Download CHAPTER 1 Meteorites, Asteroids, and Comets

CHAPTER 1
Meteorites, Asteroids,
and Comets
Astronomy: The Solar System and Beyond
5th edition
Michael Seeds
CHAPTER 1
Chapter 10
Meteorites, Asteroids,
and Comets
When they shall cry “PEACE, PEACE”
then cometh sudden destruction!
COMET’S CHAOS?—
What Terrible events will the Comet bring?
- From a religious pamphlet
predicting the end of the world
because of the appearance
of comet Kohoutek, 1973
CHAPTER 1
Meteorites, Asteroids,
and Comets
• Of course, you are not afraid of comets.
• However, not long ago, people viewed
them with terror.
– In 1910, Comet Halley was spectacular.
– On the night of May 19, Earth actually passed through
the tail of the comet—and millions of people
panicked.
CHAPTER 1
Meteorites, Asteroids,
and Comets
– The spectrographic discovery of cyanide gas in the
tails of comets led many to believe that life on Earth
would end.
– Householders in Chicago stuffed rags around doors
and windows to keep out the gas, and bottled oxygen
was sold out.
– Con artists in Texas sold comet pills and inhalers to
ward off the noxious fumes.
– An Oklahoma newspaper reported, in what was
apparently a hoax, that a religious sect tried to
sacrifice a virgin to the comet.
CHAPTER 1
Meteorites, Asteroids,
and Comets
• Throughout history, bright comets have
been seen as portents of doom.
• Even the more recent appearance of
bright comets has generated predictions of
the end of the world.
– Comet Kohoutek in 1973, Comet Halley in 1986, and
Comet Hale-Bopp in 1997 all caused concern among
the superstitious.
CHAPTER 1
Meteorites, Asteroids,
and Comets
• A bright comet moving slowly through the
night sky is such an unusual sight that you
should not be surprised if it generates
some instinctive alarm.
CHAPTER 1
Meteorites, Asteroids,
and Comets
• Comets are not just graceful and beautiful
visitors to our skies but they are also
useful in the study of the solar system.
– Astronomers think of comets as messengers from the
age of planet building.
– By studying comets, you can learn about the
conditions in the solar nebula from which planets
formed.
CHAPTER 1
Meteorites, Asteroids,
and Comets
• However, comets tell only part of the
story.
– They are only the icy remains of the outer solar
nebula.
• The asteroids are the rocky debris left
over from terrestrial planet building.
CHAPTER 1
Meteorites, Asteroids,
and Comets
• However, you cannot easily visit comets
and asteroids.
• Nevertheless, you can learn about them
by discussing the fragments of those
bodies that fall into our atmosphere—the
meteorites.
CHAPTER 1
Meteorites
Meteorites, Asteroids,
and Comets
• You learned about meteorites when you
studied the age of the solar system.
– You learned that the solar system is filled with small
particles called meteoroids, which can fall into Earth’s
atmosphere at speeds of 10 to 40 km/s.
– Friction with the air heats the meteoroids to glowing,
and they vaporize as meteors streaking across the
night sky.
– If a meteoroid is big and strong enough, it can survive
its plunge through the atmosphere and reach Earth’s
surface.
CHAPTER 1
Meteorites
Meteorites, Asteroids,
and Comets
• Once the object strikes Earth’s surface, it
is called a meteorite.
– The largest can blast out giant craters on Earth’s
surface.
• However, such impacts are rare.
– The vast majority are too small to form craters.
• These meteorites fall all over Earth, and
their value is in what they can reveal about
the origin of the planets.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Meteorites can be divided into three
broad categories.
– Iron meteorites are solid chunks of iron and nickel.
– Stony meteorites are silicate masses that resemble
Earth rocks.
– Stony-iron meteorites are mixtures of iron and stone.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Iron meteorites are dense and heavy.
• They often have dark, rusted surfaces and fluted
shapes caused by their passage through the
atmosphere.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• When they are sliced open, polished,
and etched with nitric acid, they reveal
regular bands called
Widmanstätten
patterns.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• The patterns arise from crystals of
nickel-iron alloys that have grown very
large over time.
– This indicates that the meteorite cooled from a molten
state no faster than a few degrees
per million years.
– Explaining how iron meteorites
could have cooled so slowly will
be a major step in analyzing their
history.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Stony meteorites called chondrites have
chemical compositions that resemble a
cooled lump of matter from the sun with
the volatile gases removed.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Although there are many kinds of chondrites, most
contain chondrules, rounded bits of glassy rock
ranging from microscopic to pea-sized.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• The origin of chondrules is unknown.
• They appear to have formed in the young
solar system as droplets of molten rock
that cooled and hardened rapidly.
– As subsequent melting of the meteorite would have
destroyed chondrules, their presence in meteorites
indicates that the meteorites have not been melted
since they formed.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Nevertheless, some kinds of chondrites
show signs that they have been heated
slightly.
– Those meteorites are poor in volatiles such as carbon
compounds and water.
– Solids condensing out of the solar nebula should
have incorporated volatiles.
– However, if they were heated slightly, they could lose
them.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Yet, some chondrites are rich in water.
– This means that they formed in the presence of
volatiles.
• The form and composition of chondrites
dates back to the formation of the solar
nebula.
– Only minor changes, if any, have occurred since.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• The carbonaceous chondrites generally contain
both chondrules and volatile compounds,
including significant amounts of carbon.
– Heating would have modified and driven off these fragile
compounds.
– The carbonaceous chondrites are, along with certain kinds of
chondrites, among the least modified bodies in our solar system.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• Some stony meteorites contain no
chondrules, and they are called
achondrites.
– They also lack volatiles and appear to have been
subjected to intense heat that melted chondrules and
drove off volatiles, leaving behind rocks with
compositions similar to Earth’s lavas.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• 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.
CHAPTER 1
Inside Meteorites
Meteorites, Asteroids,
and Comets
• The meteorites carry hints about the
origin of our solar system, as well as
an older secret.
– Among the smallest grains in meteorites are specks
of minerals whose abundance of isotopes brands
them as star dust—grains of interstellar matter that
predate our solar system.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• You can find evidence of the origin of
meteors through one of the most pleasant
observations in astronomy.
• You can observe a
meteor shower, a display
of meteors that are
clearly related in a
common origin.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• On any clear, moonless night of the year,
you could see 5 to 15 meteors an hour.
• However, they are not related to each
other and may streak in
any direction across the
sky.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• During a meteor shower, you could see as
many as 50 meteors an hour.
• These meteors would all
seem to come from the
same part of the sky.
– For example, the Perseid
meteor shower occurs each
year in August and seems to
come from a spot in the
constellation Perseus.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• That is because all the meteors in a
shower are traveling in the same direction
as they meet Earth’s atmosphere.
– Like railroad tracks extending from a point on the
horizon, the meteors appear to approach from a point
in space.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• Meteor showers occur when Earth
passes near the orbit of a comet.
– So, the meteors must
be dust and debris left
behind by the head of
the comet.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• The orbits of comets are filled with such
debris.
– The telescope aboard the
Infrared Astronomy
Satellite detected the
dusty orbits of a
number of comets
glowing in the
far-infrared
because of sun-warmed
dust scattered
along the orbits.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• Like any natural phenomenon, a meteor
shower is more enjoyable when you know
more about it.
• Actually, most of the meteors you see on
any given night are cometary material that
has been scattered out of the original
orbits of the comets and spread
throughout the solar system.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• Comet debris is small, delicate, and weak.
• No comet material has ever been found on
Earth’s surface.
– It all burns up in Earth’s atmosphere.
• Meteorites that reach Earth’s surface are
structurally much stronger than cometary
material.
– They are iron and stone, and appear to be fragments
of larger solid bodies.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• Some meteorites appear to be fragments
of planetesimals that were large enough
to grow hot from radioactive decay, melt,
and differentiate to form iron-nickel cores
and rocky mantles.
– The molten iron cores would have been well
insulated by the thick rocky mantles.
– The iron would
have cooled
slowly to produce
Widmanstätten
patterns.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• Collisions could have broken such bodies
up and produced different kinds of
meteorites.
– Iron meteorites
appear to be
fragments from
the iron cores.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
– Some stony meteorites that have been strongly
heated appear to have come from the mantles and
surfaces of such bodies.
– The stony-iron meteorites apparently come from the
boundary where the stony mantle meets the iron core.
– Chondrites are probably fragments of smaller bodies
that never melted.
– The carbonaceous chondrites may have formed in
small, colder bodies further from the sun.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• These theories trace the origin of
meteorites to planetesimallike parent
bodies.
• However, the small meteorites in the solar
system cannot be fragments of the
planetesimals that formed the planets.
– They would have been swept up by the planets in
only a billion years or less.
– They could not have survived for 4.6 billion years.
CHAPTER 1
Origin of Meteors and Meteorites
Meteorites, Asteroids,
and Comets
• When astronomers study the orbits of
objects seen to fall to Earth as meteorites,
the orbits lead back into the asteroid belt.
– Thus, astronomers have good evidence to believe
that the meteorites now in museums all over the world
must have been broken off asteroids within the last
billion years.
– Although nearly all meteors are pieces of comets, the
meteorites are pieces of asteroids.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• How can meteors come from comets but
meteorites come from asteroids?
– This is a revealing argument, because it contains a
warning that seeing is not enough in science.
– Thinking about seeing is critical.
• A selection effect can determine what you
notice when you observe nature.
– A very strong selection effect prevents people from
finding meteorites that originated in comets.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• Cometary particles are physically weak.
• They vaporize in Earth’s atmosphere
easily.
– Very few ever reach the ground.
– People are unlikely to find them.
• Even if a particle reached the ground, it
would be so fragile that it would weather
away rapidly.
– Again, people would be unlikely to find it.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• Asteroidal particles are made from rock
and metal, and so are stronger.
– They are more likely to survive their plunge through
the atmosphere and to survive erosion on the ground.
• Meteors from the asteroid belt are rare.
– Almost all the meteors you see come from comets.
– However, not a single meteorite is known to be
cometary.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• The meteorites are valuable because they
provide hints about the process of planet
building in the solar nebula.
• Build a new argument but, as always, think
carefully about what you see.
– Why do meteors in showers seem to come from a
point?
CHAPTER 1
Asteroids
Meteorites, Asteroids,
and Comets
• According to old-time pulp-fiction, space
pirates lurk in the asteroid belt.
• However, astronomers have found that
there isn’t much in the asteroid belt for a
pirate to stand on.
– Most are quite small.
– Given its vast size, the asteroid belt between Mars
and Jupiter is mostly empty.
CHAPTER 1
Asteroids
Meteorites, Asteroids,
and Comets
• Nevertheless, you must consider the nature
of the asteroids.
– You have learned that they were the last remains of
material that was unable to form a planet between Mars
and Jupiter.
– You will now examine these small worlds in more detail.
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• Asteroids are distant objects too small to
study in detail with Earth-based
telescopes.
• Yet astronomers have learned a surprising
amount about these little worlds.
• Spacecraft have provided a few close-ups
too.
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• Most asteroids are irregular in shape
and battered by impact cratering.
– In fact, some appear to be rubble piles of
broken fragments.
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• Some asteroids are double objects or
have small moons in orbit around them.
– This is further evidence of collisions among the
asteroids.
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• A few larger asteroids show signs of geological
activity on their surfaces that may have been
caused by volcanic activity when the asteroid
was young.
CHAPTER 1
Properties of Asteroids
• Asteroids can be
classified by their
albedo and color,
which reveal
clues to their
compositions.
Meteorites, Asteroids,
and Comets
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• Not all asteroids lie in the asteroid belt.
• A few thousand objects larger than 1 km
follow orbits that cross Earth’s orbit.
• A number of searches are under way to
locate these near-Earth objects (NEOs).
– For example, Lowell Observatory Near Earth Object
Search (LONEOS) is searching the entire sky once a
month and should be able to locate a thousand NEOs
over the next 10 years.
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• Astronomers are searching for these
asteroids not only because they want to
understand the asteroids better but
because these collide with Earth
occasionally.
– Although such collisions occur very rarely, a single
impact could cause planetwide devastation.
CHAPTER 1
Properties of Asteroids
Meteorites, Asteroids,
and Comets
• You have learned about the icy bodies of
the Kuiper belt.
– Roughly 70,000 of these orbit in the outer solar
system from the orbit of Neptune out to about 50 AU.
• Some astronomers think of them as
asteroids.
– However, it is clear that they formed from the
outermost parts of the solar nebula.
CHAPTER 1
Origin of the Asteroids
Meteorites, Asteroids,
and Comets
• An old theory proposed that asteroids are
the remains of a planet that exploded.
• Planet-shattering death rays may make for
exciting science-fiction movies.
• However, in reality, planets do not
explode.
CHAPTER 1
Origin of the Asteroids
Meteorites, Asteroids,
and Comets
• The gravitational field of a planet holds the
mass tightly, and disrupting the planet
would take tremendous energy.
– Shattering Earth would require all the energy
generated by the sun over a period of two weeks.
• In addition, the total mass of the asteroids
is only about one-twentieth the mass of
the moon—hardly enough to be the
remains of a planet.
CHAPTER 1
Origin of the Asteroids
Meteorites, Asteroids,
and Comets
• Astronomers believe that the asteroids are
the remains of material that was unable to
form a planet at 2.8 AU from the sun—due
to the gravitational influence of Jupiter, the
next planet outward.
– If this is true, then the asteroids are the remains of
ancient
planetesimals
fragmented by
collisions with
one another.
CHAPTER 1
Origin of the Asteroids
– This would explain
why the C-type
asteroids, which
appear to be
carbonaceous, are
more common in the
outer asteroid belt.
– It is cooler there, and
the condensation
sequence predicts that
carbonaceous material
would form there more
easily than in the inner
belt.
Meteorites, Asteroids,
and Comets
CHAPTER 1
Origin of the Asteroids
Meteorites, Asteroids,
and Comets
• As in the case of Vesta, a few asteroids
may have been geologically active, with
lava flowing on their surfaces when they
were young.
– Perhaps they incorporated short-lived radioactive
elements such as aluminum-26.
– Such elements could have been produced by a
supernova explosion.
– Such an explosion might have triggered the formation
of the sun and planets.
CHAPTER 1
Origin of the Asteroids
Meteorites, Asteroids,
and Comets
• However, not all asteroids have been
active.
– Ceres, 900 km in diameter, is almost twice as
big as Vesta, but it shows no spectroscopic
sign of past activity and is rich in water.
CHAPTER 1
Origin of the Asteroids
Meteorites, Asteroids,
and Comets
• Although there are still mysteries to solve,
you can now understand the compositions
of the meteorites.
• They are fragments of planetesimals,
some of which developed molten cores,
differentiated, and then cooled slowly.
– The largest asteroids astronomers see today may be
nearly unbroken planetesimals.
– However, the rest are just the fragments produced by
4.6 billion years of collisions.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• What evidence makes you think that
the asteroids have been fragmented?
– Perhaps the best scientific arguments test the
interpretation of evidence.
– If you understand the evidence, you hold the key to
the science.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• To begin, you might note that the solar
nebula theory of the formation of the solar
system predicts that planetesimals collided
and either stuck together or fragmented.
– This is suggestive, but it is not evidence.
– A theory can never be used as evidence to support
some other theory or hypothesis.
– Evidence refers to observations or the results of
experiments.
– So, you need to turn to observations of asteroids.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• Spacecraft photographs of asteroids such
as Ida, Gaspra, and Eros show irregularly
shaped little worlds heavily scarred by
impact craters.
– Observations of some asteroids show what may be
pairs of bodies in contact.
– The Galileo image of Ida reveals its small satellite,
Dactyl.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• Furthermore, some meteorites appear to
come from the asteroid belt, and a few
have been linked to specific asteroids
such as Vesta.
• There are even families of asteroids that
seem to be fragments from a single
collision.
• All this evidence suggests that the
asteroids have been broken up by violent
impacts.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• The impact fragmentation of asteroids has
been important, but it has not erased all
traces of the original planetesimals from
which the asteroids formed.
• Build another argument based on
evidence.
– What evidence can you cite that reveals what those
planetesimals were like?
CHAPTER 1
Comets
Meteorites, Asteroids,
and Comets
• Of all the fossils left behind by the solar
nebula, comets are the most beautiful.
– Asteroids are dark, rocky worlds.
– Meteors are flitting specks of fire.
– However, comets move with
the grace and beauty of a
great ship at sea.
– For example, Comet HaleBopp was visible for weeks
in 1996 and 1997.
CHAPTER 1
Comets
Meteorites, Asteroids,
and Comets
• Scientifically, comets are interesting
because observations of the tail and head
of a comet reveal facts about the small icy
nucleus and the
ancient solar nebula
from which our solar
system’s wealth of
comets is inherited.
CHAPTER 1
Properties of Comets
Meteorites, Asteroids,
and Comets
• As always, you should begin your study of
a new kind of object by summarizing its
observational properties.
– What do comets look like?
– How do they behave?
• The observations are the evidence that
reveal the secrets of the comets.
CHAPTER 1
Properties of Comets
• Comets have three
important
properties.
• One, they have two
kinds of tails,
shaped by the
solar wind and
solar radiation.
Meteorites, Asteroids,
and Comets
CHAPTER 1
Properties of Comets
• The two kinds of
tails show that the
nucleus contains
ices of water and
other compounds
plus rocky material
most evident as
dust.
Meteorites, Asteroids,
and Comets
CHAPTER 1
Properties of Comets
Meteorites, Asteroids,
and Comets
• Two, comets contain dust.
– This not only produces dust tails but spreads
throughout the solar system.
CHAPTER 1
Properties of Comets
Meteorites, Asteroids,
and Comets
• Finally, comet nuclei are fragile and can
break into pieces.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Astronomers can put these and other
observations together to discuss the
structure of comet nuclei.
– The nuclei of comets are quite small and cannot be
studied in detail from Earth-based telescopes.
– Nevertheless, astronomers are beginning to
understand the geology of these peculiar worlds.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Comet nuclei contain ices of water and
other volatile compounds such as carbon
dioxide, carbon monoxide, methane, and
ammonia.
– These ices are the kinds of compounds that should
have condensed from the outer solar nebula.
– That makes astronomers think that comets are
ancient samples of the gases and dust from which the
outer planets formed.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• When the nuclei of comets approach the
sun, the ices absorb energy from sunlight
and sublime—change from a solid directly
into a gas—to produce the observed tails.
– As the gases break down and combine chemically,
they release many compounds found in comet tails.
– Vast clouds of hydrogen gas observed around the
heads of comets are derived from the breakup of
molecules from the ices.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Five spacecraft flew past the nucleus
of Comet Halley when it visited the
inner solar system in 1985 and 1986.
• The Deep Space 1 spacecraft flew
past the nucleus of Comet Borrelly in
2001.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• The Stardust spacecraft flew past Comet
Wild 2 in 2004.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Photos show that these comet
nuclei are irregular in shape and
very dark.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• In general, the nuclei are darker than a
lump of coal, suggesting the composition
of the carbon-rich meteorites called
carbonaceous chondrites.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• From the gravitational influence of a
nucleus on a passing spacecraft,
astronomers can find the mass and
density of the nucleus.
– Comet nuclei appear to have densities of 0.1 to 0.25
g/cm3, much less than the density of ice.
– From these observations, astronomers can conclude
that comet nuclei are not solid balls of ice but must be
fluffy mixtures of ices and dust with significant
amounts of empty space.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Photos of the comas of comets often show
jets springing from the nucleus and being
swept back by the pressure of sunlight and
by the solar wind to form the tail.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Studies of the movements of these jets as
the nucleus rotates and the photographs of
the nucleus of Comet Halley reveal that the
jets originate from active regions that may
be faults or vents.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• As the rotation of a cometary nucleus
carries an active region into sunlight, it
begins venting gas and dust.
• As it rotates into darkness, it shuts down.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• In 2005, the Deep Impact spacecraft
released an instrumented impactor,
weighing 816 lb on Earth, into the path of
comet Tempel 1.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• The nucleus of the comet, traveling at
almost 10 km/s (23,000 mph), collided with
the impactor exactly as planned.
– The impact penetrated the
crust of the nucleus and
blasted material out into
space, where the mothership
could analyze it as it flew past.
– The burst of vapor and dust
was detected by the mothership
and also by Earth-based
telescopes.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Analysis of the data from the Deep Impact
mission and images of other comet nuclei
show that they are not solid objects.
• To call them dirty snowballs or icy mud
balls is misleading.
• Rather, they appear to be irregular bodies
with large voids.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• Although comet nuclei seem to have
porous crusts of dark material, the ice and
rock are not uniformly mixed through the
interior.
– Breaks in the crust can expose pockets of highly
volatile ices and cause sudden bursts of gas
production.
CHAPTER 1
The Geology of Comet Nuclei
Meteorites, Asteroids,
and Comets
• The nuclei of comets range from a few
kilometers to a few tens of kilometers in
diameter.
• Each passage near the sun costs a
nucleus many millions of tons of ices.
– So, the nucleus slowly wastes away until there is
nothing left but dust and rock falling along an orbit
around the sun.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• The fate of a comet is clear.
• The mystery is its origin.
– Family relationships among the comets
provide clues to their origin.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Most comets have long, elliptical orbits
with periods greater than 200 years.
• These are known as long-period comets.
– Their orbits are randomly inclined, with comets falling
into the inner solar system from all directions.
– As many circle the sun clockwise as
counterclockwise.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• In contrast, about 100 of the 600 wellstudied comets have orbits with periods
less than 200 years.
• These short-period comets follow orbits
that lie within 30° of the plane of the solar
system.
– Most revolve around the sun counterclockwise—the
same direction the planets orbit.
– Comet Halley, with a period of 76 years, is a shortperiod comet.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Comets cannot survive long before the
heat of the sun drives away their ices and
reduces them to inactive bodies of rock
and dust.
– A comet may last only 100 to 1,000 orbits around the
sun.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• The comets seen in our skies can’t have
survived 4.6 billion years since the
formation of the solar system.
• So, there must be a continuous supply of
new comets.
– Where do they come from?
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• In the 1950s, Dutch astronomer Jan Oort
proposed that the long-period comets are
objects that fall in from the Oort cloud.
– This is a spherical cloud
of icy bodies believed to
extend from 10,000 to
100,000 AU from the sun.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Astronomers estimate that the cloud
contains several trillion icy bodies.
• Far from the sun, they are very cold, lack
comas and tails, and are invisible.
– The gravitational influence of occasional passing
stars could perturb a few of these objects to fall into
the inner solar system.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
– In the inner solar system, the sun’s heat warms their
ices and transforms them into the brilliant display
you see in the night sky.
– As the Oort cloud is
spherical, these longperiod comets fall
inward from random
directions.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Some of the short-period comets,
including Comet Halley, appear to have
originated in the Oort cloud and probably
had their orbits altered by a close
encounter with Jupiter.
• However, many of the short-period comets
cannot have begun in the Oort cloud.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Interactions with a planet can’t put objects
from the Oort cloud into the orbits that
some short-period comets occupy.
• There must be another source of icy
bodies in our solar system.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• In 1951, Dutch-American astronomer
Gerard P. Kuiper proposed that the
formation of the solar system should have
left behind a belt of small, icy
planetesimals beyond the Jovian planets
and in the plane of the solar system.
– Those objects could not be detected with the
telescopes and instruments available in those days.
– Nevertheless, the proposed band of icy bodies
became known as the Kuiper belt.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Modern astronomers have found
hundreds of Kuiper-belt objects in orbits
extending from Neptune at 30 AU out to
about 50 AU from the sun.
• Few are seen further away.
– Those that are found there were probably scattered
outward by interactions with other belt objects.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• The entire Kuiper belt would be
hidden behind the yellow dot.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Astronomers can detect similar belts
around other stars.
– A number of nearby stars such as Beta Pictoris are
surrounded by disks of dust believed to be released
by icy bodies in the equivalent of Kuiper belts around
these stars.
• Thus, the detection of the dust implies the
presence of Kuiper belts.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• The Kuiper-belt objects found so far are
mostly too big to become comets.
• However, there must be lots of smaller
objects that can’t be seen.
• When a small Kuiper-belt object is
perturbed into the inner solar system, it
can interact with planets and be captured
into orbit as a short-period comet.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• If comets are icy planetesimals from
the Oort cloud and the Kuiper belt,
how did those planetesimals form?
– The Kuiper-belt objects appear to have formed as icy
planetesimals in the outer solar nebula, not much
farther from the sun than the outer planets.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
– However, the objects in the Oort cloud lie much
farther from the sun, and they can’t have formed
there.
– The solar nebula would have been too tenuous
at such great distances.
– Also, you would expect objects that formed from
the nebula to be confined to a disk and not
distributed in a sphere.
CHAPTER 1
Origin of Comets
Meteorites, Asteroids,
and Comets
• Astronomers think the objects now in the
Oort cloud formed in the outer solar
system among the present orbits of the
Jovian planets.
– As the Jovian planets grew more massive, they swept
up some of these planetesimals and ejected others to
form the Oort cloud.
• If this idea is true, then the long-period
comets are the same kind of icy
planetesimals that make up the shortperiod comets.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• For centuries, superstitious people have
associated comets with doom, which
seems silly.
• Of course, comets and asteroids must hit
planets now and then.
• So, you might wonder just how dangerous
such impacts would be.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Earthlings watched in awe, during the
summer of 1994, as the fragmented
head of a comet slammed into
Jupiter, producing impacts equaling
millions of megatons of TNT.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Such impacts on Jupiter probably occur
every century or so, and you might expect
similar impacts on Earth—smaller and with
less gravitational power—to occur much
less often.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Nevertheless, these impacts do occur.
– On some moons, you can find chains of craters that
seem to have been formed by fragmented comets.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Small meteorite impacts occur quite often.
– A building is damaged by a falling meteorite every few
years.
• Larger impacts are less common.
• Truly large impacts are rare.
• A large impact could have devastating
consequences.
CHAPTER 1
Impacts on Earth
• Earth is marked by
about 150 meteorite
craters that illustrate
the power of an
impacting object.
Meteorites, Asteroids,
and Comets
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Studies of sediments laid down all over the
world 65 million years ago at the time of
the extinction of the dinosaurs have found
overabundances of the element iridium—
common in meteorites but rare in Earth’s
crust.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• This finding, along with other evidence,
suggests that the impact of a large meteorite
may have altered the atmosphere and
climate on Earth so dramatically that the
dinosaurs and over 75 percent of the
species then on Earth became extinct.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Mathematical models and observations of
the impact of the comet fragments on
Jupiter in 1994 have combined to create a
plausible scenario of the events following
a major impact on Earth.
– Creatures living near the site of the impact would
probably die in the initial shock.
– An impact at sea would create tsunamis (tidal waves)
many hundreds of meters high that would devastate
coastal regions halfway around the world for many
kilometers inland.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• However, the worst effects would
begin after the initial explosion.
– On land or sea, a major impact would excavate large
amounts of pulverized rock, heat it to high
temperatures, and loft it high above the atmosphere.
– As this material fell back, Earth’s atmosphere would
be turned into a glowing oven of falling red-hot
meteors.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• The heat would trigger massive forest fires
around the world.
– Soot from such fires has been detected in the layers
of clay laid down at the end of the Cretaceous period.
• Once the firestorms cooled, the remaining
dust in the atmosphere would block
sunlight and produce deep darkness for a
year or more, killing most plant life.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• At the same time, large amounts of carbon
dioxide locked in limestone deposits and
released into the atmosphere by the
impact would produce intense acid rain.
• All these consequences make it surprising
that any life could have survived such an
impact.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Geologists have located a crater at least
150 km in diameter centered near the
village of Chicxulub in the northern
Yucatán.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Although the crater is totally covered by
sediments, mineral samples show that it
contains shocked quartz typical of impact
sites and that it is the right age.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• The impact of an object 10 to 14 km in
diameter formed the crater about 65
million years ago, just when the dinosaurs
and many other species died out.
• Many Earth scientists now believe that this
is the scar of the impact that ended the
Cretaceous period.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Chicxulub may not be the only impact that
triggered an extinction.
• The biggest extinction on record occurred
250 million years ago at the end of the
Permian period, when 95 percent of life in
the oceans and 80 percent of life on land
died out.
– The Permian extinction is also called the
Great Dying.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Core samples from the ocean floor
northwest of Australia reveal shattered
rock, meteoric fragments, and shocked
quartz.
• Seismic and gravity data support the idea
that a major impact occurred there at
about the right time to cause the Great
Dying.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• Could such impacts happen again?
– Earth gets hit by small meteorites every day and by
larger objects less often.
– Impacts by large asteroids may happen many millions
of years apart, but they continue to happen.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• In mid-March 1998, newspaper headlines
announced ‘Mile-Wide Asteroid to Hit
Earth in October 2028.’
• However, the news media did not report
that there was uncertainty in the orbit.
– Within days, astronomers found the asteroid on old
photographic plates, recalculated the orbit adding the
new data, and concluded that the asteroid—known as
1997XF11—would miss Earth by 600,000 miles.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• There will be no impact by this
asteroid in 2028.
• However, there are plenty more
asteroids that haven’t been
discovered.
– It is just a matter of time.
CHAPTER 1
Impacts on Earth
Meteorites, Asteroids,
and Comets
• A solar system is a dangerous place to put
an inhabited planet.
• Comets, asteroids, and meteoroids
constantly rain down on the planets, and
Earth gets hit relatively often.
– Chicxulub isn’t the only large impact scar on Earth.
– About 150 are known, including giant craters buried
under sediment in Iowa and another underlying most
of Chesapeake Bay.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• How do comets help explain the formation
of the planets?
– This scientific argument pulls together several ideas.
• According to the solar nebula hypothesis,
the planets formed from planetesimals
that accreted in a disk-shaped nebula
around the forming sun.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• In the outer solar nebula, it was cold, and
the planetesimals would have contained
large amounts of ices.
• Many of these planetesimals were
destroyed when they fell together to make
the planets.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• However, some of the planetesimals
survived.
• The icy bodies of the Oort cloud and the
Kuiper Belt may be the last surviving icy
planetesimals in our solar system.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• When these bodies fall into the inner solar
system, they become comets.
• The gases they release reveal that they
are rich in volatile materials such as water,
carbon dioxide, carbon monoxide,
methane, and ammonia.
– These are the ices you would expect to find in the icy
planetesimals.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• Furthermore, comets are rich in dust, and
the planetesimals must have included
large amounts of dust frozen into the ices
when they formed.
• The nuclei of comets seem to be frozen
samples of the ancient solar nebula.
CHAPTER 1
Building Scientific Arguments
Meteorites, Asteroids,
and Comets
• Nearly all the mass of a comet is in the
nucleus, but the light you see comes from
the coma and the tail.
• Build an argument pulling together ideas
about cometary nuclei.
– What kind of spectra do comets produce?
– What does that tell you about their nuclei?