Comets Extraterrestrial Life

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Comets and the possibility of extraterrestrial
life
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Comets Basic Structure

• Dust tail
• Away from the Sun, but curved slightly
• Usually white
• Scattered light
• Ion tail
• Directed away from the Sun
• Blueish color
• Charged water and carbon monoxide molecules
  picked up by the solar wind

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Comet Hale-Bopp
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Comets More on the Basic Structure

• Coma
• The inner fuzzy region
• Million km across
• Nucleus
• The actual surface (possibly an icy dirtball)
• A few km across

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Nucleus of Comet Halley as seen by the Giotto
spacecraft
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Structure and Composition

• Solid object
• Not a sandbank
• Originally thought to be a dirty ice ball
• Observations of Tempel 1 show surprisingly small
  amounts of water ice (in blue in the picture)
• Icy dirt ball?

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Coma density and Jets

• The comets coma is actually very tenuous
• visible because its very deep (i.e. it exists
  over a large, extended region)
• Coma particles about the size of smoke particles
• Jets have higher density, but would still appear
  transparent without overexposure
• Still dont fully understand what causes jets
• Increased sublimation?
• Rocket effect?

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Deep Impact

• On July 4, 2005, a space probe was intentionally
  crashed into comet Tempel 1.
• Impactor was about the size of a normal coffee
  table
• The impact was monitored from a mother ship
  nearby
• Flyby craft is about the size of a Volkswagon
• The resulting impact could be seen by Hubble
  Space Telescope
• Gave us exceptional closeup views of a comet
• Fine powdery dust, like talcum powder
• This was a big surprise

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Comets eventually break apart, and their
fragments give rise to meteor showers
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Meteor Showers

• Due to Earths passage through the remains of an
  extinct comet
• Named after the constellation they appear to come
  from (which depends on the time of year)
• For example, the Leonids and Perseids
• Sand-grain sized objects

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Sun Grazers

• Sun grazers are comets that pass extremely close
  to the Sun at perihelion
• 70 times closer to the Sun than Mercury

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The Search for Extraterrestrial Life

• What is the possibility of life elsewhere?
• What did the Viking landers find on mars?
• What is the story of ALH84001?
• Have we been visited by aliens in the past?

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Are we Alone ?

• No spacecraft have found evidence for life
  elsewhere in the solar system
• Nor has any evidence of life been found in
  meteorites
• There have been no signals detected from alien
  civilizations
• UFOs ?

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The chemical building blocks of life are
foundthroughout space

• All life on Earth, and presumably on other
  worlds, depends on organic (carbon-based)
  molecules
• These molecules occur naturally throughout
  interstellar space
• The organic molecules needed for life to
  originate were probably brought to the young
  Earth by comets or meteorites

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• Another likely source for organic molecules is
  chemical reactions in the Earths primitive
  atmosphere
• Similar processes may occur on other worlds
• In the 1950s Stanley Miller and Harold Urey
  performed a famous experiment (Miller-Urey
  experiment)
• showed that organic compounds (combinations of C
  and H) and amino acids (building blocks of
  proteins) could be created from a simulated
  Earths early atmosphere and lightning

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Is There Life Elsewhere ?

• What about our own solar system ?
• Mars ?
• Europa or Enceladus ?
• There are billions and billions and billions (and
  billions and billions) of stars
• Surely there is life out there
• Or is there ?
• Technologically advanced ?

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The Drake equation
Proposed by astronomer Frank Drake An estimate
of the number of advanced civilizations capable
of sending us signals Based on the
multiplication of probabilities
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Put in the numbers here

• Textbook s
• R 1/year (star formation)
• fp 1 (fraction that have planets)
• ne 0.1 (fraction of Earth-like planets in the
  system)
• fl 1 (fraction on which life actually evolves)
• fi 1 (fraction of those that evolve into
  intelligent beings)
• fc 1 (fraction of those that develop the right
  technology to send
• us signals)
• L 100 years (lifetime of advanced civilization)
• N 1/year x 1 x 0.1 x 1 x 1 x 1 x 100 years 10

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The Drake Equation

• The actual numbers that go in the Drake equation
  are poorly known
• Beyond the first two (the rate of star formation,
  and the fraction that have planets), the rest are
  not known at all no observational constraints !
• The textbook gives a conservative estimate of 10
  technologically advanced civilizations
• Could be just one (us only) or many

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Have we Been Visited by Aliens ?
Check out http//www.ufoevidence.org/photographs/
photohome.asp