Download Ch18 Life - UCF Physics

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Start with the Early Earth…
• Hot ~ 230C
• Oceans (at about 4.2 By)
• CO2 atmosphere with ammonia,
methane, water vapor, and nitrogen
• Lots of UV-radiation (no ozone)
• Reducing conditions
• Lots of lightning
Miller-Urey Experiment
• Idea was that conditions
on the primitive Earth
could produce chemical
reactions that made
organic compounds from
inorganic material.
• Used water (H2O),
methane (CH4), ammonia
(NH3), and hydrogen (H2)
• Made up to 22 different
amino acids
• After a week about 10–
15% of the carbon within
the system was now in
the form of organic
So we have organics….
• Turns out that there are
lots of other possible
origins for organics
– Deep sea vents
– Spontaneous formation of
– Radioactive beaches
– And many, many more…
• Now you need to make
• There are, of course,
piles of theories on the
origin of cells
PAHs: Self Organizing Building
• Polycyclic Aromatic
Hydrocarbons (PAHs) are
amphiphilic (they have parts
that are both hydrophilic
and hydrophobic).
• In solution, they tend to self
organize themselves in
stacks, with the
hydrophobic parts
• In this self ordering stack,
the separation between
rings is 0.34 nm, the same
separation found in RNA
and DNA.
• Smaller molecules will
naturally attach themselves
to the PAH rings.
However it happened…
• We think prokaryote cells
(single-cell organisms that
lack a nucleus) developed
as early as ~ 3.85 Billion
years ago
• WE KNOW that by 3.5 Billion
years ago we had bacteria
and blue-green algae
• By 2 Billion years ago we
had eukaryotes (organism
whose cells have a nucleus)
• By 1 Billion years ago we
had multicellular life
• By 600 million years ago we
had simple animals
By 2.5 Billion years ago plankton were
altering the oxygen content of the atmosphere
What are the requirements for Life
• Liquid Water
– Too close….water boils off
– Too far….water freezes
• A source of Energy
– Solar
– Tidal
• Available Organic Molecules
– Carbon Compounds….abundant in comets and some
• Enough Time
– A stable environment
– Evolve Complexity
• This comes together in the concept of a Habitable
But there are a few other things…
Stable Sun
Near-circular planetary orbits
Earth-like planetary mass
Night and Day
No major orbital disruptions
Occasional mass extinctions are OK
– But not too often….
Galactic Habitable Zones
• It is all about stability
• If it takes stability for over 4 billion years
to develop intelligent life, you need to
be in the Galactic suburbs
• Stay away from
Black holes
High star density areas (comets)
Star forming regions
• For a start, stay away from the Galactic
• No planets have been found around stars
with less than 40% of the Sun’s metal ratio
• Too high metallicity is also a problem (we
– Tend to larger, more volatile-rich, lower-relief
– Water-covered
– Easier to form gas-giants…could be bad for
terrestrial Planets
• Metallicity increases steadily toward the
Galactic center
– More matter, faster star formation
• Another thing to avoid is
transiting spiral arms
• These are areas of high
stellar density and high
star formation
– Increases probability of
close gravitational
– Or being to close to
• Our Sun’s galactic orbital
period is about the same
as rotation period the
nearby spiral arm
The Drake Equation
• R*Fp*Ne*Fl*Fi*Fc*L = N
– R = The number of suitable stars, effectively F, G, and K stars,
that form in our galaxy per year (about 1)
– Fp= The fraction of these stars that have planets (about 0.5)
– Ne = The number of Earth-like planets (planets with liquid water)
within each planetary system (we are learning about this
now…..expect an answer in 3-5 years)
– Fl = The fraction of Earth-like planets where life develops (we
could have some idea in 20 years)
– Fi = The fraction of life sites where intelligent life develops (how
are we ever going to know this?)
– Fc = The fraction of intelligent life sites where communication
develops (one would do….)
– L = "The "lifetime" (in years) of a communicative civilization
(how long have we been a communicative civilization?)
– N = The number of communicative civilizations within the Milky
Way today
The Drake Equation
• R*Fp*Ne*Fl*Fi*Fc*L = N
• Drake thinks that N is about 10,000
for our Galaxy.
• I really doubt that…..
– Throw into the equation the limitations
of metallicity, local star density, near-by
supernova, and binary systems
• But a few would not be unreasonable
How can we tell if there is life?
• Look at the
• Life uses the
atmosphere as a
source of energy and
a sink for waste
• We should know
about nearby
systems in ~20 years
But we haven’t we found any
communicative civilizations
• Well….….there may be nothing to find.
• Think about it… would an
advanced civilization communicate?
– How long has it been since Marconi
invented radio?
– Transatlantic commercial service was
established in 1907
Big Questions…
• Is there life elsewhere in our solar
– There is no evidence
• Is there intelligent life elsewhere
in the Universe?
– There is no evidence