chapter 8 looking for life beyond earth. organic molecules in the universe complex molecules and...
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Chapter 8Looking for LifeBeyond Earth
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Organic Molecules in the Universe
• Complex molecules and carbon may be the basis for life.
• Atoms that can bond to only two other atoms, like the atoms denoted X shown here, can form a chain of atoms called a linear molecule.
• The chain stops where we introduce an atom, such as those labeled Y and Z, that can bond to only one other atom.
• A carbon atom (denoted C) can bond with up to four other atoms and can form more complex, nonlinear molecules like glucose.
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Evidence of Carbon from the Early Solar System
• If life is based on organic molecules, then these molecules must be present on a planet’s surface in order for life to arise from nonliving matter.
• Evidence for this comes from ancient meteorites that date from the formation of the solar system and that are often found to contain a variety of carbon-based molecules.
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The Miller-Urey Experiment• Demonstrated that simple chemicals can combine
to form the chemical building blocks of life. • In a closed container, they prepared a sample of
the most common molecules in the solar system: – hydrogen (H2)
– ammonia (NH3)
– methane (CH4)
– water vapor (H2O)
They then exposed this mixture of gases to an electric arc (to simulate lightning) for a week.
• The inside of the container had become coated with a reddish-brown substance rich in amino acids and other compounds essential to life.
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Finding Liquid Water Is Essential
Water has been found on both Europa and the Moon.
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Searching for Life on Mars
• In 1976 Viking 1 and Viking 2 were sent to Mars to search for water and signs of life.
• Results were inconclusive.
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New Evidence for Martian Water
• Visible-light images show the south polar cap of Mars, but do not indicate its chemical composition.
• By using a camera tuned to different wavelengths of infrared light, the Mars Express spacecraft was able to identify the distinctive reflections of an upper layer of carbon dioxide ice and a deeper layer of water ice.
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New Evidence for Martian Water
• In this formation, some layers are made of dust deposited by the Martian winds; others were laid down by minerals that precipitated out of standing water.
• The bluish color shows the presence of millimeter-sized spheres of gray hematite, which forms in water-soaked deposits. (False-Color)
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Meteorites from Mars
• 12+ meteorites formed on Mars have managed to make their way to Earth.
• These SNC meteorites are named after the first three examples found (Shergotty, Nakhla, and Chassigny).
• SNC meteorites are identified by the chemical composition of trace amounts of gas trapped within them.
• This composition is very different from that of the Earth’s atmosphere, but is a nearly perfect match to the composition of the Martian atmosphere found by the Viking Landers.
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The Drake Equation
The number of technologically advanced civilizations in the galaxy may be estimated by a single mathematical sentence.
N = number of technologically advanced civilizations in the galaxy whose messages we might be able to detect
R* = the rate at which solar-type stars form in the galaxyfp = the fraction of stars that have planetsne = the number of planets per solar system that are Earthlike (i.e., suitable for life)fl = the fraction of those Earthlike planets on which life actually arisesfi = the fraction of those life-forms that evolve into intelligent speciesfc = the fraction of those species that develop adequate technology and then
choose to send messages out into spaceL = the lifetime of a technologically advanced civilization
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Searching for Planets Using “Wobbles”
• A planet and its star both orbit around their common center of mass, always staying on opposite sides of this point.
• Even if the planet cannot be seen, its presence can be inferred if the star’s motion can be detected.
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Searching Using Images and Spectra
• A Sun-like star, and orbiting planets• The simulated infrared spectrum of one of the planets,
showing absorption lines of water vapor, ozone, and carbon dioxide
• The infrared spectrum of such planets will make it possible to identify worlds on which life may have evolved.
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Radio Searches for Alien
Civilizations
• The Water Hole: a range of radio frequencies from about 103 to 104 megahertz (MHz) in which there is little noise and little absorption by the Earth’s atmosphere.
• This noise-free region may be well suited for interstellar communication.