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Chapter 18Life in the Universe
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Life on Earth
• What is “Life?”
• Life on Earth – When did life arise?
– How did life arise?
• Life in our Solar System?
• Is there Intelligent life in Space – that we can find?
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What is Life?
• Atoms, Molecules – no
• Random linear chains of Molecules – no
• Random non-linear chains – no!
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What is Life?
Non-linear chains of silicon & oxygen => inert gels or liquids OR
=> inert rigid rocks
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What is Life?
• Complex non-linear chains of carbon – no, but getting closer!
• Complex non-linear chains of carbon, hydrogen, nitrogen, & oxygen – no, but getting closer still!
Lysine – an amino acid
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Common to all life on Earth?
• Chemically interacts with environment
– Takes in nutrients selectively
– Expels waste products chemically
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Common to all life on Earth?
• Transforms food into energy for metabolic processes
– Add cream of wheat to a rock?
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Common to all life on Earth?
• Transforms food into energy for metabolic processes
– Add cream of wheat to a rock?
=> No transformation!
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Common to all life on Earth?
• Transforms food into energy for metabolic processes
– Add cream of wheat to a person?
– Transformation!
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Common to all life on Earth?
• Reproduction!
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Common to all life on Earth?
• DNA!
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What is Life?
Sequence of nucleo base-pairs tied together with
sugar & phosphates?
YES!!
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Necessities for Life
• Nutrient source
• Energy (sunlight, chemical reactions, internal heat)
• Liquid water (or possibly some other liquid)
Hardest to find on other planets
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Earliest Life Forms
• Life probably arose on Earth more than 3.85 billion years ago, shortly after the end of heavy bombardment.
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Earliest Life Forms
• Life probably arose on Earth more than 3.85 billion years ago, shortly after the end of heavy bombardment.
• Evidence comes from fossils and carbon isotopes.
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Fossils in Sedimentary Rock
• Rock layers of the Grand Canyon record more than 500 million years of Earth’s history.
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Earliest Fossils• The oldest fossils show
that bacteria-like organisms were present over 3.5 billion years ago.
• Carbon isotope evidence pushes the origin of life to more than 3.85 billion years ago.
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How did life arise on Earth?
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When did life arise on Earth?
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Origin of Life on Earth
• Life evolves through time.
• All life on Earth shares a common ancestry.
• We may never know exactly how the first organism arose, but laboratory experiments suggest plausible scenarios.
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The Theory of Evolution• Fossil record shows evolution
occurred through time.
• Darwin’s theory tells HOW evolution occurs: through natural selection.
• Theory supported by discovery of DNA: evolution proceeds through mutations.
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Tree of Life
• Plants and animals are a small part of the tree.
• Suggests likely characteristics of common ancestor.
• Mapping genetic relationships has led biologists to discover this new “tree of life”.
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• Genetic studies suggest earliest life on Earth may have resembled bacteria
• Today found near deep ocean volcanic vents (black smokers)
• Geothermal hot springs.
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Laboratory Experiments
• Miller–Urey experiment show building blocks of life form easily and spontaneously under conditions of early Earth.
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Microscopic, enclosed membranes or “pre-cells” have been created in the lab.
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Origin of Oxygen
• Cyanobacteria paved the way for more complicated life-forms
• Release oxygen into atmosphere via photosynthesis.
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Could life have migrated to Earth?
• Venus, Earth, and Mars have “exchanged” tons of rock (blasted into orbit by impacts).
• Meteorites & Comets are known to carry organic materials.
• Some microbes can survive years in space.
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Could there be life on Mars?
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Searches for Life on Mars
• Mars had liquid water in the distant past.• Mars still has subsurface ice—possibly
subsurface water near sources of volcanic heat.
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In 2004, NASA’s Spirit and Opportunity rovers sent home new mineral evidence of past liquid water on Mars.
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In 2012 and 2013, NASA’s Mars Science Lab explorer confirmed mineral evidence of past liquid water!
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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
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Does the meteorite contain actual fossil evidence of life on Mars?
Most scientists are not yet convinced.
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Could there be life on Europa or other jovian moons?
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• Ganymede, Callisto also show some evidence for subsurface oceans
• Relatively little energy available for life, but still… • Intriguing prospect of THREE potential homes for life
around Jupiter alone
Ganymede Callisto
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Saturn’s Moon Titan
• Surface too cold for liquid water (but deep underground?)
• Liquid ethane/methane on surface
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Saturn’s Moon Enceladus
Ice fountains suggest that it might have
liquid water below the surface
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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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How many are possible?
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How many are possible?
If each star was a grain of sand…
• There are more stars in our galaxy than grains of sand on a beach…
• AND, as many galaxies in the universe as grains of sand on a beach…
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But not every star is viable!
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 where a planet of the right size could support liquid water
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Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.
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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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Spectral Signatures of Life
Earth
Venus
Mars
oxygen/ozone
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Are Earth-like planets rare or common?
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Elements and Habitability
• Some argue that proportions of heavy elements need to be just right for formation of habitable planets.
• If so, Earth-like planets are restricted to a habitable zone in the galaxy…
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Impacts and Habitability
• Some argue that Jupiter-like planets are necessary to reduce the rate of impacts.
• If so, then Earth-like planets are restricted to star systems with Jupiter-like planets.
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Climate and Habitability
• Some argue that plate tectonics and/or a large moon are necessary to keep the 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 of stars have sizable habitable
zones, but we don’t yet know how many have terrestrial planets in those zones.
• Are Earth-like planets rare or common?— We don’t yet know because we are still
trying to understand all the factors that make Earth suitable for life.
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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
= NHP flife fciv fnow
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The Drake Equation
Number of civilizations with whom we could potentially communicate
= NHP flife fciv fnow
NHP = total number of habitable planets in galaxy
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The Drake Equation
Number of civilizations with whom we could potentially communicate
= NHP flife fciv fnow
NHP = total number of habitable planets in galaxy
flife = fraction of habitable planets with life
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The Drake Equation
Number of civilizations with whom we could potentially communicate
= NHP flife fciv fnow
NHP = total number of habitable planets in galaxy
flife = fraction of habitable planets with life
fciv = fraction of life-bearing planets with civilization at some time
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The Drake Equation
Number of civilizations with whom we could potentially communicate
= NHP flife fciv fnow
NHP = total number of habitable planets in galaxy
flife = fraction of habitable planets with life
fciv = fraction of life-bearing planets with civilization at some time
fnow = fraction of civilizations around now
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Drake Equation: Number of communicating civilizations in the
Galaxy = NHP flife fciv fnow
We do not know the following values:
NHP : probably billions
flife : ??? Hard to say (near 0 or near 1)
fciv : ??? It took 4 billion years on Earth
fnow : ??? Can civilizations survive long-term?
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Are we “off-the-chart” smart?
• Humans have comparatively large brains.
• Does that mean our level of intelligence is improbably high?
Insert ECP6 Figure 18.18
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How does SETI work?
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SETI experiments look for deliberate signals from E.T.
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We’ve even sent a few signals ourselves…
Earth to globular cluster M13: Hoping we’ll hearback in about 42,000 years!
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Your computer can help! SETI @ Home: a screensaver with a purpose
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How difficult is interstellar travel?
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Current Spacecraft• Current spacecraft travel at <1/10,000c;
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.• There are social complications of time dilation.
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Where are the aliens?
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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 haven’t we detected them?
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Possible solutions to the paradox
1. 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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2. Civilizations are common, but interstellar travel is not, perhaps because:• interstellar travel is more difficult than we think.
• the desire to explore is rare.
• civilizations destroy themselves before achieving interstellar travel.
These are all possibilities, but they are not very appealing.
Possible solutions to the paradox
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3. There IS a galactic civilization…
… and someday we’ll meet them.
Possible solutions to the paradox