Download astro20 chap27 - Las Positas College

Life In The Universe
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Use a particular definition of “life”
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organisms that can react to environment
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organisms that take nourishment from the
environment
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organisms that can reproduce
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organisms that can genetically evolve
•
Earth examples
– viruses
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•
crystalline and inert when outside host
•
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
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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.)
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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
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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
•
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
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•
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
•
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.)
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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
•
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.)
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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
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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
•
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 !
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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