Life in the Universe and Cosmic Catastrophes
Review for Astronomy 123 Midterm Exam 4

March 8, 2011, jeb

Outline



• Life in the Universe
• Cosmic Catastrophes

Life in the Universe

Our understanding of life in the universe requires we understand how life evolved in the one place we know for sure it did - on the Earth

Life on Earth emerged as the end of an evolutionary sequence:

Life appeared on Earth about 3.5 billion years ago

Life on Earth is carbon based

• based on complex (large) molecules (amino acids and nucleotide bases)
• early Earth provided energy sources which combined basic elements into complex (large) molecules
  • basic elements
    • ammonia (NH₃)
    • methane (CH₄)
    • carbon dioxide (CO₂ )
    • water (H₂O)
  • energy sources
    • natural radioactivity
    • lightning
    • volcanism
    • solar ultraviolet radiation
    • meteoritic impacts

Liquid water essential



Life Elsewhere in the Solar System?

Metereorites often contain organic molecules

• Murchison Meteorite

Three large bodies in the solar system besides the Earth have possiblities:
 

• Titan, Saturn's largest moon
• atmosphere of methane, ammonia and nitrogen
• Cassini-Huygens probe into the Titan atmosphere indicates inhospitable conditions for Earth-like life
• Europa, Jupiter's moon
• liquid water below the icy surface
• Mars
• explored by Viking -> no life
• meterorites from Mars reach Earth

So far, life has only been observed on Earth

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Origin of Life - Can It Happen Spontaneously?

Life - as we know it - is based on complex molecules built as carbon chains
 
Can these molecules be created in the lab?
• 1953 - Harold Urey and Stanley Miller
• Mixed H₂O (water), CH₄ (methane), NH₃ (ammonia), CO₂ (carbon dioxide), and applied sparks
• They found amino acids were formed - the basic building blocks of life
• More recent experiments have produced amino acid, complex sugars, nucleotide basse, and protein-like microspheres
• no DNA
• Still no life
• This step remains a mystery
• But research continues

Possible Alternate Origin of Life on Earth

Not from the primordial soup on Earth, but from organic material produced in interstellar space that came to Earth as interplanetary dust and with meteors

• Was first organic material produced in interstellar space, coming to Earth as extraterrestrial visitor?
• NASA's modern version of Urey-Miller experiment, using ultraviolet radiation to simulate energy from newborn stars
  • Found oily, hollow droplets rich in organic molecules
• Comets do carry large amounts of organic material
• Small fraction of meteorites found on surface of Earth contain organic compounds
  • Murchison Meteorite contained large amounts of amino acids and other organic material

The extraterrestrial origin is a possibility
 

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Drake Equation

1961 - A small group of astrophysicists (including Carl Sagan) gathered at the National Radio Astronomy Observatory, Green Bank, WV
 
• Frank Drake presented his equation
 
• to estimate the number of technological civilizations that presently exist in the Milky Way
 

N = R_*f_pn_ef_ef_if_cL



• R_* = rate of star formation in the Milky Way
• f_p = fraction of stars with planets
• n_e = # of habitable planets/star
  planet in habitable zone;
  system in galactic habitable zone;
  cannot include binary star systems
• f_e = fraction with life evolved
• f_i = fraction where life develops intelligence
• f_c = fraction which choose to communicate and develop advanced technology
• L = lifetime of such civilizations


(Note: This original form is a little different from the textbook)
• The Drake Equation is an excellent method for us to fomulate our thinking,
• but it does not lead to a well defined answer, because there are many uncertainties in the factors

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UFOs (Unidentified Flying Objects)

Occam's Razor 
• Fundamental principle in science - "the simplest explanation that satisfies all the evidence is the best". 

Occam's Razor leads to the conclusion that reported UFO's are not good evidence for extraterrestrial life
• Many sightings are explained by optical illusions
• Many sightings turn out to be fraud
• No sign that extraterrestrials want to contact authorities - like the U.S. President

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The Scientific Search

1. We are looking for intelligent, advanced life since we assume primitive life is too difficult to detect
• Analysis of difficulty for extraterrestrials to directly observe life on Earth shows how difficult this is
• like "biological rust" of Earth


2. We assume that intelligent life wants to communicate
• Communication will take time
  • Round trip for radio signal over 50 light years takes 100 years
  • Round trip for radio signal over 100 light years takes 200 years
  • Round trip for radio signal over 10,000 light years takes 20,000 years


3. Two avenues for discovery
1. We discover them
SETI - search for extraterrestrial intelligence
2. They discover us
CETI - communication with extraterrestrial intelligence

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We Discover Them (SETI)

SETI - search for extraterrestrial intelligence

What should we look for?

• Popular guess among scientists

- 21 cm spin-flip line of hydrogen
    (1420 MHz)
hydrogen is the simplest atom
and 21 cm is a "quiet band" known as the water hole

Searches at 21 cm (1420 MHz) have failed so far

Late 1990's - Project Phoenix used sophisticated computer controls to investigate millions of channels simultaneously

• analyzed broad band of frequencies looking for patterns which could indicate signs of intelligent life
• Still no sign of life

They Discover Us (CETI)

CETI - communication with extraterrestrial intelligence

We have been beaming radio signals to outerspace now for more than 70 years

1974 - Arecibo - Puerto Rico

message from people on earth beamed at globular cluster M13 (many stars) - distance 24,000 light years
1679 bits of information (0000001010101000000000000101......
only factors of 1679 are 23 and 73 (1679 = 23 x 73)

One motivation for this is to learn from an advanced technological civilization


Placed messages aboard space vehicles

• Pioneer
• Voyager

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Carbon Chauvinism

Usual assumption of SETI is that life is based on carbon chemistry, like on earth

Why Not Silicon?

1. We don't have an existence proof - like for carbon
2. Life on earth is based on molecules containing millions of atoms (DNA)
Silicon
• short molecules at room temperature
• and we think life would not be able to function at colder temperatures

So silicon does not look like as likely a basis for life as carbon

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Search for Extrasolar Planets

One element of the Drake equation is the existence of planets around stars

We can detect the wobble of a star caused by the pull of orbiting planets

• This search is biased toward large planets orbiting close to a star

Also look for reduced star light due to planetary eclipse

Many stars with at least one planet have been identified

Some stars with detectable multiple planets have been identified

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Cosmic Catastrophes

• Impact
• Supernova
• Gamma-ray Burst
• Ice Ages

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Impact

• Fossil record reveals history of life on Earth
  • several spectacular sudden periods of extinction are found
  • most species that ever lived are now extinct
  • dinosaurs suddenly vanished 65,000,000 years ago after 160,000,000 years of dominance
  
  
• Could a cosmic collision play a role in these extinctions?
  • the impact of a comet or asteroid likely played a role in the demise of the dinosaurs
    • evidence for this is the deposits of iridium all over the Earth at the K-T boundary (same depth as the fossil record shows dinosaurs vanish)
    • estimate size of comet or asteroid was 10-15 kilometers in diameter (about the size of one of the moons of Mars)
    • Chicxulub on the Yucatan Peninsula is likely the crater
    
    
  • Craters all over the Earth record past impacts
    • Major impacts with potential for global catastrophes occur every ten to hundred thousand (10,000-100,000) years
    • Manicouagan Reservoir in Quebec - 70 km in diameter
    • Barringer Crater (1.2 km wide) - Arizona
      • 25,000 years old
    
    
  • Tunguska event - 1908 in Siberia
    • explosion in atmosphere
  
  
• Jupiter, 1994
  • Comet Shoemaker-Levy crashed into surface of Jupiter
  • "String of Pearls"
  • Huge fireballs (hundreds of km) and surface disturbed over thousands of km
  • Another impact on Jupiter discovered in 2009
    • spot the size of the Pacific Ocean caused by impacter 200-500 meters in size
  
  
• Periodicity
  • Some scientists find evidence for periodicity of mass extinctions in the fossil record
    • 26,000,000 year period
    • coincidentally, this is approximate period of Solar System's passage through the plane of the Milky Way
      • could Oort Cloud be disrupted, increasing rain of comets within the inner Solar System?
    • this is still a controversial topic

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Supernova

• SN1987A was a supernova in the Large Magellanic Cloud
  • first supernova visible from Earth by the naked eye since invention of the telescope
  • extremely bright, and also emitted intense flux of X-rays, gamma-rays, and cosmic rays
  • too distant for radiation to harm life on Earth
  
  
• close-by supernova could be a hazard to life on Earth
  • within 10 light years
  • radiation death would be a risk
    • X-rays, gamma-rays, cosmic rays
  • atmosphere would be altered for extended period
  • frequency of such occurrences on Earth are about once per few billion years

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Gamma-ray Burst

• Most violent events in the universe
• uniformly distributed in the sky
  • therefore at very great distances (billions of parsecs)
• very small, determined by short duration
• two leading models for origin
  • merger of a binary star system (compact stars)
  • hypernova
  • both are probably responsible for different events
• Fermi Gamma-ray Space Telescope is now studying GRBs

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Ice Ages

• Ice on polar caps periodically extends far into the temperate zone on Earth
• last such period ended about 10,000 years ago
• Earth's wobble and orbital shape affects the distribution of sunlight on Earth
  • Eccentricity of Earth's orbit
  • Tilt of Earth's rotation axis
  • Precession of the equinoxes