Transcript Slides from the fourth lecture

Life in the Universe
The Elements of Life
• Organic Chemistry
– By definition, involves H,C,N,O
• Most common elements (produced by most stars)
• Well dispersed and available
– Occurs even in interstellar space
• Many organic compounds found in ISM, comets, meteors
(despite extremely harsh conditions)
– Easily delivered to early Earth, or produced locally
• Biochemistry
– Requires liquid water?
– Arises naturally when basic conditions met?
• What is “life”?
– System out of chemical equilibrium which extracts energy from its
environment to maintain itself
• Energy source could be heat, light, chemical, other?
– Reliably reproduces, with opportunity for evolution
• Able to store and decode information for this
Basic Chemistry of Life (here)
From H,C,N,O (plus some trace
amounts of heavier elements like P
and Fe) are built nucleic acids,
proteins, carbohydrates, and lipids,
which can do the chemistry
needed for both metabolism and
evolution.
Photosynthesis
6CO2 + 6H2O + E C6H12O6 + 6O2
Carbon Dioxide + Water + Energy YIELDS Glucose + Oxygen
Digestion
C6H12O6 + 6O2 6CO2 + 6H2O + E
Glucose + Oxygen YIELDS Carbon Dioxide + Water + Energy
Habitable Zones (liquid surface water)
Because the star
keeps getting
brighter, the
continuously
habitable zone
is smaller than
the habitable
zone at a given
time.
Climate on the Earth
The Sun is getting brighter, and was 30%
fainter in the beginning. We’d be frozen now
without greenhouse gases (and really frozen
then). Somehow the greenhouse effect has been
regulated to keep liquid water on the surface. In
less than a billion years, it will be hard to stop a
runaway greenhouse on Earth (like Venus).
The Atmospheres of the Terrestrial Planets
Studying Extrasolar Terrestrial Planets
Even if we can just get
a spectrum, we might
be able to detect signs
of basic life.
The Martian Desert
Evidence of Water Flows
on Mars
Water cannot be a liquid on Mars’ surface
now, but billions of years ago it flowed
(there may even have been seas).
Water Distribution on Mars
Water is concentrated near the poles, but is fairly widespread.
It is likely mostly in the form of sub-surface ice.
The Martian Meteorite
This rock IS from Mars.
Does it contains signs of life?
Most don’t think so now,
But it is intriguing….
Bacterial fossil from Earth
Europa
Europa is an ice-covered rocky moon. This “orbital
resonance” keeps both Io and Europa internally
heated. The stresses are apparent in the huge
surface cracks. There is apparently a deep ocean
under the ice, with volcanoes at the bottom.
Natural color
Titan
The “Flow of Life”
Genetic analysis gives us a window into the distant past,
and clues on how life developed. Most of the biomass on
the Earth is still bacterial, and they are best at filling
ecological niches. Extreme life is found in amazing
places. There is some indication that life could have
started at deep undersea volcanic hydrothermal vents.
Emergence of Life on the Earth
• 0.0-0.5 Gyr
Formation and intense bombardment
– surface is uninhabitable
• 0.5-1.0 Gyr
Surface stabilizes, simple life starts
– RNA, DNA; thermophilic progenitor (chemical energy)
• 1.0-2.0 Gyr
Anerobic prokaryotes, stromatolite beds
– single-celled, no nuclei; oldest fossils formed
• 2.0-2.5 Gyr
Photosynthesis invented, free oxygen
– surface life; use of sunlight; oxygen crisis
• 2.5-3.0 Gyr
Aerobic bacteria, eukaryotes
– exploit available oxygen (more energy), cell nucleus
• 3.0-3.5 Gyr
bacteria diversify
– Keep changing the mix, experiment
• 3.5-4.0 Gyr
Sexual reproduction invented
– Evolve, baby!
• 4.0-4.5 Gyr
complex organisms appear
– All current body plans appear, land colonized
The Drake Equation
How Likely is Radio Contact With Extraterrestrial Intelligences?
NIC = RIC x LIC = Rstar x Pplanets x Phabitability x
Psimple life x Pcomplex life x Pradio
signals
x Lradio era
RICxLIC
rate at which civilizations appear x their lifetime
Rstar
Pplanets
Phabitability
Astronomy
rate at which stars are formed in the Galaxy
probability a star will have planets
probability a planet will be suitable for life
Psimple life
Pcomplex life
Biology
probability bacteria will arise on a suitable planet
probability bacteria will evolve into complex life
Pradio signals
Lradio era
Sociology
probability complex life will send out radio signals
total duration during which radio is sent
Evaluating the Odds for Simple Life
NSL = RSL x LSL = Rstar x Pplanets x Phabitability x Psimple
Rstar
life
Defensible Estimates
observed rate:
10 per year
Pplanets observed discoveries:
0.2
(some systems are unsuitable, but our sensitivity is poor)
Phabitability
extreme life:
(life seems to be everywhere)
0.1
Psimple
0.1
life
rapidity of life on Earth
NSL = Lsimple life/200 = tens of millions!
Thus, many astronomers are willing to say
that a reasonable conclusion is that the
Galaxy is full of life…
Evaluating the Full Odds Optimistically
NIC = RIC x LIC = Rstar x Pplanets x Phabitability x
Psimple life x Pcomplex life x Pradio signals x Lradio era
Optimistic Estimates
Rstar
observed rate:
10 per year
Pplanets observed discoveries:
0.5
Phabitability
extreme life:
0.5
Psimple life
rapidity of life on Earth
1.0
Pcomplex life
long time on Earth
0.2
Pradio signals
who knows? (E-M is common) 0.02
NIC = Lradio era/100
pick your favorite duration…
So if Lre is greater than a few hundred years, there’s
probably somebody out there.
Lre needs to be a million years
for them to be neighbors
(meaning within 1000 ly).
The Galaxy’s a big place, and
its been around a long time!
SETI : the search for extraterrestrial intelligence
• Our only real hope of detecting ET (unless they come to us) is
by listening to the radio
– Radio travels at the speed of light, over the whole Galaxy
– Radio is a low energy way to send a message
– We already have the ability
to send and receive across the Galaxy
• Where should we listen?
– Not the currently known extrasolar systems!
– Solar-type stars? Milky Way?
• How should we listen?
– Frequencies that are relatively quiet.
– How narrow-band?The “water hole”?
• What should we listen for?
– A regular carrier pattern. Complexity.
• What are the odds we will hear
something?
– The Drake equation
Orbital Chaos
70 Vir system
…and so, we are listening!
Allen Array
2006
(Puerto Rico)
Rapid Prototype Array
Arecibo
Evaluating the Odds Pessimistically
NIC = RIC x LIC = Rstar x Pplanets x Phabitability x
Psimple life x Pcomplex life x Pradio
signals
x Lradio
era
Pessimistic Estimate
Rstar
observed rate:
10 per year
Pplanets observed discoveries:
0.1 (no terrestrials known)
Phabitability
extreme life:
0.01 (surface liquid water to
start it)
Psimple life
rapidity of Earth life 0.1 (we got lucky)
Pcomplex life
long time on Earth
0.01 (looks tough)
Pradio signals
who knows?
0.001 (what good are radios?)
NIC = Lradio era/100 million
duration doesn’t much matter…
Pessimistic Conclusion:
There’s nobody home (except for us!).
Let’s be careful, live long, and prosper!
THE END!!