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7/21/2019 24. Life in the Universe 2015
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Life in the Universe
7/21/2019 24. Life in the Universe 2015
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Life in the Solar System
Our goals for learning• How do we find/identify life?
• Could there be life in the Solar System and wherewould it be?
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Finding life in the Universe
• Solar System -close enough to visit, activeexploration possible. – Look for signs of life in likely places
• Other star systems -too far away to visit, passiveexploration dependant on naturally received dataonly.
– Finding stars with planets (preferably Earth-like),identifying right materials for habitability, looking forsignals from intelligence.
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What Does Life look Like?
• Only one known example- Earth!
• We use Earth’s fossil record and environmentalniches to gain an idea of where, what and how life
exists. (Terrestrial analogues)
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Earliest Fossils
• The oldest fossilsshow that bacteria-like organisms were
present over 3.5
billion years ago.
• Carbon isotopeevidence dates the
origin of life to morethan 3.85 billionyears ago.
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Tree of Life
• Mapping geneticrelationships has led
biologists to discover
this new “tree of life.”
• Plants and animals are
a small part of the tree.
• Suggests likely
characteristics of
common ancestor
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• These genetic studies suggest that the earliest life on
Earth may have resembled the bacteria today found
near deep ocean volcanic vents (black smokers) and
geothermal hot springs.
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Chemicals to Life?
Experiments show that the building-blocks oflife could form easily and spontaneously underthe conditions of early Earth.
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Brief History of Life on Earth
• 4.4 billion years - early oceans form
• 3.5 billion years - cyanobacteria start releasing
oxygen.• 540-500 million years - Cambrian Explosion
– (Only the last 13% of Earth's history!)
• 225-65 million years - dinosaurs and small
mammals (only last 5% of Earth’s history)
• >6 million years - earliest hominids
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Necessities for Life
• Nutrient source• Energy (sunlight, chemical reactions, internal heat)
• Liquid water (or possibly some other liquid)
• Stable environment for life to form (e.g. not devastated
by volcanoes or impacts every 5 minutes)
Hardest to find onother planets
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Looking For Life in the Solar
System
• Look for water and the other necessities
• Examine environments that life can flourish (extremophiles)
• Examining fossils to recognize past life
(identifying fossil bacteria)
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Life in Unexpected Places
(Extremophiles)Extreme cold
Extreme heat
Extreme pressure No sunlight
Low water content
Poison tolerant
Salt tolerant
Acid tolerantAlkali tolerant
Radiation tolerant
Lives in minerals
No sunlight
No oxygen
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1. Searching for Life on Mars
• Mars had liquid water in the distant past, still hassubsurface ice; possibly subsurface water •Energy sources: solar, possible old volcanic vents•Carbon compounds from meteorites
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In 2004, NASA Spirit and Opportunity Rovers sent home new
mineral evidence of past liquid water on Mars. Also looking for
evidence of micro-fossils
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2. Jovian Ice Worlds
Evidence for globalsubsurface oceans on
Europa, Ganymede
and Callisto.
Ganymede and
Callisto seem too
inactive to have
many energy sources
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3a. Saturn Ice worlds-Titan?
• Surface too cold for liquid water (but deep underground?)• Liquid ethane/methane on surface•Lots of hydrocarbons
•Relatively stable, energy sources could be a problem.
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3b. Saturn Ice worlds-Enceladus?
• Ice fountains on Saturn's
moon Enceladus suggest that
it might have liquid waterbelow the surface
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What have we learned?
• Identifying Life – The most common and longest life forms are single-
celled. They arose at least 3.85 billion years ago on Earth
and occur in a variety of environments that complex lifecannot endure.
• What are the necessities of life? – Nutrients, energy, and liquid water
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What have we learned?
• Could there be life In the Solar System? – Evidence for present or past liquid water occur on Mars,
– Any life on Mars would be simple and small though
– Ice moons are cold, but several show signs of sub-surfacewater and other liquids, plus they have hydrocarbons for
building-blocks and nutrients.
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Looking for Life Around Other Stars
Our goals for learning
• Are habitable planets likely?
• Hunting for Planets
• Are Earth-like planets rare or common?
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Are habitable planets likely?
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Habitable Planets
Definition:
A habitable world contains the basic
necessities for life as we know it, including
liquid water.
• It does not necessarily have life.
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Possible Requirements for Habitability(based on Earth)
1. Proper distance from its star
2. Volcanism – to make and maintain an
atmosphere and oceans.
3. Plate tectonics – for climate regulation.
4. Planetary magnetic field to protect the atmosphere.
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The more massive the star, the
larger the habitable zone — higher
probability of a planet in this zone.
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Finding them will be hard
• Looking for an Earth-like planet around a
nearby star is like standing on the East
Coast of the United States and looking for a pinhead on the West Coast — with a VERY
bright grapefruit nearby.
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Planet Finding IS Possible
• A variety of techniques exist to detect planets
• Most result in the preferential detection of
high-mass planets.
• Direct measuring techniques can detect Earth-
sized planets.
• Number of planets is rising into the 1000s
– A number are in the habitable zone.
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Is life rare or common?
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Impacts and Habitability
• Some scientists argue
that Jupiter-like
planets are necessary
to reduce rate ofimpacts
• If so, then Earth-like
planets are restricted
to star systems with
Jupiter-like planets
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Climate and Habitability
• Some scientists argue
that plate tectonics
and/or a large Moon
are necessary to keepthe climate of an
Earth-like planet
stable enough for life
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The Bottom Line
We don’t yet know how important or
negligible these concerns are.
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What have we learned?
• Are habitable planets likely? – Billions stars have sizable habitable zones, but
we don’t yet know how many have terrestrial planets in those zones
• Is life rare or common? – We don’t yet know because we are still trying to
understand all the factors that make Earthsuitable for life
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The Search for Extraterrestrial
IntelligenceOur goals for learning
• How many civilizations are out there?
• How does SETI work?
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How many civilizations are out
there?
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The Drake Equation
Number of civilizations with whom we could potentially
communicate
= N HP ! f
life ! f
civ ! f
now
N HP = total # of habitable planets in galaxy
f life = fraction of habitable planets with life
f civ = fraction of life-bearing planets w/ civilization at some time
f now = fraction of civilizations around now.
We have no idea about the values of any of these.
H d SETI k?
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SETI experiments look for deliberate signals from E.T.
How does SETI work?
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Your computer can help! SETI @ Home: a screensaver with a
purpose.
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What have we learned?
• How many civilizations are out there? – We don’t know, but the Drake equation gives
us a framework for thinking about thequestion
• How does SETI work? – Some telescopes are looking for deliberate
communications from other worlds
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Interstellar Travel and Its Implications
to CivilizationOur goals for learning
• How difficult is interstellar travel?
• Where are the aliens?
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How difficult is interstellar
travel?
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Current Spacecraft
• Current spacecraft travel at <1/10,000 c;
100,000 years to the nearest stars.
Pioneer plaque Voyager record
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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
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Where are the aliens?
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Fermi’s Paradox
• Plausible arguments suggest that civilizationsshould be common, for example:
• Even if only 1 in 1 million stars gets a civilizationat some time " 100,000 civilizations
• So why we haven’t we detected them?
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Possible solutions to the paradox1) We are alone: life/civilizations much rarer than
we might have guessed.• Our own planet/civilization looks all the more
precious…
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1) Civilizations are common but interstellar travelis not. Perhaps because:
! Interstellar travel more difficult than we think.
! Desire to explore is rare.
! Civilizations destroy themselves before achieving
interstellar travel
These are all possibilities, but not very appealing…
Possible solutions to the paradox
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1) There IS a galactic civilization… … and some day we’ll meet them…
Possible solutions to the paradox
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What have we learned?
• How difficult is interstellar travel? – Interstellar travel remains well beyond our
current capabilities and poses enormousdiffculties
• Where are the aliens? – Plausible arguments suggest that if interstellar
civilizations are common then at least one ofthem should have colonized the rest of the
galaxy – Are we alone? Has there been no
colonization? Are the colonists hiding?