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 HaleBopp 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