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Meteorites, Asteroids, and Comets

• Chapter 25

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Guidepost
In Chapter 19, we began our study of planetary
astronomy by asking how our solar system formed.
In the five chapters that followed, we surveyed
the planets, but we gained only limited insight
into the origin of the solar system. The planets
are big, and they have evolved as heat has flowed
out of their interiors. In this chapter, we have
our best look at unevolved matter left over from
the solar nebula. These small bodies are, in
fact, the last remains of the nebula that gave
birth to the planets. This chapter is unique in
that it covers small bodies. In past chapters, we
have used the principles of comparative
planetology to study large objects the planets.
In this chapter, we see that the same principles
apply to smaller bodies, but we also see that we
need some new tools in order to think about the
tiniest worlds in the solar system.
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Outline
I. Meteorites A. Meteoroid Orbits B. Meteorite
Impacts on Earth C. An Analysis of
Meteorites D. The Origins of Meteorites II.
Asteroids A. The Asteroid Belt B. Nonbelt
Asteroids C. Composition and Origin III.
Comets A. Properties of Comets B. The Geology
of Comet Nuclei C. The Origin of Comets
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Outline (continued)
IV. Impacts on Earth A. Impacts and
Dinosaurs B. The Tunguska Event
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Comets of History
Throughout history, comets have been considered
as portents of doom, even very recently
Appearances of comet Kohoutek (1973), Halley
(1986), and Hale-Bopp (1997) caused great concern
among superstitious.
Comet Hyakutake in 1996
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Meteorites
Distinguish between
Meteoroid small body 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...

• 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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Meteor Showers
Most meteors appear in showers, peaking
periodically at specific dates of the year.
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Radiants of Meteor Showers
Tracing the tracks of meteors in a shower
backwards, they appear to come from a common
origin, the radiant.
? Common direction of motion through space.
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Meteoroid Orbits

• Meteoroids contributing to a meteor shower are
  debris particles, orbiting in the path of a
  comet.
• Spread out all along the orbit of the comet.
• Comet may still exist or have been destroyed.

Only a few sporadic meteors are not associated
with comet orbits.
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Meteorite Impacts on Earth
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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Impact Craters on Earth
Barringer Crater 1.2 km diameter 200 m deep
Much larger impact features exist on Earth

• Impact of a large body formed a crater 180
  300 km in diameter in the Yucatán peninsula, 65
  million years ago.
• Drastic influence on climate on Earth possibly
  responsible for extinction of dinosaurs.

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Finding Meteorites
Most meteorites are small and do not produce
significant craters.
Good place to find meteorites Antarctica!
Distinguish between

• Falls meteorites which have been observed to
  fall (fall time known).

• Finds meteorites with unknown fall time.

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Analysis of Meteorites
3 broad categories

• Iron meteorites

• Stony meteorites

• Stony-Iron meteorites

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What Does a Meteorite Look Like?
Selection bias Iron meteorites are easy to
recognize as meteorites (heavy, dense lumps of
iron-nickel steel) thus, more likely to be
found and collected.
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The Allende Meteorite

• Carbonaceous chondrite, fell in 1969 near
  Pueblito de Allende, Mexico
• Showered an area about 50 km x 10 km with over 4
  tons of fragments.

Fragments containing calcium-aluminum-rich
inclusions (CAIs)
Extremely temperature-resistant materials.
Allende meteorite is a very old sample of
solar-nebula material!
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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

• Some melted by heat produced by 26Al decay
  (half-life 715,000 yr).

• 26Al possibly provided by a nearby supernova,
  just a few 100,000 years before formation of the
  solar system (triggering formation of our sun?)

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The Origins of Meteorites (2)

• Planetesimals cool and differentiate

• Collisions eject material from different depths
  with different compositions and temperatures.

• Meteorites can not have been broken up from
  planetesimals very long ago

? so remains of planetesimals should still exist.
? Asteroids
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Asteroids
Last remains of planetesimals that built the
planets 4.6 billion years ago!
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The Asteroid Belt
Small, irregular objects, mostly in the apparent
gap between the orbits of Mars and Jupiter.
Thousands of asteroids with accurately determined
orbits known today.
Sizes and shapes of the largest asteroids,
compared to the moon
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Kirkwoods Gaps

• The asteroid orbits are not evenly distributed
  throughout the asteroid belt between Mars and
  Jupiter.
• There are several gaps where no asteroids are
  found
• Kirkwoods gaps (purple bars below)

These correspond to resonances of the orbits with
the orbit of Jupiter.
Example 23 resonance
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Non-Belt Asteroids
Not all asteroids orbit within the asteroid belt.
Apollo-Amor Objects
Trojans Sharing stable orbits along the orbit
of Jupiter
Asteroids with elliptical orbits, reaching into
the inner solar system.
Trapped in the Lagrangian points of Jupiter.
Some potentially colliding with Mars or Earth.
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Colors of Asteroids
M-type Brighter, less reddish asteroids,
probably made out of metal rich materials
probably iron cores of fragmented asteroids
S-type Brighter, redder asteroids, probably made
out of rocky materials very common in the inner
asteroid belt
C-type Dark asteroids, probably made out of
carbon-rich materials (carbonaceous chondrites)
common in the outer asteroid belt
Colors to be interpreted as albedo
(reflectivity) at different wavelengths.
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The Origin of Asteroids
Distribution S-type asteroids in the outer
asteroid belt C-type asteroids in inner asteroid
belt ? may reflect temperatures during the
formation process.
However, more complex features found
Vesta shows evidence for impact crater and lava
flows.
Images of the Asteroid Vesta show a complex
surface, including a large impact crater.
Heat for existence of lava flows probably from
radioactive decay of 26Al.
Meteorite probably fragmented from Vesta
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Comets
Comet Ikeya-Seki in the dawn sky in 1965
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Two Types of Tails
Ion 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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Gas and Dust Tails of Comet Mrkos in 1957
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Build A Comet
(SLIDESHOW MODE ONLY)
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Comet Hale Bopp (1997)
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Dust Jets from Comet Nuclei
Jets of dust are ejected radially from the nuclei
of comets.
Comet Hale-Bopp, with uniform corona digitally
removed from the image.
Comet dust material can be collected by
spacecraft above Earths atmosphere.
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Fragmenting Comets
Comet Linear apparently completely vaporized
during its sun passage in 2000.
Only small rocky fragments remained.
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The Geology of Comet Nuclei
Comet nuclei contain ices of water, carbon
dioxide, methane, ammonia, etc.
Materials that should have condensed from the
outer solar nebula.
Those compounds sublime (transition from solid
directly to gas phase) as comets approach the sun.
Densities of comet nuclei 0.1 0.25 g/cm3
Not solid ice balls, but fluffy material with
significant amounts of empty space.
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Fragmentation of Comet Nuclei
Comet nuclei are very fragile and are easily
fragmented.
Comet Shoemaker-Levy was disrupted by tidal
forces of Jupiter
Two chains of impact craters on Earths moon and
on Jupiters moon Callisto may have been caused
by fragments of a comet.
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The Origin of Comets
Comets are believed to originate in the Oort
cloud
Spherical cloud of several trillion icy bodies,
10,000 100,000 AU from the sun.
Gravitational influence of occasional passing
stars may perturb some orbits and draw them
towards the inner solar system.
10,000 100,000 AU
Interactions with planets may perturb orbits
further, capturing comets in short-period orbits.
Oort Cloud
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The Kuiper Belt
Second source of small, icy bodies in the outer
solar system
Kuiper belt, at 30 100 AU from the sun.
Few Kuiper belt objects could be observed
directly by Hubble Space Telescope.
Pluto and Charon may be captured Kuiper belt
objects.
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Impacts on Earth
Comet nucleus impact producing the Chicxulub
crater 65 million years ago may have caused
major climate change, leading to the extinction
of many species, including dinosaurs.
Gravity map shows the extent of the crater hidden
below limestone deposited since the impact.
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The Tunguska Event

• The Tunguska event in Siberia in 1908 destroyed
  an area the size of a large city!
• Explosion of a large object, probably an Apollo
  asteroid of 90 190 m in diameter, a few km
  above the ground.
• Energy release comparable to a 12-megaton nuclear
  weapon!

Area of destruction from the Tunguska event
superimposed on a map of Washington, D.C. and
surrounding beltway.
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Impacts on Earth
(SLIDESHOW MODE ONLY)
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New Terms
radiant sporadic meteor fall find iron
meteorite selection effect Widmanstätten
pattern stony meteorite chondrite chondrule carbon
aceous chondrite CAI achondrite stony-iron
meteorite Kirkwoods gaps ApolloAmor
objects Trojan asteroids Hirayama families
gas (type I) tail dust (type II) tail coma Oort
cloud Kuiper belt
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Discussion Questions
1. Futurists suggest that we may someday mine the
asteroids for materials to build and supply space
colonies. What kinds of materials could we get
from asteroids? (Hint What are S-, M-, and
C-type asteroids made of?) 2. If cometary
nuclei were heated by internal radioactive decay
rather than by solar heat, how would comets
differ from what we observe? 3. From what you
know now, do you think the government should
spend money to locate near-Earth asteroids? How
serious is the risk?
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Quiz Questions
1. What type of meteorite is the most common, at
about 80 of all falls? a. Irons. b.
Stony-irons. c. Chondrites. d. Achondrites. e.
Carbonaceous chondrites.
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Quiz Questions
2. If most falls are stony meteorites, why are
most finds iron meteorites? a. Stony meteorites
are less weather-resistant. b. Stony meteorites
look more like Earth rocks. c. Stony meteorites
penetrate the ground more deeply. d. Both a and b
above. e. All of the above.
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Quiz Questions
3. How do observations of meteor showers reveal
one of the sources of meteoroids? a. The
radiants of meteor showers are at locations where
Earth crosses the debris trail of comets. b. The
meteorites that result from meteor showers
contain icy materials that we match to comets. c.
Many shower meteors can be traced back to our
moon. d. Meteor showers are usually best viewed
after midnight. e. Both a and b above.
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Quiz Questions
4. What evidence do we have that some meteorites
have originated inside large bodies? a. Some
meteorites are very large. b. Chondrules can only
form inside a large body that cools slowly. c. We
can track their orbits back to the asteroid
belt. d. The Widmanstätten patterns in iron
meteorites indicate very slow cooling. e. Both a
and c above.
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Quiz Questions
5. Which type of meteorite is rich in volatiles,
and are thus the best samples of the solar
nebula? a. Irons. b. Stony-irons. c.
Chondrites. d. Achondrites. e. Carbonaceous
chondrites.
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Quiz Questions
6. Of the following, which type of meteorites
comes from undifferentiated bodies? a. Irons. b.
Stony-irons. c. Chondrites. d. Achondrites. e.
Both a and b above.
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Quiz Questions
7. What do we suspect was the heat source that
melted planetesimals that were as small as 20 km
in diameter? a. Impact energy of the
planetesimals constituent particles. b.
Long-lived radioactive isotopes such as Uranium
238. c. Short-lived radioactive isotopes such as
Aluminum 26. d. Gravitational energy released by
differentiation. e. Electrical discharges in the
solar nebula.
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Quiz Questions
8. Why do we think that a supernova event may be
the source of the shockwave that triggered the
gravitational collapse that formed the solar
system? a. Supernova events produce Aluminum
26. d. The half-life of aluminum 26 is 715,000
years. b. Some iron meteorites cooled in
planetesimals as small as 20 km in diameter. c.
Magnesium 26 is found in meteorite minerals that
usually contain aluminum. e. All of the above.
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Quiz Questions
9. What evidence do we have that meteorites are
pieces of recently broken planetesimals? a. The
cosmic-ray-exposure ages of meteorites are
typically less than 100 million years. b. Some
meteorites are pieces of differentiated larger
bodies. c. Some meteorites are breccias. d. Both
a and b above. e. All of the above.
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Quiz Questions
10. How can most meteors be cometary if most,
perhaps all, meteorites are asteroidal? a.
Asteroids are rocky and metallic in
composition. b. Comet debris particles are small
and full of volatiles. c. In Earths vicinity the
meteoroids are mostly cometary particles. d. Both
a and b above. e. All of the above.
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Quiz Questions
11. What causes the Kirkwood gaps of the asteroid
belt? a. Orbital resonances with Earth. b.
Orbital resonances with Mars. c. Orbital
resonances with Jupiter. d. Orbital resonances
with Saturn. e. Both a and b above.
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Quiz Questions
12. Studies show that the orbits of Apollo and
Amor objects are not stable that is, these
orbits cannot have existed since the beginning of
the solar system. What is most likely source of
the Apollo-Amor objects? a. They are asteroids
that were ejected from the Kirkwood gaps. b. They
are most likely impact fragments from the
Moon. c. They are most likely impact fragments
from Mars. d. They have been perturbed from the
Kuiper Belt. e. They have been perturbed from the
Oort Cloud.
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Quiz Questions
13. In 1928 Kiyotsugu Hirayama grouped some
asteroids into families. What is similar for the
asteroids of one Hirayama family? a. The
semimajor axes of their orbits. b. The
eccentricity of their orbits. c. The inclination
of their orbits. d. Both a and b above. e. All of
the above.
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Quiz Questions
14. The small asteroid Braille and the Eucrite
meteorites are believed to be pieces of the large
asteroid Vesta. What evidence is there for this
connection? a. The Hubble Space Telescope has
detected a large impact crater on Vesta. b. The
reflectance spectrum of the two asteroids matches
the reflectance spectrum of the Eucrite meteorite
group. c. Calculating the orbits of the two
asteroids backward in time shows that they were
at the same location 50,000 years ago. d. Both a
and b above. e. All of the above.
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Quiz Questions
15. The three main classes of asteroids, based on
similarities in their infrared reflectance
spectra, are C, S, and M. How do the different
types of meteorites match up to the main asteroid
classes? a. C-type stony, S-type iron,
M-type carbonaceous chondrites. b. C-type
carbonaceous chondrites, S-type stony, M-type
iron. c. C-type iron, S-type stony, M-type
carbonaceous chondrites. d. C-type carbonaceous
chondrites, S-type iron, M-type stony. e.
C-type stony, S-type carbonaceous chondrites,
M-type iron.
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Quiz Questions
16. Why do we believe that comets are loosely
consolidated, fluffy mixtures of ice and
rock? a. A few comets are known to have broken
apart due to close passage to the Sun and
Jupiter. b. We have measured comet densities to
range from 0.1 to 0.25 grams per cubic
centimeter. c. The spectra of comet tails reveals
ionized molecules and atoms consistent with
sublimated ices. d. Comets have dust tails that
are most likely rocky bits of material like those
collected at high altitude. e. All of the above.
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Quiz Questions
17. What are the characteristics of a type I
comet tail? a. Type I tails generally point
outward, away from the Sun. b. Type I tails have
an emission line spectrum of ionized gases. c.
Type I tails have a reflected solar absorption
line spectrum. d. Both a and b above. e. Both a
and c above.
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Quiz Questions
18. If you analyze the chemical composition of
several typical long period comets and several
typical short period comets, you are likely to
find a greater abundance of low condensation
temperature ices in one group relative to the
other. Which group has the greater abundance,
and why? a. Long-period comets, because they
come from the Oort Cloud, which is at a greater
distance from the Sun. b. Short-period comets,
because they come from the Kuiper Belt, which is
closer to the Sun. c. Long-period comets, because
these bodies originally formed among the Jovian
planets. d. Short-period comets, because these
bodies originally formed beyond the orbit of
Neptune. e. None of the above reasons will prove
true.
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Quiz Questions
19. A key feature of the impact hypothesis for
the mass extinction that occurred 65 million
years ago is the presence of the metallic element
iridium in high abundance in the clay layer at
the Cretaceous-Tertiary boundary. If both the
meteoroid and Earth formed from the solar nebula,
how could the metallic element iridium be in low
abundance at Earths surface and in high
abundance in meteoritic material? a. Earths
iridium resides in its core. b. The meteoroid was
from outside the Solar System. c. The meteoroid
was undifferentiated or a piece of a
differentiated iron core. d. Both a and b
above. e. Both a and c above.
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Quiz Questions
20. Of the following, which is a major flaw with
the comet impact hypothesis for the Tunguska
Event of 1908? a. A comet would have been easily
seen in the predawn skies the morning of the
impact. b. A small comet body would vaporize much
higher in Earths dense atmosphere. c. The impact
crater was too large for a comet body impact. d.
No known comets went missing in 1908. e. No ice
was found at the impact site.
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Answers
1. c 2. d 3. a 4. d 5. e 6. c 7. c 8. e 9. e 10. e
11. c 12. a 13. e 14. d 15. b 16. e 17. d 18. d 19
. e 20. b