1. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 Comets … and the possibility of extraterrestrial life

2. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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

3. PTYS/ASTR 206Comets / 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

4. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07

5. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 Comet Hale-Bopp

6. PTYS/ASTR 206Comets / 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

7. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 Nucleus of Comet Halley as seen by the Giotto spacecraft

8. PTYS/ASTR 206Comets / 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?

9. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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?

10. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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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12. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 Comets eventually break apart, and their fragments give rise to meteor showers

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14. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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

15. PTYS/ASTR 206Comets / 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

16. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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?

17. PTYS/ASTR 206Comets / 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 ?

18. PTYS/ASTR 206Comets / 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 (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

19. PTYS/ASTR 206Comets / 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 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

20. PTYS/ASTR 206Comets / Extraterrestrial Life 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 ?

21. PTYS/ASTR 206Comets / 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

22. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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

23. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 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

24. PTYS/ASTR 206Comets / Extraterrestrial Life 4/26/07 Have we Been Visited by Aliens ? Check out: http://www.ufoevidence.org/photographs/photohome.asp