Chapter 9
Remnants of Rock and Ice
Asteroids, Comets, and Pluto
9.1 Asteroids and Meteorites
Our Goals for Learning
• Why is there an asteroid belt?
• How are meteorites related to asteroids?
Asteroid
Facts
• Asteroids are rocky leftovers of planet formation.
• Largest is Ceres, diameter ~1,000 km
• 150,000 in catalogs, and probably over a million with
diameter >1 km.
• Small asteroids are more common than large asteroids.
• All the asteroids in the solar system wouldn’t add up to
even a small terrestrial planet.
Asteroids are cratered and not round
Why is there an asteroid belt?
More than
150,000
asteroids at
their
predicted
locations for
Jan 1 2004
On this
scale,
asteroids are
much
smaller than
the dots used
to represent
them
Why are there very few asteroids
beyond Jupiter’s orbit?
A. There was no rocky material beyond Jupiter’s
orbit.
B. The heaviest rocks sank towards the center of the
solar system.
C. Ice could form in the outer solar system.
D. A passing star probably stripped away all of
those asteroids, even if they were there at one
time.
Why are there very few asteroids
beyond Jupiter’s orbit?
A. There was no rocky material beyond Jupiter’s
orbit.
B. The heaviest rocks sank towards the center of the
solar system.
C. Ice could form in the outer solar system.
D. A passing star probably stripped away all of
those asteroids, even if they were there at one
time.
Which explanation seems to be
the most plausible?
A. The belt is where all the asteroids
happened to form.
B. The belt is the remnant of a large
terrestrial planet that used to be between
Mars and Jupiter.
C. The belt is where all the asteroids
happened to survive.
Which explanation seems to be
the most plausible?
A. The belt is where all the asteroids
happened to form.
B. The belt is the remnant of a large
terrestrial planet that used to be between
Mars and Jupiter.
C. The belt is where all the asteroids
happened to survive.
But WHY didn’t they
form a little planet?
Rocky planetesimals
survived in the asteroid
belt between Mars and
Jupiter because they did
not accrete into a planet.
Jupiter’s gravity, stirs
up the asteroid orbits
and prevents their
planet formation.
How are meteorites related to
asteroids?
How are meteorites related to
asteroids?
Meteorites are pieces of asteroids - or
sometimes planets or the Moon.
Meteor: The bright tail of hot debris from the rock
Meteorite: A rock from space that reaches Earth’s surface
Peekskill, NY:
October 9, 1992
Pieces of Asteroids:Meteorite
Types
1) Primitive: Unchanged in composition
since they first formed 4.6 billion years
ago.
2) Processed: Younger, have experienced
processes like volcanism or
differentiation.
Primitive Meteorites: simple, all
ingredients mixed together
Processed Meteorites:
shattered fragments of larger objects
Iron
from a
core
Volcanic rock from a crus
or mantle
What do we learn from
meteorites?
• primitive meteorites tell us when solar
system formation began.
• Processed meteorites tell us what asteroids
are like on the inside.
• Processed meteorites provide direct proof
that differentiation and volcanism happened
on asteroids.
Meteorites from Moon and Mars
• A few meteorites arrive from the Moon and Mars
• Composition differs from the asteroid fragments.
• A cheap (but slow) way to acquire moon rocks and
Mars rocks.
• One Mars meteorite generated a stir when scientists
claimed evidence for microscopic life in it.
What have we learned?
• Why is there an asteroid belt?
• Orbital resonances with Jupiter
disrupted the orbits of
planetesimals, preventing them
from accreting into a planet.
Those that were not ejected
from this region make up the
• How are meteorites related to
asteroid belt today. Most
asteroids?
asteroids in other regions of the
inner solar system accreted into • Most meteorites are pieces of
asteroids. Primitive meteorites
one of the planets.
are essentially unchanged since
the birth of the solar system.
Processed meteorites are
fragments of larger asteroids
that underwent differentiation.
9.2 Comets
• Our Goals for Learning
• How do comets get their tails?
• Where do comets come from?
How do comets
get their tails?
Comet Facts
• Formed beyond the frostline, comets are icy
counterparts to asteroids.
• “Dirty snowballs” = the nucleus
• Most comets do not have tails.
• Most comets remain perpetually frozen in
the outer solar system. Only a few enter the
inner solar system, where they can grow
tails.
When a comet nears the Sun, its ices can sublimate into
gas and carry off dust, creating a coma and long tails.
Draw This
Picture
Comets eject small particles that follow the comet around
in its orbit
This can cause meteor showers when Earth crosses the
comet’s orbit.
Meteors in a shower appear to emanate from the same area of sky
because of Earth’s motion through space
Where do comets come from?
Only a tiny number
of comets enter the
inner solar system most stay far from
the Sun
Oort cloud:
On random orbits
extending to about
50,000 AU
Kuiper belt:
On orderly orbits
from 30-100 AU in
disk of solar
system
How did they get there?
• Kuiper belt comets align with the plane of
planet orbits
• Oort Cloud Comets were kicked out of the
solar system by the gravity from jovian
planets: random orbits
What have we learned?
• How do comets get their tails?
• The vast majority of comets do
not have tails. Only those few
comets that enter the solar
system grow tails. As the comet
approaches the Sun its nucleus
heats up. Some of the comet’s
ice sublimates into gas, and the
escaping gases carry along
some dust. The gas and dust
form a coma and two tails: a
plasma tail of ionized gas and a
dust tail. Larger particles can
also escape, becoming the
particles that cause meteors and
meteor showers on Earth.
What have we learned?
• Where do comets come from?
• Comets that enter the solar
system come from one of two
reservoirs in the outer solar
system: the Kuiper belt and the
Oort cloud. The Kuiper belt
comets still reside in the region
beyond Neptune in which they
formed during the birth of the
solar system. The Oort cloud
comets are thought to have
formed in the region of the
jovian planets, and were kicked
out to the great distance of the
Oort cloud by gravitational
encounters with the planets.
9.3 Pluto: Lone Dog
or Part of a Pack?
• Our Goals for Learning
• What is Pluto like?
• Is Pluto a planet or a Kuiper belt comet?
Pluto: the exception
• Not a gas giant like the other outer planets.
• Has a very elliptical, inclined orbit.
• By far the smallest planet, and smaller than
several moons.
• Has a surprisingly large moon Charon, probably
formed by a huge comet collision with Pluto.
Pluto will never collide with Neptune because
of a 3:2 orbital resonance.
What is Pluto like?
• 1978 discovery of Pluto’s moon Charon:
Pluto’s mass from Newton’s orbital law.
• It has a thin nitrogen atmosphere that will
refreeze onto the surface as Pluto’s orbit
takes it farther from the Sun.
• Pluto is the largest Solar System object that
has not been visited by spacecraft.
HST’s view of Pluto & Charon
Brightness variations during eclipsing orbits showed
dirty ice - like comets.
Is Pluto a planet or a Kuiper Belt
comet?
Is Pluto a planet or a Kuiper Belt
comet?
• Pluto is well beyond Neptune, in the Kuiper
Belt.
• Inclined orbit is typical of Kuiper Belt
comets.
• Composition is typical of Kuiper Belt
comets, but not any of the other planets.
Is Pluto a planet or a Kuiper Belt
comet?
• Kuiper Belt objects have been found that
approach Pluto’s size.
• Kuiper Belt comets have similar orbital
resonances with Neptune.
• Kuiper Belt comets can have moons.
• Triton (a captured moon) is even larger than
Pluto.
What have we learned?
• What is Pluto like?
• Pluto is much smaller than
any other planet, with an orbit
more elliptical and more
inclined to the ecliptic plane
than that of any other planet.
It is made mostly of ices and
has a very thin atmosphere of
gases that are expected to
freeze onto the surface as
Pluto moves farther from the
Sun in its 248-year orbit. It
has a moon, Charon, with a
slightly lower density than
Pluto, suggesting that Charon
may have been formed in a
giant impact.
• Is Pluto a planet or a Kuiper
belt comet?
• Whether Pluto should be
called a “planet” is a matter
of opinion, but its properties
suggest that it is a Kuiper belt
comet. Its composition and
orbital properties match those
of other Kuiper belt comets
and do not fit in with the
other planets. It is the largest
known Kuiper belt comet
today, but there may be larger
ones still awaiting discovery.
9.4 Cosmic Collisions: small
bodies vs. the planets
Our Goals for Learning
• Have we ever witnessed a major impact?
• Did an impact kill the dinosaurs?
• Is the impact threat a real danger or just media
hype?
• How do other planets affect impact rates and life
on Earth?
Have we ever witnessed a major
impact?
Comet SL9 caused a string of
violent impacts on Jupiter in 1994,
reminding us that catastrophic
collisions still happen.
Tidal forces tore it apart during
previous encounter with Jupiter
Impact plume rises
high above Jupiter’s
surface
Did an impact kill the dinosaurs?
Mass Extinctions
• Large dips in total species diversity in the
fossil record.
• The most recent was 65 million years ago,
ending the reign of the dinosaurs.
Was it caused by an impact?
How would it have happened?
No dinosaur
fossils in these
rock layers
Thin layer
containing iridium
from impactor
Dinosaur fossils
in lower rock
layers
Iridium - evidence of an impact
• Iridium is very rare in Earth surface rocks
but often found in meteorites.
• Luis and Walter Alvarez found a worldwide
layer containing iridium, laid down 65
million years ago.
Comet or
asteroid about
10km in
diameter
approaches
Earth
An iridium-rich
sediment layer and
an impact crater on
the Mexican coast
65 million years
ago.
shows that a large
impact occurred
at the time the
dinosaurs died out,
The Impact Threat:
Real danger or media hype?
Facts
• Asteroids and comets have hit the Earth.
• A major impact is only a matter of time: not IF but
WHEN.
• Major impact are very rare.
• Extinction level events ~ millions of years.
• Major damage ~ tens-hundreds of years.
Tunguska, Siberia: June 30, 1908
The ~40 meter object disintegrated and exploded in the
atmosphere
Meteor Crater, Arizona: 50,000 years ago (50 meter object)
This slide is placeholder in case you would like to
download and show the movie noted below (notes
field).
An object 1km in size would produce a mile-high tidal wave
Impacts will certainly occur in the future, and while the
chance of a major impact in our lifetimes is small, the
effects could be devastating.
The asteroid with our name on it
We haven’t seen it yet.
Deflection is more probable with years of
advance warning.
Control is critical: breaking a big asteroid into
a bunch of little asteroids is unlikely to help.
We get less advance warning of a killer
comet…
What are we doing about it?
• Stay tuned to
http://impact.arc.nasa.gov
How do other planets
affect impact rates
and life on Earth?
Gravity from
Jovial planets
can influence
the path of
comets and
Asteroids.
They could
protect us or
steer one in
our direction
Fig 9.20
Was Jupiter necessary for life
on Earth?
Impacts can extinguish
life.
But were they
necessary for ‘life as
we know it’?
What have we learned?
• Have we ever witnessed
a major impact?
• In 1994, we observed
the impacts of comet
Shoemaker–Levy 9 on
Jupiter. The comet had
fragmented into a string
of individual nuclei, so
there was a string of
impacts that left
Jupiter’s atmosphere
scarred for months
What have we learned?
• Did an impact kill the
dinosaurs?
• We are not certain whether an
impact was the sole cause, but
a major impact clearly
coincided with the mass
extinction in which the
dinosaurs died out, about 65
million years ago. Sediments
from the time show clear
evidence of an impact, and an
impact crater of the right age
has been found near the coast
of Mexico.
What have we learned?
• Is the impact threat a
real danger or just
media hype?
• Impacts certainly pose
a threat, though the
probability of a major
impact in our lifetimes
is fairly low.
What have we learned?
• How do other planets affect impact rates and life
on Earth?
Impacts of asteroids and comets are always linked in at
least some way to the gravitational influences of
Jupiter and the other jovian planets. These
gravitational influences have shaped the asteroid belt,
the Kuiper belt, and the Oort cloud, and sometimes
still help determine when an object is flung our way.