early oceans on Earth were probably filled with
  a rich mixture of organic compounds the
  Primordial Soup
• Chemical evolution leads to the formation and
  survival of the most stable of the more complex
  compounds.

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Extraterrestrial Origin of Life on Earth

• Alternative theory Most primitive living
  entities transported to Earth in meteorites or
  comets.

• Some meteorites do show traces of amino acids.

• Theory of extraterrestrial origin of life is
  currently untestable.

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Formation of Cells
First cell membranes may have formed before the
beginning of life
Single amino acids can be assembled into long
protein-like molecules, which form microspheres
when they cool in water.

• Cell membranes

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The Earliest Fossils
Oldest fossils known stromatolites
Built up layer by layer from single-celled
creatures, similar to bacteria, 3.5 billion
years ago.
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Geologic Time
In geologic terms, higher life forms, in
particular mammals and humans, have evolved only
very recently.
Humans have existed for only 3 million years.
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Earth Calendar
(SLIDESHOW MODE ONLY)
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Three Questions About the Evolution of Life
1) Could life originate on another world if
conditions were suitable?
Miller experiment etc. indicate probably yes.
2) Will life always evolve toward intelligence?
If intelligence favors one species over another
probably yes.
3) How common are suitable conditions for the
beginning of life?
? Investigate conditions on other planets and
statistics of stars in our Milky way
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Some Requirements of Life

• Liquid water (for chemical reactions and as
  transport medium).

• Atmosphere (to avoid rapid vaporization of water
  gasses needed for organic compounds)

• Moderate temperatures (keep water liquid avoid
  disintegration of organic compounds activate
  complex chemical reactions)

• Time for life to evolve from simple organic
  compounds into higher life forms several billion
  years.

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Life in Our Solar System
Other planets or their moons are unlikely to have
ever provided suitable conditions for life.
Most promising candidate Mars.
Claimed traces of microscopic fossils may well be
regular mineral formations in the rock.
Meteorite ALH84001,0 probably originated on Mars.
Possibly some evidence of past life on Mars, but
questionable.
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Requirements for Life in Other Planetary Systems

• Planetary systems are probably common.

• Stable orbit around the star ? consider only
  single stars.

• Time for evolution ? consider only F5 or less
  massive stars.

• Moderate temperatures ? Life zone around the star

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Interstellar Communications
(SLIDESHOW MODE ONLY)
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Communication with Distant Civilizations

• Direct space travel to other stars not feasible
  due to large distances (long travel times).

• Viable alternative Radio communication.

• Even for radio communication Long answer times
  due to light-travel time.

• Messages can be arranged in blocks of certain
  length that is a product of two prime numbers ?
  Only two ways to arrange them in a rectangle.

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The Arecibo Message
At dedication of Arecibo Radio Observatory,
blocks of 1679 pulses were emitted, which can be
arranged in only two ways
23 rows of 73or 73 rows of 23.
Resulting 23 x 73 grid contained basic
information about our human society.
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The Search for Extraterrestrial Intelligence
(SETI)
In addition to sending messages to possible
extraterrestrial civilizations, there are also
programs to listen for intelligent messages from
space SETI.
Signals would be overwhelmed by background noise
Only certain wavelength ranges are suitable for
this search.
SETI program is highly controversial because of
the uncertain prospects of positive results.
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The Drake Equation
Factors to consider when calculating the number
of technologically advanced civilizations per
galaxy
Nc N fp nLZ fL fl FS
Most of the factors are highly uncertain.
Possible results range from 1 communicative
civilization within a few dozen light years to us
being the only communicative civilization in the
Milky Way.
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Drake Equation
(SLIDESHOW MODE ONLY)
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New Terms
DNA (deoxyribonucleic acid) amino
acid protein enzyme RNA (ribonucleic
acid) chromosome gene natural selection mutant Cam
brian period Miller experiment primordial
soup chemical evolution stromatolite life
zone water hole SETI
Drake equation
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Discussion Questions
1. What would you change in the Arecibo message
if humanity lived on Mars instead of Earth? 2.
What do you think it would mean if decades of
careful searches for radio signals for
extraterrestrial intelligence turned up nothing?
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Quiz Questions
1. Which element is the physical basis for life
chemistry on Earth? a. Hydrogen. b. Helium. c.
Oxygen. d. Carbon. e. Silicon.
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Quiz Questions
2. On Earth why is carbon-based life more
successful than silicon-based life? a. Silicon
molecules from rings rather than chains. b.
Carbon molecule chains are stronger and more
stable. c. Most of the silicon on Earth is locked
up in silicate rocks. d. Carbon molecule chains
can store, extract, and utilize energy. e.
Silicon molecule chains require a higher
temperature to be stable.
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Quiz Questions
3. How are the instructions for life stored in
DNA molecules? a. As a sequence of protons and
electrons. b. As Digital Numeric Analogs
(DNA). c. As a sequence of base pairs. d. In base
ten code. e. In binary code.
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Quiz Questions
4. How many different bases (A, C, etc.) make up
the base pairs of DNA molecules? a. 2. b. 4. c.
23. d. 26. e. 52.
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Quiz Questions
5. When a cell divides, how does a DNA molecule
duplicate its stored information for the two new
cells? a. The D, N, and A are highlighted,
copied, and then pasted into the new molecule. b.
The original DNA molecule lines up with the new
blank cell nucleus and burns a copy of itself
into the new cell. c. The DNA molecule divides
along the long dimension of the ladder, splitting
the base pairs. Corresponding new bases attach to
each half to form two new identical DNA
molecules. d. The DNA molecule places its
information in temporary storage, in RNA
molecules that later reinstall the information
into the two new cell nuclei. e. A tightly wound
1.5 meter segment of the DNA molecule containing
about 4.5 billion base pairs breaks off and goes
into the new cell nucleus, as a duplicate copy of
the entire sequence of base pairs.
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Quiz Questions
6. How can the instructions stored in DNA
molecules be changed? a. By chance mismatching
of base pairs during cell division. b. By
exposure of reproductive cells to radioactive
isotopes. c. By exposure of reproductive cells to
cosmic rays. d. By cutting the DNA and adding
base pairs. e. All of the above.
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Quiz Questions
7. Why must the DNA instructions change for a
species to survive? a. Without changes, the DNA
instructions will degrade. b. Without variety
some species will die of boredom. c. To produce
mutants for experimentation. d. To adapt to
changes in the environment. e. To create larger
and stronger life forms.
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Quiz Questions
8. How do the instructions stored in a DNA
molecule at the nucleus of a cell get out to
where they are needed to conduct the business of
life? a. DNA molecules transport the information
throughout the cell. b. The instructions are
copied and transported by RNA molecules. c. The
information is not needed at any location other
than the cell nucleus. d. Short strands of DNA
sequences break loose and follow outward-flowing
currents. e. None of the above.
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Quiz Questions
9. What evidence do we have that life on Earth
began in the sea? a. The base pairs of DNA
molecules sequences always begin with base C. b.
The oldest fossils are of the remains of ocean
creatures. c. About 75 of Earth's surface is
covered with water. d. Fish are more primitive
than mammals. e. Water is the best common solvent.
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Quiz Questions
10. What was the significance of the Miller-Urey
experiment? a. It was the first time that humans
created life. b. It verified the suspected
conditions of primeval Earth. c. It formed new
amino acids that had not been found before. d. It
formed proteins in simulated conditions of
primeval Earth. e. It shows that complex organic
molecules can form naturally.
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Quiz Questions
11. What is chemical evolution? a. The process
of binding molecules together to form
progressively more complex molecules that cannot
reproduce themselves. b. The process of forming
progressively more complex life forms that derive
their energy from sources other than sunlight. c.
The evolution of life that has occurred since the
first cell divided and reproduced itself. d. The
development of the periodic table. e. The process
that forms stromatolites.
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Quiz Questions
12. How may intelligence have begun? a. By
species developing multiple escape strategies. b.
By developing an external skeleton for
protection. c. By developing lungs for
respiration. d. By developing the ability to make
and use tools. e. By collecting the seeds of wild
plants and planting them for later harvest.
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Quiz Questions
13. Why do we think that liquid water is
necessary for the origin of life? a. Water is
the best common solvent. b. Amino acids can
easily form in water. c. Amino acids need a
medium in which to link together to form
proteins. d. Organisms need a medium to transport
nutrients and waste. e. All of the above.
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Quiz Questions
14. Other than Earth, where in the solar system
is the most likely place to find life? a.
Earth's moon. b. Mercury. c. Mars. d. Europa. e.
Titan.
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Quiz Questions
15. If a planet is to remain in the life zone of
a G or K main sequence star for 4 to 5 billion
years, it must form near _____ of the life
zone. a. the outer edge b. the middle c. the
inner edge d. Either a or b above. e. Either b or
c above.
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Quiz Questions
16. Why are upper main sequence stars unlikely
sites for intelligent civilizations? a. The
short lifetime of such stars allows little time
for intelligent life to evolve. b. These stars
are too hot for life to evolve on a planet that
is 1 AU away. c. Upper main sequence stars are
very rare. d. Both a and b above. e. All of the
above.
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Quiz Questions
17. Why do we suspect that travel between stars
is nearly impossible for intelligent life
forms? a. The distance between stars in the disk
of our galaxy is typically several light
years. b. The maximum speed that can ever be
approached is the speed of light. c. To
accelerate to and from high speeds requires a
tremendous amount of fuel. d. Both a and b
above. e. All of the above.
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Quiz Questions
18. Why do SETI programs only observe at
wavelengths between 1 and 30 centimeters? a. The
first commercial radio stations transmitted in
this range of wavelengths, and we expect that any
communicating extraterrestrial that has picked up
our early signals will attempt to contact us at
the same wavelength. b. Funding for SETI projects
is so low that military surplus radar antennae
that operate at this range of wavelengths are all
that can be afforded. c. We expect that
intelligent beings must have feet the size of
ours, and thus would communicate at wavelengths
around this natural unit of length. d. At
wavelengths longer than 30 cm our galaxy emits a
lot of interference, and at wavelengths less than
1 cm our atmosphere is opaque. e. The FCC
reserves this range of wavelengths for SETI
programs.
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Quiz Questions
19. Where is the water hole that we suspect is
a good place to find extraterrestrials? a.
Devils Tower. b. Roswell New Mexico. d. At the
poles of Mercury. e. Beneath the frozen icy crust
of Europa. c. Between 18-cm and 21-cm wavelengths.
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Quiz Questions
20. Which variable in the modified Drake equation
would be likely to give us the greatest
difficulty in estimating an accurate and precise
value? a. The number of planets per star that
lie in the life zone for longer than 4 billion
years. b. The fraction of a star's life during
which a technological society survives. c. The
fraction of life forms that evolve to
intelligence. d. The fraction of suitable planets
on which life begins. e. The fraction of stars
with planets.