comets … and the possibility of extraterrestrial life
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Comets … and the possibility of extraterrestrial life. Announcements. 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! - PowerPoint PPT PresentationTRANSCRIPT
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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
Comets … and the possibility of
extraterrestrial life
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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
Announcements
• 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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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
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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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
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
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Nucleus of Comet Halley as seen by the Giotto spacecraft
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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
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?
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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
Coma density and Jets
• The comet’s coma is actually very tenuous– visible because it’s 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 don’t fully understand what causes jets– Increased sublimation?– Rocket effect?
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PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07
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 Earth’s 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 Earth’s primitive atmosphere
• Similar processes may occur on other worlds
• 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 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