Transcript Lecture21 - University of Waterloo

Extraterrestrial life
Life: Definitions
(from Michael’s four primary sources)
• Google: the organic phenomenon that distinguishes living organisms
from nonliving ones
• Wikipeda: “something” that exhibits:
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Organization
Metabolism
Growth
Adaptation
Reponse to stimuli
Reproduction
• Hartmann: a series of chemical reactions using carbon-based
molecules, by which matter is taken into a system and used to
assist the system’s growth and reproduction, with waste products
expelled.
• Woody Allen: full of loneliness, and misery, and suffering, and
unhappiness – and it’s all over much too quickly.
The Chemistry of Life
• Based on Carbon-Hydrogen bonds
• Commonly include Oxygen and Nitrogen
• Sometimes phosphorous
• Silicon?
• Water also appears to be important.
Early Earth
• Solar nebula rich in C, H, O, N and H2O
• Earth forms, with hot interior
 Volcanic activity releases many gases, including water vapour.
• By 4 Gyr ago there were probably bodies of surface water
 Atmosphere rich in H, ammonia, methane, water, N2 and CO2.
Miller experiment
• In the 1950s, Miller and
Urey conducted an
experiment
 Gaseous mix of H,
ammonia, methane,
H2O over a pool of
liquid water
 Passed electric sparks
through the gas
 After some time,
amino acids appeared
in the water
Amino acids
• Amino acids are molecules from which proteins are built up
• E.g. : as produced in the original Miller-Urey experiment
NH 3  2CH 4  2 H 2O  C2 H 5O2 N  5H 2
• In a less primitive atmosphere,
HCN is easily produced, and this
can also produce (e.g.) Glycine
3HCN  2 H 2O  C2 H 5O2 N  CN 2 H 2
Extraterrestrial Amino acids
• Amino acids found in several carbonaceous chondrite meteorites
• The Murchison meteorite (September 28, 1969 over Murchison,
Australia).
• High (12%) water content of 12% and more than 92 different
amino acids
• Only nineteen of these are found on Earth.
The next step?
Proteinoids: simple, dry heating of amino
acids can produce protein molecules. When
water is added, they form a non-living
structure very similar to bacteria.
Coacervates: cell-sized clusters,
produced spontaneously when
proteins are mixed in solution with
other complex molecules
Earliest cells
Prokaryote cells: the simplest and
most primitive, they appeared around
3.6-3.7 Gyr ago. Contain DNA, and
formed bacteria and blue-green algae
(cyanobacteria)
• Include archaebacteria, which may
have been among the first life forms
to appear, and from which we are
descended
 May have been better suited to
oxygen-poor environment
 Can still be found in oxygen-poor,
extreme environments
Extremophiles
• High-temperature, smoker vents on
the sea-floor
 Mineral-rich columns of hot water
are released from geothermal
vents
 A form of archaebacteria exist
there, collecting biological
materials from the vents, and not
the Sun
• Life may have begun in geothermal
environments rather than tidal pools
• Pompeii worm colony, near a
hydrothermal vent
First signs of life
Stromatolites: sedimentary growth
structures, formed by
cyanobacteria
• Some are 2.7-3.5 Gyr old,
suggestive of early life
• Some forms of cyanobacteria
began to produce oxygen and
change the atmosphere.
Modern
Precambrian
Atmospheric development
• Photosynthesis by
cyanobacteria, and later by
plants, caused the oxygen
content to rise dramatically
about 2.5 Gyr ago.
 Rocks formed and buried more
than 2.5 Gyr show signs of an
oxygen-poor environment
• Sunlight dissociated O2 and
allowed formation of ozone (O3).
 This protected surface from
UV rays which tends to break
up complex molecules
Break
Requirements for (complex) life to start?
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Presence of amino acids (seems to be fairly easy)
Liquid water?
Stability
Moderate temperatures?
Moderate pressures?
Habitable zone
• Stars with 4000<T<7000 K
 Live long enough for complexity to evolve
 Emit some UV but not too much
 Cooler stars may suffice? Important because common.
• Habitable zone should be stable in time
• Star content should be rich in heavy elements
Drake Equation
A guess at the number of civilizations in our galaxy, with which we
might hope to communicate
N  R  f p  ne  fl  f i  f c  L
*
where:
• Reasonably well-known are:
 R* is the rate of star formation in our galaxy
 fp is the fraction of those stars which have planets
 ne is average number of planets which can potentially support life per star that
has planets
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Wild guesses are:
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fl is the fraction of the above which actually go on to develop life
fi is the fraction of the above which actually go on to develop intelligent life
fc is the fraction of the above which are willing and able to communicate
L is the expected lifetime of such a civilization
Solar System
• Mars: cannot rule out presence of microbes
 Possibly hidden at the base of the permafrost
 Tantalizing evidence for methane, though there could be nonbiological explanations
• Europa: may have
liquid water
beneath crust of
young ice
• Titan: organic molecules are
present
 Geothermal heat sources
Mars: Viking landers
• Tested soil for organic
material and found it
completely sterile
Allan Hills 84001
• Martian rock, formed about 4500 Myr ago
• Indications that liquid water percolated through
rock in the past
• Organic molecules (polycyclic aromatic hydrocarbons
and amino acids) were found inside
• Peculiar, microbe-like structures found.
 Not clear if they are microbes
 Hard to rule out terrestrial contamination
Methane?
• Methane detected in Mars’ atmosphere (10 parts per billion)
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Has short half life (few hundred years), so there must be a source
Active volcanism? (but no sulfur dioxide makes this unlikely)
UV-driven reactions involving CO2?
Meteors?
Or biological processes?
• Recent suggestions that the methane is correlated with
underground water, strengthening the biological interpretation
Exogenesis (panspermia)
• Hypothesis that life began elswhere in the Universe and was
transported to Earth (e.g. via comets)
 Conversely, could life from Earth be transported to other worlds?
 Calculations show sufficiently large impact on Earth could have
propelled material to Titan, and that microbes might survive.
• Hoyle and Wickramasinghe: hypothesize that viral molecules are
synthesized in space and transported by comets
• Extremophiles good candidates for making the journey
• Could explain why life began so quickly after Earth formed,
following an era of heavy bombardment
SETI
• Listening for extraterrestrial radio signals
• Where?
 Focus on Sun-like stars
 About 1000 such stars within 100 light years
• At what radio frequency should we listen?
 21 cm is an important frequency, because it is emitted by neutral
hydrogen
 Increases the chance of being detected “by accident”
• Why no contact?
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Nearest civilization may be too far away
Desire to contact other worlds may not be common
Synchronization of evolutionary clocks
Requires several thousand times the Earth’s current powergenerating capacity to transmit a radio signal in all directions, out to
100 light years