Transcript The Milky Way - Department of Political Science

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Chapter 25
Meteorites, Asteroids, and Comets
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.
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
Outline (continued)
IV. Impacts on Earth
A. Impacts and Dinosaurs
B. The Tunguska Event
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
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).
Meteor Showers
Most meteors appear in showers, peaking
periodically at specific dates of the year.
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.
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.
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
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.
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.
Analysis of Meteorites
3 broad categories:
• Iron meteorites
• Stony meteorites
• Stony-Iron
meteorites
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.
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-aluminumrich inclusions (CAIs)
Extremely temperatureresistant materials.
Allende meteorite is a very old sample of solar-nebula material!
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?)
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
Asteroids
Last remains of
planetesimals
that built the
planets 4.6
billion years
ago!
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
Kirkwood’s 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:
Kirkwood’s gaps (purple bars below)
These correspond
to resonances of
the orbits with the
orbit of Jupiter.
Example:
2:3 resonance
Non-Belt Asteroids
Not all asteroids orbit within the asteroid belt.
Apollo-Amor
Objects:
Asteroids
with elliptical
orbits,
reaching into
the inner
solar system.
Some
potentially
colliding with
Mars or
Earth.
Trojans:
Sharing
stable orbits
along the
orbit of
Jupiter:
Trapped in
the
Lagrangian
points of
Jupiter.
Colors of Asteroids
M-type: Brighter,
less reddish
asteroids,
probably made
out of metal rich
materials;
probably iron
cores of
fragmented
asteroids
C-type: Dark
asteroids, probably
made out of
carbon-rich
materials
(carbonaceous
chondrites);
common in the
outer asteroid belt
S-type:
Brighter,
redder
asteroids,
probably made
out of rocky
materials; very
common in the
inner asteroid
belt
“Colors” to be interpreted as albedo
(reflectivity) at different wavelengths.
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:
Images of the
Asteroid Vesta
show a complex
surface,
including a large
impact crater.
Meteorite probably fragmented from Vesta
Vesta shows
evidence for
impact crater
and lava flows.
Heat for
existence of
lava flows
probably from
radioactive
decay of 26Al.
Comets
Comet Ikeya-Seki in
the dawn sky in 1965
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
sun’s radiation pressure.
Lagging behind the
comet along its
trajectory
Gas and Dust Tails of
Comet Mrkos in 1957
Build A Comet
(SLIDESHOW MODE ONLY)
Comet Hale Bopp (1997)
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 Earth’s atmosphere.
Fragmenting Comets
Comet Linear
apparently completely
vaporized during its
sun passage in 2000.
Only small rocky
fragments remained.
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.
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 Earth’s
moon and on Jupiter’s
moon Callisto may
have been caused by
fragments of a comet.
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.
Oort Cloud
Interactions with
planets may perturb
orbits further,
capturing comets in
short-period orbits.
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.
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.
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.
Area of destruction from the Tunguska
event superimposed on a map of
Washington, D.C. and surrounding
beltway.
•Energy release
comparable to a 12megaton nuclear
weapon!
Impacts on Earth
(SLIDESHOW MODE ONLY)
New Terms
radiant
sporadic meteor
fall
find
iron meteorite
selection effect
Widmanstätten pattern
stony meteorite
chondrite
chondrule
carbonaceous chondrite
CAI
achondrite
stony-iron meteorite
Kirkwood’s gaps
Apollo–Amor objects
Trojan asteroids
Hirayama families
gas (type I) tail
dust (type II) tail
coma
Oort cloud
Kuiper belt
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 Earth’s vicinity the meteoroids are mostly cometary
particles.
d. Both a and b above.
e. All of the above.
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.
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.
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.
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.
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.
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.
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.
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.
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 Earth’s surface and in
high abundance in meteoritic material?
a. Earth’s 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.
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 Earth’s
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.
Answers
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
c
d
a
d
e
c
c
e
e
e
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
c
a
e
d
b
e
d
d
e
b