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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
PTYS/ASTR 206
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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
Comet Hale-Bopp
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
Nucleus of Comet Halley as seen by the
Giotto spacecraft
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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?
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
• 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?
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
• 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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
– This was a big surprise
Deep Impact
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
Comets eventually break apart, and their
fragments give rise to meteor showers
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
• 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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
Sun Grazers
• Sun grazers are comets
that pass extremely close
to the Sun at perihelion
– 70 times closer to the
Sun than Mercury
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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?
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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 ?
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
• Another likely source for organic
molecules is chemical reactions in
the Earth’s primitive atmosphere
• Similar processes may occur on
other worlds
PTYS/ASTR 206
• 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
Comets / Extraterrestrial Life
atmosphere
and lightning
4/26/07
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 ?
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
• 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
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07
Have we Been Visited by Aliens ?
Check out: http://www.ufoevidence.org/photographs/photohome.asp
PTYS/ASTR 206
Comets / Extraterrestrial Life
4/26/07