Meteorites, Asteroids, and Comets

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Meteorites, Asteroids, and Comets
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• Chapter 19

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• Meteoroid fragment of a comet or asteroid in
  space
• Meteor meteoroid colliding with Earth and
  producing a visible light trace in the sky
• Meteorite meteor that survives the plunge
  through the atmosphere to strike the ground

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Comets leave a trail of debris behind them as
they orbit the sun. Meteoroids contributing to a
meteor shower are debris particles, orbiting in
the path of a comet.
A meteor shower occurs when Earth passes through
the orbital path of a comet. The comet may still
exist or have been destroyed.
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Meteor Showers
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Most meteors appear in showers, peaking
periodically at specific dates of the year. All
of the meteors in a given shower have the same
origin.
Shower Date R.A. Dec. Associated
Comet Perseids Aug. 10-14 3h4m
58o 1982 III Leonids Nov. 14-19 10h12m 22o 18
66 I Temp Geminids Dec. 10-13 7h28m 32o
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• Most meteors we see, whether or not there is a
  shower, come from comets. Therefore, they are
  small specks of matter that burn up in the
  atmosphere.

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Meteorites
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Sizes from microscopic dust to a few centimeters.
About 2 meteorites large enough to produce
visible impacts strike the Earth every day.
Statistically, one meteorite is expected to
strike a building somewhere on Earth every 16
months.
Typically impact onto the atmosphere with 10 30
km/s ( 30 times faster than a rifle bullet).
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Analysis of Meteorites
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3 broad categories

• Iron meteorites

• Stony meteorites

• Stony-iron meteorites

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• Iron Meteorites
• Dense and heavy
• Dark rusted surfaces
• When sliced, polished, and etched with nitric
  acid, they reveal Widmanstatten patterns caused
  by crystals of nickel-iron alloys that have grown
  large. This indicates that the meteorite cooled
  slowly.
• Stony-iron meteorites are a mixture of iron and
  stone. They appear to have formed when a mixture
  of molten iron and rock cooled and solidified.

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• Stony Meteorites
• Chondrites
• Contain chondrules (rounded bits of glassy rock
  ranging from microscopic to pea size.)
• They formed from droplets of molten rock that
  cooled and hardened rapidly when the solar system
  was young.
• Their presence indicates that the meteorites have
  not melted since they formed.
• Some chondrites only have a few volatiles
  indicating they were heated slightly, which
  caused them to lose their volatiles, but not
  heated enough to destroy the chondrules.
• Carbonaceous chondrites contain both chondrules
  and volatile compounds including carbon. They
  have not been heated since the formation of the
  solar system.
• Achondrites contain no chondrules and lack
  volatiles. They appear to have been heated.
  They are similar to Earths lavas.

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The Origins of Meteorites
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• Probably formed in the solar nebula, 4.6
  billion years ago.

• Almost certainly not from comets (in contrast to
  meteors in meteor showers!).

• Probably fragments of stony-iron planetesimals

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Asteroids
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Small, irregular objects, mostly in the apparent
gap between the orbits of Mars and Jupiter.
Last remains of planetesimals that built the
planets 4.6 billion years ago!
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Evidence for Collisions
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Hirayama families Groups of asteroids sharing
the same orbits and spectroscopic characteristics
apparently result of common origin through
collisions.
Radar images of asteroids reveal irregular
shapes, sometimes peanut-like shapes
Evidence for low-velocity collisions between
asteroids on very similar orbits.
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• Not all asteroids are in the asteroid belt.
• A few thousand asteroids larger than 1 km cross
  Earths orbit.
• Near Earth Objects (NEOs)
• Searches are underway to find these NEOs.

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The Origin of the Asteroids

• Ray blasts from Death Stars are unlikely to cause
  planets to explode as in Star Wars.
• Besides, the total mass of all the asteroids is
  only 1/20 that of the moon.
• The asteroids probably are not the result of a
  planet exploding.
• Asteroids are probably the remains of a planet
  that did not form at 2.8 Au from the sun due to
  Jupiters gravity.
• Therefore, asteroids are probably fragments of
  left over planetesimals.
• The ones in the outer belt formed where the solar
  nebula was cooler so carbon could condense.
  Thats why type C asteroids are in the outer belt
  and type S are in the inner belt.

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Comets
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Comet C/2001 Q4
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Throughout history, comets have been considered
as portents of doom, even until very recently
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Appearances of comet Kohoutek (1973), Halley
(1986), and Hale-Bopp (1997) caused great concern
among superstitious.
Comet Hyakutake in 1996
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Comet Hale-Bopp in 1997
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Comet NcNaught (2007) was visible in the southern
sky. It will never return.
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• When a comet is far from the sun, its just
  the nucleus. When it gets close enough to the
  sun, it begins to sublime and a coma and tail
  form.
• The coma of a comet is the cloud of gas and dust
  that surrounds the nucleus. It can be over a
  million km in diameter, which is bigger than the
  sun.

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Two Types of Tails
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gas tail Ionized gas pushed away from the comet
by the solar wind. Pointing straight away from
the sun.
Dust tail Dust set free from vaporizing ice in
the comet carried away from the comet by the
suns radiation pressure. Lagging behind the
comet along its trajectory
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• Comet tails point generally away from the sun,
  but their precise direction depends on the flow
  of the solar wind and the orbital motion of the
  nucleus.

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Comet Mrkos in 1957 shows how The gas tail
can change from night to night due to changes in
the magnetic field in the solar wind.
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• Comets cannot last more than 100 to 1000 orbits
  around the sun before all their ice is gone and
  there is nothing left but dust and rock.
• The comets we see today cannot have been orbiting
  close to the sun for 4.6 billion years.
• Where do new comets come from?

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Impacts on Earth

• Small meteorite impacts occur quite often.
• Every few years a building is damaged by a
  meteorite.
• A few years ago, a car was hit by a meteorite and
  then auctioned off for 10,000,000.
• Really large impacts are rare.

In 1954 Mrs. E. Hulitt Hodges of Sylacauga,
Alabama was hit by a meteorite while napping in
her living room. This is the only known person
to have been injured by a meteorite.
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Over 150 impact craters found on Earth.
Famous example Barringer Crater near Flagstaff,
AZ
Formed 50,000 years ago by a meteorite of 80
100 m diameter
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Barringer Crater 1.2 km diameter 200 m deep
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• Sediments from all over the Earth from 65 million
  years ago have an overabundance of iridium, an
  element common in meteorites but rare in the
  Earths crust.
• The impact of a large meteorite at that time may
  have altered the atmosphere and climate on Earth,
  which caused the extinction of the dinosaurs and
  75 of the other species on the planet.

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• The biggest extinction we know of occurred 250
  million years ago The Great Dying.
• 95 of life in the oceans died out.
• 80 of life on land died out.
• Data indicates that a large impact occurred off
  the shore of Australia 250 million years ago.

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The 1908 Tunguska event in Siberia destroyed an
area the size of a large city. Here the area of
destruction is superimposed on a map of
Washington, D.C., and its surrounding beltway. In
the central area, trees were burned in the outer
area, trees were blown down pointing away from
the center of the blast for as far as 30 km.
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The Effects of a Large Impact on Earth

• If on land, the initial shock would be deadly.
• If on sea, there would be tidal waves hundreds of
  meters high that would devastate coastal regions.
• Lots of dust would be thrown into the atmosphere.
• The hot dust falling back to Earth could start
  fires.
• The dust left in the atmosphere would block
  sunlight, making temperatures cooler for a time.

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• In 1998, newspaper headlines read Mile Wide
  Asteroid to Hit Earth in October 2028.
• Rumors of Earths demise were greatly
  exaggerated. The asteroid will miss Earth by
  600,000 miles.
• Now rumor is a 430 mile wide asteroid named
  Apophis will hit in 2029 or 2036.
• Actually Apophis is not 430 miles in diameter but
  more like 250 METERS.
• The future for Apophis on Friday, April 13 of
  2029 includes an approach to Earth no closer than
  29,470 km (18,300 miles, or 5.6 Earth radii from
  the center, or 4.6 Earth-radii from the surface)
  over the mid-Atlantic, appearing to the naked eye
  as a moderately bright point of light moving
  rapidly across the sky.
• Updated computational techniques and newly
  available data indicate the probability of an
  Earth encounter on April 13, 2036, for Apophis
  has dropped from one-in-45,000 to about four-in-a
  million.
• http//www.nasa.gov/home/hqnews/2009/oct/HQ_09-232
  _Apophis_Update.html