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Astrobiology: the origin, evolution,
distribution, and future of life in the universe
Outline of this class:
Life, extreme life on earth
Where else in solar system could life exist? Mars, Titan& Europa,
Habitable zone (review), difficulty with estimating probability of life,
Drake equation for estimating likelihood
SETI: Search for Extraterrestrial Intelligence
No class this Wednesday, Happy Thanksgiving!
Next Monday: primarily review
Next Wed: YOU each do class presentation (30 % of your final)
Mon, Dec 9 Final exam. Would you all like to follow this with a
potluck supper? (Colette and I will contribute major items!) No
need to cook something!
What defines life?
• the capacity to grow,
• metabolize (convert food
to energy)
• respond (to stimuli),
• adapt
• reproduce
What is necessary? Recent discoveries of life under extreme conditions on
earth (extremophiles) show that neither sunlight nor oxygen are required
Yellowstone National Park: microbes live in boiling water
(90 C). Other pools are extremely acidic, yet microbes
and bacteria thrive there
Life in extreme
conditions on earth
Black smoker, deep in the ocean: an
example of life that has no need of
From vents deep in the ocean
hydrogen sulfide provide energy for
bacteria, which in turn feed clams,
tube worms (up to 10 ft long)
Bacteria up to a mile
underground: water seeps in,
and bacteria generates energy
from chemical reactions
A NASA favorite: Tardigrade
(water bear) that survive at
temps from absolute zero to
above boiling, pressures up
to 6x that of deepest ocean
trenches, ionizing radiation.
They can go without food or
water for more than 10 years
and then revive.
(Less than 1 mm long)
Are there other places in our solar system that might harbor life?
Temperatures that allow liquid water may be very important
We discussed the Goldilocks idea for Venus (too hot), Mars (too cold), Earth
(just right)
Could there have been life
on Mars in the distant past?
1996: Martian meteorite found on earth, could it be possible fossil life from Mars?
Current thinking is that this is not a fossil, but it raises interesting questions
What about other places? The moons of gas giants
Moons of Jupiter (Ganymede, Europa) and Saturn (Titan)
NASA missions in past 20 years have revealed a great deal:
Europa: covered with ice, possibly liquid beneath the ice
Titan: has atmosphere, and liquid surface (but not water…)
Europa: One of 4 Jupiter moons easily
seen with small telescope
- About the size of earth’s moon
- Orbits Jupiter in about 4 days – so
relatively close to Jupiter
NASA Galileo mission, launched
1989, reached Jupiter 1995, orbited
with flybys of moons until 2003
Found the surface of Europa is
covered in ice, long fractures –
suggests liquid water underneath (
heated by strong tides from Jupiter)
Liquid water: raises possibility of
primitive life?
Next, an Aside on tides:
Aside on tides:
Earth- moon: ocean tides caused
by gravity
( force = mass, inverse distance)
Tides consume energy
(friction): lead to tidal locking
Example of tidal locking: moon keeps
one face to the earth all the time)
Surface of Europa,
from the Galileo
mission, showing
“ice rafts”
Scale: This image is 20 by 50 miles
A theoretical model of possible
ocean on Europa. (Rick
Greenberg, UA)
NASA Cassini mission to Saturn:
Launched 1997, arrived 2004
Cassini made multiple flybys of Venus,
Earth and Jupiter to gain the required
energy to reach Saturn
It carried a probe, named Huygens, that parachuted to the surface
of the moon Titan in 2005, sending back images of the descent
Cassini is still orbiting Saturn, sending back data
Saturn’s largest moon: Titan
View of surface from Huygens probe, which
parachuted to the surface
Surface from about 30 km
Titan: further exploration suggest lakes of liquid methane, ethane: a
“water cycle” than involves no water!
While these are interesting places, we have no evidence of any form of life on them.
Let’s turn to the stars.
Is extrasolar intelligent life likely? Let’s start with a statistical
estimate exercise:
how many left-handed, 8 year old boys are there is the US right now?
how many left-handed, 8 year old boys are there is the US right
Population of the US, P:
Fraction of males, Fm
Fraction of people who are left handed Fl
Fraction of population who are 8 years old F8
•Answer = P * Fm* Fl* F8
Scientific Notation: or handling big numbers
scientific notation:
= 103 = one thousand
1,000,000 = 106 = one million
1,000,000 = 109 = one billion
100 = 102 1000 = 103 ,
102 x 103 = 105 (add the exponents)
(2 x 102) x (3 x 103) = 6 x 105
105 / 103= 102 (subtract the exponents)
Our CCD at 0.9m was 4 x 103 by 4 x 103 pixels. How many pixels
The Drake Equation: statistical estimate of the number
of intelligent, communicating civilizations in our galaxy
right now
Number of stars in our galaxy
Fraction of stars that have planets around them
Number of planets per star that are capable of supporting life (see
habitable zone)
Fraction of planets where life evolves
Fraction of these planets where intelligent life evolves
Fraction of intelligent life that communicates
Fraction of a planet’s lifetime during which the civilizations
N equals the product of all these factors!
1. How can we measure the number of stars in our galaxy?
(This isn’t an actual picture of our galaxy.
How do we measure the number of stars in our galaxy?
We can use the law of gravity
to measure how much mass is
within our galactic orbit.
Vc = velocity of sun around
galactic center
r = distance from sun to
galactic center
We divide this mass by the
average mass per star to get
the number of stars
Current best number:
2-4x1011 stars
(200 to 400 billion)
2. What is the fraction of stars that have planets?
Kepler Project: indicates that
practically all sun-like stars have
Although Kepler looks at a very
small fraction of the Milky Way
galaxy, it should be representative
of most
Is is appearing that the
majority of planets are more
earth-like than Jupiter-like
3. What number of planets are able to support life?
• Recall the habitable zone concept – warm enough for liquid water
“one in five stars like the sun is home to a planet up to twice the size of Earth,
orbiting in a temperate environment. “
The other factors ( 4 through 7 ) are up to you:
Values you get?
Note that this ONLY addresses our galaxy: there are about as
many galaxies in the known universe as there are stars in our
So, how far might the nearest earth-like planet be?
If there is intelligent life there, do they know about us? (the “I Love Lucy” effect)
Listening for intelligence: from Project Ozma
(1960) to SETI (today)
SETI Project: search for intelligent signals
[email protected]: 1998, citizen science program, using personal computers to help
with data reduction, also support from Planetary Society (private group)
Small percent of time devoted to this
SETI Project: the Allen Telescope array
• Need for more telescope time:
proposal to build up to 350 small radio
supported by Paul Allen (Microsoft
founder), located at Hat Creek Obs, CA
• Went on line in 2007, only 3 telescopes
in place
Survey 1,000,000 “nearby”stars for SETI
Survey the galactic plane for very
powerful transmitters
What sort of signal is SETI looking for?
•The Arecebo telescope does not track the sky, so
a exterrestrial signal will drift through its beam.
•We might expect an intelligent exterrestrial
signal to be narrow in frequency, rather than
covering a broad range
•If the signal contains information, it will be
•Since planets (like us) probably rotate, it may
show a Doppler shift , or change in frequency –
and this would include pulses
•If we detect a signal, how will we decode it?
Needless to say, we haven’t heard
do you think we will? How will we
decode it?