Meteorites_ Asteroids_ and Comets - HRSBSTAFF Home Page by xiaopangnv

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									      Chapter 19:

Meteorites, Asteroids,
    and Comets
• 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
  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.
                 Meteor Showers
   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    1866 I Temp

Geminids     Dec. 10-13   7h28m         32o
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.
                 Meteorites

 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).
Analysis of Meteorites
                3 broad categories:
               • Iron meteorites
               • Stony meteorites
               • Stony-iron meteorites
• 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.
• 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
    Earth’s lavas.
           The Origins of Meteorites
• 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
     Asteroids
     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!
Evidence for Collisions
            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.
• Not all asteroids are in the asteroid belt.
• A few thousand asteroids larger than 1 km
  cross Earth’s orbit.
  – Near Earth Objects (NEOs)
  – Searches are underway to find these NEOs.
     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 Jupiter’s 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. That’s why type C asteroids are
     in the outer belt and type S are in the inner belt.
Comets




    Comet C/2001 Q4
 Throughout history, comets have been considered
   as portents of doom, even until very recently:

  Appearances of comet Kohoutek (1973), Halley
(1986), and Hale-Bopp (1997) caused great concern
               among superstitious.


       Comet Hyakutake in 1996
Comet Hale-
Bopp in 1997
Comet NcNaught (2007) was visible in
the southern sky. It will never return.
When a comet is far from the sun, it’s 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.
Two Types of Tails
    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
       sun’s radiation
      pressure. Lagging
  behind the comet along
         its trajectory
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.
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.
• 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?
 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.
            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
Barringer Crater: ~ 1.2 km diameter; 200 m deep
• 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
  Earth’s 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.
• 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.
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.
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.
• In 1998, newspaper headlines read “Mile Wide Asteroid to
  Hit Earth in October 2028.”
• Rumors of Earth’s 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

								
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