Transcript PPT

Comets …
and the possibility of
extraterrestrial life
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
Announcements
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Reading Assignment – none!
Final activity today
course evaluations today
Last Lecture – Tuesday
Final exam – Thursday, May 10 11AM-1PM
– Brief review on Tuesday
• Pick up past assignments by the end of next week!
• Office Hours
– Jade: not in next week
– John: as usual
– Me: as usual
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Comets / Extraterrestrial Life
4/26/07
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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• The comet’s coma is actually
very tenuous
– visible because it’s very deep
(i.e. it exists over a “large,
extended region)
Coma density and
Jets
• Coma particles about the size of
smoke particles
• Jets have higher density, but
would still appear transparent
without overexposure
• Still don’t fully understand what
causes jets
– Increased sublimation?
– Rocket effect?
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• 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
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– This was a big surprise
Deep Impact
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Comets eventually break apart, and their
fragments give rise to meteor showers
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• Due to Earth’s passage
through the remains of an
extinct comet
Meteor Showers
• 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
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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 found
throughout space
• All life on Earth, and
presumably on other worlds,
depends on organic (carbonbased) 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 Earth’s primitive atmosphere
• Similar processes may occur on
other worlds
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• In the 1950’s 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” Earth’s early
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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
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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 actual numbers that go in
the Drake equation are poorly
known
The Drake Equation
• 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
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