ast1001.ch9
TRANSCRIPT
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Chapter 9Asteroids, Comets, and Dwarf Planets
Their Nature, Orbits, and Impacts
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Why is there an asteroid belt?
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Asteroids are cratered and not round.
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Asteroids with Moons• Some large asteroids
have their own moon.
• Asteroid Ida has a tiny moon named Dactyl.
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Asteroid Orbits• Most asteroids orbit
in a belt between Mars and Jupiter.
• Trojan asteroids follow Jupiter’s orbit.
• Orbits of near-Earth asteroids cross Earth’s orbit.
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Orbital Resonances• Asteroids in orbital
resonance with Jupiter experience periodic nudges.
• Eventually those nudges move asteroids out of resonant orbits, leaving gaps in the belt.
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Origin of Asteroid Belt• Rocky planetesimals
between Mars and Jupiter did not accrete into a planet.
• Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.
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How are meteorites related to asteroids?
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Origin of Meteorites
• Most meteorites are pieces of asteroids.
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Meteor Terminology
• Meteorite: A rock from space that falls through Earth’s atmosphere.
• Meteor: The bright trail left by a meteorite.
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Meteorite Types
1. Primitive: Unchanged in composition since they first formed 4.6 billion years ago
2. Processed: Younger, have experienced processes like volcanism or differentiation
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Primitive Meteorites
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Processed Meteorites
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Meteorites from Moon and Mars
• A few meteorites arrive from the Moon and Mars
• Composition differs from the asteroid fragments
• A cheap (but slow) way to acquire moon rocks and Mars rocks
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How do comets get their tails?
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Nucleus of Comet
• A “dirty snowball”
• Source of material for comet’s tail
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Anatomy of a Comet• Coma is atmosphere
that comes from heated nucleus.
• Plasma tail is gas escaping from coma, pushed by solar wind.
• Dust tail is pushed by photons.
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Growth of Tail
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Deep Impact• Mission to study
nucleus of Comet Tempel 1
• Projectile hit surface on July 4, 2005
• Many telescopes studied aftermath of impact
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Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.
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Meteors in a shower appear to emanate from the same area of sky because of Earth’s motion through space.
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Where do comets come from?
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Kuiper belt:On orderly orbits from 30–100 AU in disk of solar system
Oort cloud:On random orbits extending to about 50,000 AU
Only a tiny number of comets enter the inner solar system; most stay far from the Sun.
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How big can a comet be?
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Pluto’s Orbit
• Pluto’s orbit is tilted and significantly elliptical.• Neptune orbits three times during the time Pluto orbits
twice — resonance prevents a collision.
Orbits of Neptune and Pluto
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Discovering Large Iceballs
• In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto.
• Eris even has a moon: Dysnomia.
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Other Icy Bodies• There are many icy
objects like Pluto on elliptical, inclined orbits beyond Neptune.
• The largest ones are comparable in size to Earth’s Moon.
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Kuiper Belt Objects• These large, icy
objects have orbits similar to the smaller objects in the Kuiper Belt that become short period comets.
• So are they very large comets or very small planets?
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What are Pluto and other large objects of the Kuiper belt like?
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HST’s view of Pluto and moons
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Is Pluto a Planet?
• Much smaller than the eight major planets• Not a gas giant like the outer planets• Has an icy composition like a comet• Has a very elliptical, inclined orbit• Pluto has more in common with comets than with the
eight major planets.
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Is Pluto a Planet?
• In 2006, the International Astronomical Union decided to call Pluto and objects like it “dwarf planets.”
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Have we ever witnessed a major impact?
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Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen.
Tidal forces tore it apart during a previous encounter with Jupiter.
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This crater chain on Callisto probably came from another comet that tidal forces tore to pieces.
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Impact plume from a fragment of comet SL9 rises high above Jupiter’s surface
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Dusty debris at an impact site
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Artist’s conception of SL9 impact
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Several impact sites
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Impact sites in infrared light
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Did an impact kill the dinosaurs?
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Mass Extinctions
• Fossil record shows occasional large dips in the diversity of species: mass extinctions.
• The most recent was 65 million years ago, ending the reign of the dinosaurs.
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Iridium: Evidence of an Impact
• Iridium is very rare in Earth surface rocks but is often found in meteorites.
• Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact.
• Dinosaur fossils all lie below this layer.
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Iridium Layer
Dinosaur fossils in lower rock layers
No dinosaur fossils in upper rock layers
Thin layer containing the rare element iridium
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Consequences of an Impact
• A meteorite 10 km in size would send large amounts of debris into the atmosphere.
• Debris would reduce the amount of sunlight reaching Earth’s surface.
• The resulting climate change may have caused mass extinction.
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Likely Impact Site• Geologists found a
large subsurface crater about 65 million years old in Mexico.
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Comet or asteroid about 10 km in diameter approaches Earth
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Is the impact threat a real danger or just media hype?
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Facts About Impacts
• Asteroids and comets have hit the Earth.• A major impact is only a matter of time: not IF but
WHEN.• Major impacts are very rare.• Extinction level events ~ millions of years• Major damage ~ tens to hundreds of years
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Tunguska, Siberia: June 30, 1908 A ~40 meter object disintegrated and exploded in the atmosphere
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Meteor Crater, Arizona: 50,000 years ago (50 meter object)
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Frequency of Impacts• Small impacts
happen almost daily.
• Impacts large enough to cause mass extinctions are many millions of years apart.
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The Asteroid with Our Name on It
• We haven’t seen it yet.• Deflection is more probable with years of
advance warning.• Control is critical: breaking a big asteroid
into a bunch of little asteroids is unlikely to help.
• We get less advance warning of a killer comet.
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What are we doing about it?
• Stay tuned to http://impact.arc.nasa.gov
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How do other planets affect impact rates and life on Earth?
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Influence of Jovian Planets
The gravity of a jovian planet (especially Jupiter) can redirect a comet.
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Influence of Jovian Planets
Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud.
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Was Jupiter necessary for life on Earth?
Impacts can extinguish life.
But were they necessary for “life as we know it”?