Chapter 23
• Life in the Universe
Cosmic Evolution
• What is LIFE?
– Not so easy to answer, especially if we allow for types of life
that are not found on Earth
• These are some generally agreed-upon characteristics that any
life form should have:
– Ability to react to environment
– Ability to grow by taking in nourishment and processing it into
energy
– Ability to reproduce, with offspring having some characteristics
of parent
– Ability to evolve
Brief History of Life on Earth
• 4.4 billion years - early oceans form
• 3.5 billion years - cyanobacteria start releasing
oxygen
• 2.0 billion years - oxygen begins building up in
atmosphere
• 540–500 million years - Cambrian Explosion
• 225–65 million years - dinosaurs and small mammals
(dinosaurs ruled)
• Few million years - earliest hominids
Necessities for Life – As We Know It.
• A nutrient source
• Energy (sunlight, chemical reactions, internal heat)
• Liquid water (or possibly some other liquid)
Could there be life on Mars?
Searches for Life on Mars
• Mars had liquid water in the distant past.
• Still has subsurface ice; possibly subsurface
water near sources of volcanic heat
In 2004, NASA Spirit and Opportunity rovers sent home new
mineral evidence of past liquid water on Mars.
Latest Martian Probe: Curiosity will explore Mars as a
potential habitat for life, past or present.
Previous twin rovers, Spirit and Opportunity found water on mars.
The Martian Meteorite debate
Composition indicates
origin on Mars.
• 1984: meteorite ALH84001 found in Antarctica
• 13,000 years ago: fell to Earth in Antarctica
• 16 million years ago: blasted from surface of Mars
• 4.5 billion years ago: rock formed on Mars
Does the meteorite contain fossil evidence of life on Mars?
Meteorite ALH84001
Nanobacteria Fossils?
Most scientists
are not yet
convinced.
Terrestrial Nanobacteria
Fossils
Could there be life on Europa or other jovian
moons?
• Ganymede, Callisto also show some evidence for
subsurface oceans.
• Relatively little energy available for life, but there still
may be enough.
• Intriguing prospect of THREE potential homes for life
around Jupiter alone.
Ganymede
Callisto
Titan
• The surface is too cold for liquid water (but there may be
some deep underground).
• Has lakes of liquid ethane/methane on its surface.
Enceladus
• Ice fountains suggest that Enceladus may have a subsurface
ocean.
Are There Habitable Planets Around Other Stars?
Definition:
• A habitable world contains the basic necessities
for life as we know it, including liquid water.
• It does not necessarily have life.
Constraints on star systems:
1) Old enough to allow time for evolution (rules
out high-mass stars - 1%)
2) Need to have stable orbits (might rule out
binary/multiple star systems - 50%)
3) Size of “habitable zone”: region in which a
planet of the right size could have liquid water
on its surface
Even with these constraints, billions of stars in the Milky
Way could potentially have habitable worlds.
The more massive the star, the larger its habitable zone—
and the higher probability of a planet existing in this zone.
Search for Extrasolar Planets
• Extrasolar: Orbiting stars other
than our sun. (The term
exoplanet is also used.)
• Kepler (launched in 2009) will
monitor 100,000 stars for transit
events for 4 years.
• 105 so far
• Later: SIM and TPF
interferometers may
obtain spectra and crude
images of Earth-size
planets.
Spectral Signatures of Life
Venus
Earth
Mars
Oxygen/ozone
Are Earth-like planets rare or common?
Elements and Habitability
• Some scientists argue
that the proportions of
heavy elements need
to be just right for the
formation of habitable
planets.
• If so, then Earth-like
planets are restricted
to a galactic habitable
zone.
Impacts and Habitability
• Some scientists argue
that Jupiter-like
planets are necessary
to reduce rate of
impacts.
• If so, then Earth-like
planets are restricted
to star systems with
Jupiter-like planets.
Climate and Habitability
• Some scientists argue
that plate tectonics
and/or a large moon
are necessary to keep
the climate of an
Earth-like planet
stable enough for life.
The Bottom Line
We don’t yet know how important or
negligible these concerns are.
The Search for Extraterrestrial Intelligence
SETI
The Drake Equation
Factors to consider when calculating the number of
technologically advanced civilizations per galaxy:
N c  N  f p  nLZ  f L  f l  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.
SETI experiments look for deliberate signals from extraterrestrials
The Arecibo Message
At dedication of
Arecibo Radio
Observatory, blocks
of 1679 pulses were
emitted toward
globular cluster M13.
The pulses can be
arranged in only two
ways: 23 rows of 73
or 73 rows of 23.
Resulting 23x73 grid
contained basic
information about our
human society.
Communication Through Leakage!!
We are also communicating—although not deliberately—through
radio waves emitted by broadcast stations.
These have a 24-hour
pattern, as different
broadcast areas
rotate into view.
The Water Hole
If we were to deliberately broadcast signals that we wished to be
found, what would be a good frequency?
There is a feature called the
“water hole” around the
radio frequencies of
hydrogen (21 cm) and the
hydroxyl molecule (18
cm). The background is
minimal there, and it is
where we have been
focusing many of our
searches.
How difficult is interstellar travel?
Current spacecraft travel at <1/10,000c; 100,000
years to the nearest stars
Pioneer
Plaque
Voyager I and II Phonograph Records
Difficulties of Interstellar Travel
•
•
•
•
Far more efficient engines are needed.
Energy requirements are enormous.
Ordinary interstellar particles become like cosmic rays.
Social complications of time dilation.
Where are the aliens?
Fermi’s Paradox
•
•
Plausible arguments suggest that civilizations
should be common. For example, even if only 1 in
1 million stars gets a civilization at some time 
100,000 civilizations
So why we haven’t we detected them?
Possible solutions to the paradox
1) We are alone: life/civilization is much rarer than
we might have guessed.
•
Our own planet/civilization looks all the more
precious…
Possible solutions to the paradox
2) Civilizations are common, but interstellar travel is
not because:




interstellar travel is more difficult than we think.
the desire to explore is rare.
civilizations destroy themselves before achieving
interstellar travel.
they are simply avoiding us.
These are all possibilities, but not very appealing…
Possible solutions to the paradox
3) There IS a galactic civilization…
… and some day we’ll meet them.
The End.
Remember, astronomy is something you can continue on
your own.
Go outside and look up.
Have a good life.