ptys/astr 206comets / extraterrestrial life 4/26/07 comets … and the possibility of...

PTYS/ASTR 206 Comets / Extraterrestrial Life4/26/07

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!

• Office Hours

– Jade: not in next week

– John: as usual

– Me: as usual


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


Comet Hale-Bopp


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


Nucleus of Comet Halley as seen by the Giotto spacecraft


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?


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?


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


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


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


Sun Grazers

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

Sun than Mercury


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?


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 ?


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


• 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


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 ?


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


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


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


Have we Been Visited by Aliens ?

Check out: http://www.ufoevidence.org/photographs/photohome.asp

ptys/astr 206comets / extraterrestrial life 4/26/07 comets … and the possibility of...

Documents

comets extraterrestrial

comets coma

possibility of life

usual slide

mercury slide

big surprise slide

solar wind slide

giotto spacecraft slide