Transcript DTU_9e_ch09 - University of San Diego Home Pages

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 9
Vagabonds of the Solar System
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
Are the asteroids a former planet that was
somehow destroyed? Why or why not?
How far apart are the asteroids on average?
How are comet tails formed? Of what are they
made?
In which directions do a comet’s tails point?
What is a shooting star?
In this chapter you will discover…
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the properties of dwarf planets and small solar system
bodies
asteroids and meteoroids—pieces of interplanetary rock
and metal
comets—objects containing large amounts of ice and
rocky debris
space debris that falls through Earth’s atmosphere
the asteroid belt and the Kuiper belt, both filled with a
variety of debris, including orbiting pairs of objects
the impacts from space 250 million and 65 million years
ago that caused mass extinctions of life on Earth
wayward asteroids that could again threaten life on Earth
Different Classifications of Solar System Objects
Some of the definitions of the
different types of objects in the
solar system overlap. For
example, the largest asteroids
are also being classified as
dwarf planets; various transNeptunian objects (TNOs) are
asteroids or comets; some
comets are satellites of Jupiter;
some Kuiper belt objects
(KBOs) are satellites of other
KBOs. Furthermore, TNOs
exist in two groups: Kuiper belt
objects and Oort comet cloud
bodies.
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Dwarf Planet is an ADDITIONAL classification for an
object which also falls into another category.
Eris: Kuiper Belt Object
Pluto: Kuiper Belt Object
Makemake [mah-ke-mah-ke]: Kuiper Belt Object
Ceres: asteroid
Haumea: Kuiper Belt Object
A dwarf planet orbits the Sun and has enough gravity
to pull itself spherical, but does NOT clear the
vicinity of its orbit.
Pluto
These three Hubble Space Telescope images of Pluto show
little detail but indicate that the major features of Pluto’s surface
each cover large amounts of its area. Comparing these
observations to previous ones reveals that the surface changes
in color and brightness seasonally.
Discovery of Pluto
Pluto was discovered in 1930 by searching for a dim, starlike
object that slowly moved against the background stars. These
two photographs were taken 1 day apart.
Orbit of Pluto
(a) The high-eccentricity orbit of dwarf planet /KBO) Pluto stands
out compared to those of the planets. Notice how many
significant events occurred on Earth during Pluto’s present orbit
of the Sun. (b) Pluto’s passes inside the orbit of Neptune, but the
two will never collide due to a 3:2 synchronization.
Orbit of Pluto
(c) A nearly edge-on view of the ecliptic and Pluto’s
orbit compared to it.
Discovery of Charon
Long ignored as just a
defect in the photographic
emulsion, the bump on the
upper left side of this
image of Pluto led
astronomer James Christy
to discover the moon
Charon.
Pluto’s Small Moons Nix and Hydra
The Hubble Space Telescope in 2005 revealed two small
moons, each about 5000 times dimmer than Pluto. Named Nix
and Hydra, they are between 2 and 3 times farther from Pluto
than is its moon Charon. The lines radiating from Pluto and
Charon are artifacts of the exposure.
Pluto was de-planetized because
 a) international sentiment turned against
the U. S.
 b) Pluto is spherical in shape.
 c) Pluto does not gravitationally
dominate its neighborhood.
 d) Pluto is inside the Oort Cloud.
 e) Pluto has only three satellites.
Comparison of Ceres with the Moon and Earth
Ceres, the Moon, and Earth are shown here to scale. Dwarf
planet Ceres is the largest asteroid but too small to be
considered a planet. This image of Ceres suggests it has
regions of ice and rock on its surface. The asteroid will be
visited by the Dawn spacecraft in 2015, which visited Vesta.
Dwarf Planet Eris
Three perpendicular views of the orbit of Eris and Dysnomia
are compared to the planets and Pluto. Eris and Dysnomia’s
orbit around the Sun ranges from 38 to 98 AU, with orbital
eccentricity, e = 0.44, and an orbital inclination of 44°.
Dwarf Planet Eris
This is a Keck Telescope image of dwarf planet Eris and its moon Dysnomia.
Asteroid Orbits
(a) The orbits of belt asteroids Ceres, Pallas, and Juno are indicated to
scale in this diagram. (b) These are the actual positions of all known
asteroids at Jupiter’s orbit or closer. The locations of the belt asteroids are
indicated by green dots. Objects passing closer than 1.3 AU to the Sun are
shown by red circles.. Jupiter’s Trojan asteroids are deep blue squares.
Discovering Asteroids
In 1998, the Hubble Space Telescope found this asteroid while
observing objects in the constellation Centaurus. The exposure,
tracking stars, shows the asteroid as a 19-arcsec streak. This asteroid
is about 2 km in diameter and was located about 140 million km (87
million mi) from Earth.
The Kirkwood Gaps
This graph displays the number of asteroids at various distances from
the Sun. Note that few asteroids have orbital periods that correspond to
such simple fractions as 1⁄3, 2⁄5, 3⁄7, and 1⁄2 of Jupiter’s orbital period.
Resonant orbits with Jupiter have deflected asteroids away from these
orbits. The Trojan asteroids accompany Jupiter as it orbits the Sun.
Collision Between Two Asteroids
Observed in 2010, this X-shaped “object” (inset) is believed to be the
collision of two small asteroids. The event created dust that was
pushed away from the Sun, which is to the left and below this image.
The collision occurred 2 AU from the Sun and 1 AU from Earth.
The Kirkwood gaps in the asteroid belt are
 a) produced by very large asteroids
sweeping out paths in the asteroid belt.
 b) produced by a gravitational interaction
with Jupiter.
 c) present because asteroids only
formed at certain distances from the Sun.
 d) temporary features that are constantly
changing their distance from the Sun.
Ida and Its Satellite
The 55-km-long rocky asteroid Ida, shown here with its
satellite Dactyl. Inset: Dactyl is also heavily cratered.
Jupiter’s Trojan
Asteroids
Groups of asteroids orbit at
the two stable Lagrange
points along Jupiter’s orbit,
trapped by the combined
gravitational forces of
Jupiter and the Sun.
Asteroid 1994 XM1
This image was obtained on December 9, 1994, shortly
before the near Earth asteroid arrived in Earth’s vicinity.
When it passed by Earth just 12 h later, asteroid 1994 XM1
was less than half the distance from Earth to the Moon.
Asteroids Mathilde and Itokawa
Reflecting only half as much light
as a charcoal briquette,
Mathilde is half as dense as
typical stony asteroids. Size:
46 x 48 x 66 km.
The near-Earth asteroid Itokawa
was visited by the Japanese space
probe Hayabusa. Samples of dust
particles from it are now being
analyzed.
Asteroid Eros was landed on
The Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft took
these images of asteroid Eros in February 1999. (a) Eros’s dimensions are
33 km x 13 km x 13 km (21 mi x 8 mi x 8 mi) and it rotates every 5¼ h. Its
density is 2700 kg/m3, close to the average density of Earth’s crust. (b)
image looking into the large crater near the top of (a), which is 5.3 km (3.3
mi) across. (c) This is the penultimate image taken by NEAR Shoemaker
Just before it gently landed on Eros,, from an altitude of 250 m (820 ft), the
image is only 12 m across. You can see rocks and boulders.
Current Positions of Known Dwarf Planets
and SSBs in the Outer Solar System
Kuiper Belt Objects
(a,
b) These 1993 images show the
discovery (white lines) of 1 of more than
1524 known KBOs. These two images of
KBO 1993 SC were taken 4.6 h apart,
during which time the object moved
against the background stars.
(c) The KBO 1998
WW31 and its moon
(lower left).
Sedna’s Orbit: Oort Cloud?
(a)
The farthest known body in the solar system is in a highly
elliptical orbit (b) that ranges from the outer reaches of the Kuiper
belt and possibly extends to the inner Oort comet cloud.
Comet Nuclei – Halley and Borrelly
Composite Picture of Comet Wild 2
Anatomy of a Comet
Comet Wild 2
A substance called aerogel was used to capture particles from Comet
Wild 2’s dust tail. A piece of space debris pierced the aluminum foil
holding the aerogel and embedded in it, along with pieces of the foil.
Comet Wild 2
A 2-µm piece of comet dust, composed of a mineral called forsterite.
On Earth this mineral is used to make gems called peridot.
Comet Hale-Bopp
In 1997, Comet Hale-Bopp had a hydrogen envelope 1 AU in diameter
(blue ovals). This gas was observed in the ultraviolet. The visible light
inset shows the scale of the visible tails (see also the image at the
opening of this chapter).
Comet West
Astronomer Michael M. West first noticed this comet on a photograph
taken with a telescope in 1975. After passing near the Sun, Comet
West became one of the brightest comets of the 1970s. This
photograph shows the comet in the predawn sky in March 1976.
The Orbit and Tails of a Comet
The sunlight and solar wind blow a comet’s dust particles and
ionized atoms away from the Sun. Consequently, comets’ tails
always point away from the Sun.
The Two Tails of Comet Mrkos
(a) Comet Mrkos dominated the evening sky in August 1957.
These three views, taken at 2-day intervals, show dramatic
changes in the comet’s gas tail. In contrast, the slightly
curved dust tail remained fuzzy and featureless.
Comet Tails and Smoke from Forest Fires
Which statement about comet tails is
correct?
 a) comet tails point in the direction from
which the comet came.
 b) comet tails point in the direction the
comet is heading
 c) tails point away from the Sun
 d) comet tails are caused by friction
The Head of Comet Brooks
This comet had an exceptionally large, bright coma. Named
after its discoverer, William R. Brooks, it dominated the night
skies in October 1911.
The Tail of Comet Ikeya-Seki
Named after its codiscoverers in Japan, this comet dominated
the predawn skies in late October 1965. The yellow in the tail
comes from emission by sodium atoms in the dust that was
released by the comet. Although its coma was tiny, its tail
spanned over 1 AU.
Comet Tempel 1 – Struck by Deep Impact
(a) This composite image of Comet Tempel 1 has higher resolution at
the bottom, as the projectile from Deep Impact headed in that
direction. The smooth regions on the comet have yet to be explained.
(b) This image was taken 30 s before the projectile struck the comet.
Comet Tempel 1 – Impact Sequence
(c) Seconds after impact, hot debris explodes away from the comet
nucleus. The white horizontal half-ellipses are areas where the
CCDs were overloaded with light from the event. (d) Moments later,
the gases and dust were expanding outward. (e) This is an image
taken 67 s after impact. Within minutes, the cloud of debris
eventually became much larger than the entire nucleus.
Transformation and
Evolution of a LongPeriod Comet
The gravitational force of a giant
planet can change a comet’s
orbit. Comets initially on highly
elliptical orbits are sometimes
deflected into more circular
paths that keep them in the
inner solar system. (b–d) These
figures show the evolution of a
comet into gas, dust, and
rubble, and why debris from
some of these comets strikes
Earth.
The Fragmentation of Comet Schwassmann-Wachmann-3
This comet, with a 5.4-year orbit, has been coming apart for decades.
In 2006, it further fragmented after passing perihelion. One piece,
Fragment B, shed at least 30 smaller pieces, shown here.
Comet Hale-Bopp
Discovered in 1995, this was the Great Comet of 1997. Inset: Jets of gas and
debris were observed shooting out from Comet Hale-Bopp several times. This
image shows the comet nucleus (lower bright region), an ejected piece of the
comet’s surface (upper bright region), and a spiral tail. The ejected piece
eventually disintegrated, following the same spiral pattern as the tail.
Sungrazing Comet
Comet SOHO LASCO C3 is shown in the smaller box and magnified in the
larger one. Discovered in March 2004, it was the 750th sungrazing comet
discovered from the SOHO data. It completely sublimated near perihelion.
Meteor
This brilliant meteor is seen lighting up the dark skies of the
California desert area of Joshua Tree National Park. Just to the
right of the meteor trail are the Pleiades.
Meteorite Impact,
Poughkeepsie, NY, 1992
Meteor Crater near Winslow Arizona
An iron meteor measuring 50 m across struck the ground in Arizona
50,000 years ago, resulting in this beautifully symmetric impact crater.
The Origin of Meteor Showers
As comets dissipate, they leave debris behind that spreads out along their
orbits. When Earth plows through such material, many meteors can be seen
emanating from the same place within a very short time—a meteor shower.
As shown in this diagram, many comets have high orbital inclinations.
Meteor Shower
This time exposure, taken in 1998, shows meteors streaking away
from the constellation Leo Major. They are part of the Leonid meteor
shower. This shower occurs because Earth is moving through debris
left by comet Temple-Tuttle.
Recent Impacts on the Moon
The locations A–F are places on the Moon where impacts were observed
from Earth in 1999 during the Leonid meteor shower. The impacting bodies
hit the Moon at around 260,000 km/hr (160,000 mph) and had masses of
between 1 and 10 kg. Each impact created a short-lived cloud that
momentarily heated to between 5 x 104 and 10 x 104 K, much hotter than the
surface of the Sun.
The Mass of Impacts on Earth
The Vatican Obelisk is about 300 tons, the amount of mass that
strikes Earth daily. As a result, Earth’s mass increases by this
amount every day.
Stony Meteorites
(a) Most meteorites that fall to Earth are
stones. Many freshly discovered
specimens, like the one shown here,
are coated with thin, dark crusts. This
stony meteorite fell in Morocco.
(b) Some stony meteorites
contain tiny specks of iron, which
can be seen when the stones are
cut and polished. This specimen
was discovered in Ohio.
Iron Meteorites
(a) Irons are composed almost entirely of
iron-nickel minerals. The surface of a
typical iron is covered with thumbprint-like
depressions created as the meteorite’s
outer layers vaporized during its highspeed descent through the atmosphere.
This specimen was found in Argentina.
(b) When cut, polished, and etched
with a weak acid solution, most iron
meteorites exhibit interlocking crystals
in designs, called Widmanstätten
patterns. This meteorite was found in
Australia.
Stony-Iron Meteorite
Stony-irons account for about 1% of all meteorites that fall to Earth. This
specimen, a variety of stony-iron called a pallasite, was found in Antarctica. It
is thinly cut and appears to glow because of a light located behind it.
Pieces of the Allende Meteorite
(a) This carbonaceous chondrite fell near
Chihuahua, Mexico, in February 1969. Note
the meteorite’s dark color, caused by a high
abundance of carbon. Geologists believe that
this meteorite is a specimen of primitive
planetary material. The ruler is 15 cm long.
(b) Sliced open, the
Allende meteorite shows
round, rocky inclusions
called chondrules in a
matrix of dark rock.
Finding a Meteorite in Antarctica
Good places to find meteorites include deserts and ice-covered regions,
such as Antarctica. By surveying such areas, astronomers and geologists
can accurately determine the correct percentage of each of the different
types of meteorites.
Meteor showers originate
 a) from meteoroids in space whose
mutual gravity causes them to form a
small bunch.
 b) from a trail of debris left behind by a
comet.
 c) from groups of meteoroids that have
randomly bunched together.
 d) from an asteroid that was long ago
pulverized in a collision.
Aftermath of the Tunguska (Siberia) Event 1908
In 1908, a stony asteroid traveling at supersonic speed struck Earth’s
atmosphere and exploded over the Tunguska region of Siberia. Trees
were blown down for many kilometers in all directions from the impact site.
Iridium-Rich Layer of Clay
This photograph of strata in the Apennine Mountains of Italy shows a darkcolored layer of iridium-rich clay sandwiched between white limestone
(bottom) from the late Mesozoic era and grayish limestone (top) from the
early Cenozoic era. The coin is the size of a U.S. quarter.
Confirming an Extinction-Level Impact Site
By measuring slight variations in the
gravitational attraction of different materials
under Earth’s surface, geologists create
images of underground features. Concentric
rings of the underground Chicxulub Crater
(right inset) lie under a portion of the Yucatán
Peninsula. This crater has been dated to 65
million years ago and is believed to be the
site of the impact that led to the extinction of
the dinosaurs. A piece of 65-million-year-old
meteorite discovered in the middle of the
Pacific Ocean in 1998 is believed to be a
fragment of that meteorite. The fragment,
about 0.3 cm (0.1 in.) long, was cut into two
pieces for study (left inset).
Summary of Key Ideas
Asteroids
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Pieces of solar system debris larger than 10 m and
composed primarily of rock and metal are called
asteroids.
Tens of thousands of belt asteroids with diameters larger
than a kilometer are known to orbit the Sun between the
orbits of Mars and Jupiter. The gravitational attraction of
Jupiter depletes certain orbits within the asteroid belt.
The resulting Kirkwood gaps occur at simple fractions of
Jupiter’s orbital period.
Jupiter’s and the Sun’s gravity combine to capture Trojan
asteroids in two locations, called stable Lagrange points,
along Jupiter’s orbit.
Asteroids
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The Apollo asteroids move in highly elliptical orbits that
cross the orbit of Earth. Many of these asteroids will
eventually strike the inner planets.
A belt asteroid, Ceres, along with four KBOs (Pluto, Eris,
Haumea, and Makemake) are classified as dwarf
planets.
Pluto, a KBO and dwarf planet, is an icy world that may
well resemble the moon Triton.
Comets
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Comet nuclei are fragments of ice and rock often orbiting
at a great inclination to the plane of the ecliptic. In the
Kuiper belt and Oort cloud, comets have fairly circular
orbits. When close to the Sun, they generally move in
highly elliptical orbits.
Many comet nuclei orbit the Sun in the Kuiper belt, a
doughnut-shaped region beyond Pluto. Billions of
cometary nuclei are also believed to exist in the
spherical Oort cloud located far beyond the Kuiper belt.
As an icy comet nucleus approaches the Sun, it
develops a luminous coma surrounded by a vast
hydrogen envelope. A gas (or ion) tail and a dust tail
extend from the comet, pushed away from the Sun by
the solar wind and radiation pressure.
Meteoroids, Meteors, and Meteorites
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Boulder-sized and smaller pieces of rock and metal in
space are called meteoroids. When a meteoroid enters
Earth’s atmosphere, it produces a fiery trail, and it is then
called a meteor. If part of the object survives the fall, the
fragment that reaches Earth’s surface is called a
meteorite.
Meteorites are grouped in three major classes according
to their composition: iron, stony-iron, and stony meteorites.
Rare stony meteorites, called carbonaceous chondrites,
may be relatively unmodified material from the primordial
solar nebula. These meteorites often contain organic
hydrocarbon compounds, including amino acids.
Fragments of rock from “burned-out” comets produce
meteor showers.
Meteoroids, Meteors, and Meteorites
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An analysis of the Allende meteorite suggests that a
nearby supernova explosion may have been involved in
the formation of the solar system some 4.6 billion years
ago.
An asteroid that struck Earth 65 million years ago probably
contributed to the extinction of the dinosaurs and many
other species. Another impact may have caused the
“Great Dying” of life 250 million years ago. Such
devastating impacts occur on average every 100 million
years.
Key Terms
amino acid
Apollo asteroid
asteroid belt
belt asteroid
carbonaceous chondrite
chondrites
coma (of a comet)
dust tail
dwarf planet
gas (ion) tail
hydrogen envelope
impact crater
iron meteorite
Kirkwood gaps
long-period comet
meteor
meteor shower
meteorite
meteoroid
nucleus (of a comet)
Oort cloud
planet
radiation (photon)
pressure
short-period comet
small solar-system
bodies (SSSBs)
stable Lagrange
points
stony meteorite
stony-iron
meteorite
Trojan asteroid
Widmanstätten
Patterns
WHAT DID YOU THINK?
Are the asteroids a former planet that was
somehow destroyed? Why or why not?
 No. The gravitational pull from Jupiter
prevented a planet from ever forming in
the asteroid belt. Also, the total mass of
the asteroids is much less than even the
mass of tiny Pluto, a dwarf planet.
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WHAT DID YOU THINK?
How far apart are the asteroids on
average?
 The distance between asteroids averages
10 million km.
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WHAT DID YOU THINK?
How are comet tails formed? Of what are
they made?
 Ices in comet nuclei are turned into gas by
absorbing energy from the Sun. Debris is
released in this process. Sunlight and the
solar wind push on the gas and dust,
creating the tails.
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WHAT DID YOU THINK?
In what directions do a comet’s tails point?
 Comets’ gas tails point directly away from
the Sun; their dust tails make arcs pointing
away from the Sun.
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WHAT DID YOU THINK?
What is a shooting star?
 A shooting star is a piece of space debris
plunging through Earth’s atmosphere—a
meteor. It is not a star.
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