astro20 chap27 - Las Positas College
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Transcript astro20 chap27 - Las Positas College
Life In The Universe
Use a particular definition of “life”
•
organisms that can react to environment
•
organisms that take nourishment from the
environment
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organisms that can reproduce
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organisms that can genetically evolve
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Earth examples
– viruses
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crystalline and inert when outside host
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flourish in host body
•
grow and reproduce in host body
Arguments for life follow Principles of Mediocrity
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life on Earth depends on a few molecules
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elements forming molecules are common and found in
all stars
•
if the laws of science apply everywhere, life must
occur elsewhere
Arguments against life contest that life here is a
product of a series of fortunate accidents (or not)
Life In The Universe (cont.)
Clues to the formation of life on Earth are scarce
•
active geology ensures that initial conditions have
been erased
•
belief is that Earth was barren, with water
•
outgassing from geologic events produced
atmosphere rich in hydrogen, nitrogen, carbon
compounds
– little oxygen
– ammonia, water and methane formed as Earth cooled
Building blocks of life might form out of initial
conditions
•
natural sources of energy molded initial contents of
atmosphere into complex organic molecules
– sources included radioactivity, lightning, solar UV
radiation
– resulted in first appearance of amino acids and
nucleotide bases
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organic molecules are the base of life as we know it
– amino acids synthesize proteins
– nucleotide bases are the steps in the ladder of DNA
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sequences form genes
Life In The Universe (cont.)
•
Miller/Urey experiment in 1953 showed how amino
acids could result from natural conditions
– took mixture of primordial gases - water, methane, CO2,
and NH3
– sent electric discharges (lightning) through mixture
– few days of exposure resulted in production of amino
acids
•
A decade later, others produced nucleotide bases in a
similar manner
•
no DNA has ever been produced but these
demonstrate the rather common occurrence of
building blocks
Some now believe building blocks came from
interstellar sources
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arrived as cargo on meteors and comets
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lots of evidence
– long chain PAH’s (polycyclic aromatic hydrocarbons)
seen in interstellar clouds
– amino acid glycine seen in free space
– organic matter detected directly in both Comet Halley and
Hale-Bopp
Life In The Universe (cont.)
Other planets in the solar system seem to have the
right conditions for carbon based life
•
Moon and Mercury lack water, atmospheres, magnetic
fields
– UV, solar wind, and cosmic rays are a problem for the
formation of building blocks
•
Venus has too thick an atmosphere
– greenhouse effect and lack of water preclude formation of
building blocks
•
Jovian planets (Jupiter/Saturn/Neptune/Uranus) have
no solid surface
– water may be present below surface of Europa
– Titan has an atmosphere thought to be much like ours
soon after formation, and may harbor building blocks
•
Pluto is too cold
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Mars is most likely candidate
– but liquid water is scarce
– atmosphere is thin
– no magnetic field or ozone layer
– there is evidence for a thick atmosphere and ground
water in the past
– no lander so far (Viking, Pathfinder) has found life
– recent meteorite results may give clues to Martian life
Life In The Universe (cont.)
The probability of finding life in our Galaxy at all can
be guessed via the Drake equation
# technological civilizations =
rate of star formation X
fraction of those stars having planets X
average number of planets per system supporting life X
fraction of planets which result in life X
fraction of planets which result in intelligent life X
fraction of those planets that develop technology X
average lifetime of a technological society
Let’s look at each term
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rate of star formation
– 100 billion stars / 10 billion years ~ 10 stars / year
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fraction of stars having planets
– recent observations lead to conclusion ~ 1
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number of habitable planets per system
– depends on the habitable zone around a star
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determined by the boiling and freezing temperature of water
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massive stars don’t live long enough for life to develop
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small stars have very small zones
Life In The Universe (cont.)
– Venus, Earth and Mars lie within this zone in our Solar
System
– need to remove fraction of binary stars ~ 0.5
– need to remove stars that don’t live long enough ( ~ 1
billion years : 0.5
– need to remove all stars whose zones are small enough
so that a planet is not likely to orbit within ~ 0.2 - 0.3
– alltogether this should be ~ 0.1
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fraction of habitable planets on which life arises
– Miller/Urey experiment and space observations show that
life’s building blocks form readily easily
– assuming similar physical processes work everywhere
this would force us to set this ~ 1
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fraction of life bearing planets on which intelligence
arises
– assume natural selection (like on Earth) works
everywhere
– this assures that this number is ~ 1
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fraction of planets with intelligent life that develops
technology
– don’t know how many early human civilizations failed to
develop technology
– the fact the many independent early civilizations did
develop technology makes us believe ~ 1
Life In The Universe (cont.)
•
Average lifetime of a technological civilization
– problems defining the starting point
– use ourselves as an example an project a lifetime of ~
1000 years
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putting it all together
number of technological civilizations =
10 x 1 x 0.1 x 1 x 1 x 1 x 1000 = 1000 !
We now look for signs of intelligent life on other
worlds
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sent our own greetings via the music and maps on
satellites Pioneer 10, and Voyagers 1 & 2
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look for electromagnetic signatures of civilizations
– use radio since there is no extinction
– look at Earth for characteristic signals
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from a distance Earth would appear to be a periodic emitter
of radio waves more powerful than the Sun
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mostly due to US and Europe
– look for signs near hydrogen and OH (21 and 18 cm)
•
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hoping civilizations depend on water and know about the
relative wavelengths of the transmissions
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so-called “water hole” is a natural minimum of background
noise
SETI is the project which now looks for these signals