Transcript 13. Remnants of Rock and Ice: Asteroids, Comets, and Pluto

13. Remnants of Rock and Ice:
Asteroids, Comets, and Pluto
As we look out into the Universe and
identify the many accidents of physics and
astronomy that have worked to our benefit,
it almost seems as if the Universe must in
some sense have known that we were
coming.
Freeman Dyson (1923 – )
physicist
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13.1 Remnants from Birth
Our goals for learning:
• What are the three major groups of small bodies
in the Solar System?
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Small Bodies in the Solar System
• Small bodies, the leftover “scraps” from the formation
of the Solar System, fall into three distinct groups:
1 asteroids
• rocky or metallic in composition
• most are located between the orbits of Mars and Jupiter
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Small Bodies in the Solar System
2 Kuiper belt comets
• made mostly of ice
• orbit the Sun beyond Neptune
• orbit in same direction and plane
as the planets
3 Oort cloud comets
• made mostly of ice
• orbit at the outer fringe of the
Solar System
• spherically distributed about the
Sun
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A Note on Names
• asteroid – a rocky leftover planetesimal orbiting the
Sun
• comet – an icy leftover planetesimal orbiting the Sun,
regardless of its size or whether it has a tail
• meteor – a flash of light in the sky caused by a particle
entering the atmosphere, regardless of its origin
• meteorite – any piece of rock that fell to the ground
from space, regardless of its origin
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13.2 Asteroids
Our goals for learning:
• Describe asteroid sizes, shapes, and orbits.
• Why didn’t a planet form in the region of the
asteroid belt?
• How do we measure asteroid properties?
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Properties of Asteroids
• They are small in size.
• the largest one, Ceres, is only
1,000 km across
• They are not spherical in
shape.
• shaped more like “potatoes”
• gravity not strong enough to
compress rocky material
• odd shapes imply that some
are fragments from asteroid
collisions
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Properties of Asteroids
• Asteroid orbits are more
elliptical & inclined than
planetary orbits.
• Most asteroids are located
in the asteroid belt.
• between the orbits of Mars &
Jupiter
• Some share Jupiter’s orbit.
• two swarms at 60º in front of
and behind Jupiter
• known as Trojan asteroids
• A few cross Earth’s orbit.
• they are called Near-Earth
asteroids
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The Asteroid Belt
• Jupiter’s gravity disrupted
the orbits of those asteroids
whose periods were an
integer fraction of Jupiter’s.
• orbital resonances created
gaps… like in Saturn’s rings
• known as the Kirkwood gaps
• This explains why no planet formed in the asteroid belt.
• tugs from Jupiter’s gravity prevented the planetesimals from
accreting into a planet
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Measuring Asteroid Properties
• Size
• the larger the asteroid, the more sunlight it will reflect
• measuring the brightness and knowing reflectivity &
distance gives us the size.
• reflectivity is calculated from the visual & IR brightness
• Mass
• measure the effect gravity has on
a passing spacecraft or a moon
• use Kepler’s & Newton’s laws
• Density
• calculate from mass & size
• mass  volume
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Measuring Asteroid Properties
• Shapes
• measure the asteroid’s changes in brightness as it rotates
• the asteroid’s shape can be reconstructed from this
• we can bounce radar signals off of Near-Earth asteroids
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Measuring Asteroid Properties
• Composition
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examine the spectrum of sunlight reflected off the asteroid
look for non-Solar absorption lines in the spectrum
Three categories of asteroid composition:
1. very dark asteroids which contain Carbon-rich materials
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found in outer regions of the asteroid belt
2. brighter asteroids which contain rocky materials
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found in inner regions of the asteroid belt
3. asteroids which contain metals such as Iron
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13.3 Meteorites
Our goals for learning:
• How is a meteor different from a meteorite?
• How are meteorites categorized?
• Why do meteorites differ from one another?
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Rocks Falling from the Sky
• meteor – a flash of light caused by a particle which
enters Earth’s atmosphere.
• most of these particles are the size of a pea
• they completely burn up in Earth’s atmosphere
• meteorite – a rock which is large enough to have
survived its fall to Earth
• they caused a brighter meteor…sometimes called a fireball
• How can you tell that you have a meteorite?
– they have a higher metal content than terrestrial rocks
– they contain Iridium and other isotopes not found in
terrestrial rocks
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Types of Meteorites
Based on composition, meteorites fall into two basic categories:
• primitive
• about 4.6 billion years old
• accreted in the Solar nebula
• processed
• younger than 4.6 billion years
• matter has differentiated
• fragments of a larger object
which processed the original
Solar nebula material
• Each type of meteorite can be divided into two subcategories:
• primitive meteorites can be either stony, containing rocky minerals & metals,
or Carbon-rich, containing Carbon compounds or even water
• processed meteorites can be either metallic, high-density Iron/Nickel like
Earth’s core, or rocky, containing low-density material similar to earth’s crust
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Origin of Meteorites
• Primitive meteorites condensed and accreted directly
from the Solar nebula.
• the stony ones formed closer than 3 AU from the Sun
• the Carbon-rich ones formed beyond 3 AU from the Sun,
where it was cold enough for Carbon compounds to condense
• Processed meteorites come from large objects in the
inner Solar System.
• the metallic ones are fragments of the cores of asteroids which
were shattered in collisions
• the rocky ones were chipped off the surfaces of asteroids,
Mars, and the Moon by impacts
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13.4 Comets
Our goals for learning:
• What are comets made of?
• What happens to a comet as it approaches the
Sun?
• How do we know that vast numbers of comets
reside in the Oort cloud and Kuiper belt?
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Comets
• One of the most beautiful sights in the sky.
• Throughout human history, these “hairy” stars would appear.
• like planets, they moved with respect to the fixed stars
• unlike planets, they were not confined to the ecliptic and disappeared after
several weeks
• They were taken as omens of good or bad fortune.
Recent Comets:
Hale-Bopp
Hyakutake
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1986 Halley’s Comet
1996 Comet Hyakutake
1997 Comet Hale-Bopp
Dozens per year too dim
to be seen by eye
Comets
• Edmund Halley (1656 – 1742)
• first to realize that comets orbit the Sun
• predicted the return of a comet which had
been seen every 76 years
• the comet returned in 1758 and now bears his
name
• The Orbits of Comets:
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Movie. Click to play.
Composition and Structure of Comets
• Comets are “dirty snowballs”…ice mixed with rock and dust.
• ices are H2O, CO2, CO, NH3, CH4
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nucleus
• the “dirty snowball”
• how the comet appears far
from the Sun
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coma
• surrounds nucleus when near
the Sun
• sublimated gas & dust
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plasma tail
• ionized gas swept back by
Solar wind
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dust tail
• dust particles swept back
more slowly by radiation
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A Comet’s Journey
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A Comet’s Journey
• The SOHO telescope observed
Comet NEAT (C/2002 V1) round
the Sun on Feb 16, 2002.
• courtesy of SOHO/LASCO
consortium. SOHO is a project of
ESA and NASA.
• A comet can only visit the Sun a few hundred times before
losing all its ice to sublimation.
• the comet may then disintegrate
• or the rocky remains may stick together as an asteroid
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The Origin of Comets
• We can tell where comets
originate by measuring their
orbits as they visit the Sun.
• Most approach from random
directions and do not orbit in
the same sense as the planets.
• they come from the Oort cloud
• Others orbit along the ecliptic
plane in the same sense as the
planets.
• they come from the Kuiper belt
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13.5 Pluto: Lone Dog or Part of a Pack?
Our goals for learning:
• Why don’t Pluto and Neptune collide?
• What is surprising about Pluto’s density, and
how might it have come to be?
• Why do we think Pluto is a Kuiper belt comet?
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Pluto and Neptune
• Pluto has the most elliptical and inclined orbit of any planet.
• For 20 of its 248-year orbital period, it is actually closer to the Sun
than Neptune.
• such was the case between 1979 and 1999
• An orbital resonance between the two planets keeps them from
ever colliding!
Resonance
Neptune completes
three orbits for
every two orbits
that Pluto makes.
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Pluto and Charon
• Pluto’s moon, named Charon, was
discovered in 1978.
• it orbits Pluto every 6.4 days
• This allows us to measure the mass
of Pluto using Kepler’s Law #3.
• Eclipses allow us to measure the
diameters of both Pluto & Charon.
Hubble ST image of Pluto & Charon
• Pluto’s density (2 gm/cm3) is larger
than expected for an icy world.
• Charon is less dense (1.6 gm/cm3)
• Explanation similar to Earth/Moon
• Charon formed from large impact
• Pluto lost lower density outer layers
• Reconstructed image of Pluto
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Planet or Kuiper Belt Comet?
• The classification of Pluto has recently come into question.
• Pluto has many properties in common with Kuiper belt comets.
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it orbits in the vicinity of the Kuiper belt
several Kuiper belt comets have orbital resonances with Neptune
its composition of ice and rock is similar to comets
it has an atmosphere of Nitrogen which sublimes when Pluto is closest to
the Sun
• some Kuiper belt comets have moons
• Pluto has some properties which differ from Kuiper belt comets.
• its surface is much brighter; presumable because the Nitrogen atmosphere
refreezes on the surface rather than escaping
• it is much larger than most Kuiper belt comets
• But…it is smaller than Triton, which presumably once roamed
the Kuiper belt!
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13.6 Cosmic Collisions: Small Bodies
Versus the Planets
Our goals for learning:
• What happened to Jupiter in 1994?
• How often do small particles impact Earth?
• Why do we think the dinosaurs were driven
extinct by an impact?
• Do future impacts pose a real threat to our
civilization?
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Comet Shoemaker-Levy 9
• This comet was discovered in orbit about Jupiter in 1992.
• a previous encounter with Jupiter broke the nucleus into a string of fragments
• the comet was on a collision course with Jupiter
• Something similar
had happened to
Callisto.
• This crater chain is
evidence that a
string of nuclei once
impacted it.
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Comet Shoemaker-Levy 9
• One by one, each fragment collided with Jupiter in July 1994.
• infrared cameras observed hot plumes ejected from the planet
• material from deep inside Jupiter was ejected, and fell… left dark spots
• Such impacts probably occur on Jupiter once every 1,000 years.
This was a reminder to us that impacts still occur in the present!!
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Meteor Showers
• Earth is impacted by an estimated 25 million small particles
each day which cause meteors.
• When the Earth passes through the trail of a comet, the number
of particles impacting the Earth’s atmosphere increases.
• We call this a meteor shower.
• You can see upward of 1 meteor per minute from one location.
• Showers occur on the same dates each year, corresponding to
when the Earth crosses a given comet’s orbit.
• The meteors appear to emanate from one point in the sky.
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Meteor Showers
Meteors appear
to shoot from
the point
directly ahead
in the direction
that the Earth
is moving.
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Meteor Showers
• Meteor showers are named after the constellation from which the
meteors appear to emanate
• i.e., the constellation which lies in the direction of the Earth’s motion.
Halley
Swift - Tuttle
Halley
Encke
Temple - Tuttle
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Impacts and Mass Extinctions on Earth
• We know that larger objects
have impacted Earth
• Meteor Crater in northern Arizona
• caused by a 50-meter asteroid
• impact occurred 50,000 years ago
• 65 million years ago, many
species, including dinosaurs,
disappeared from earth
• Sedimentary rock layer from
that time shows:
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Iridium, Osmium, Platinum
grains of “shocked quartz”
spherical rock droplets
soot from forest fires
Impacts and Mass Extinctions on Earth
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Elements like Iridium, rare on Earth, are found in meteorites.
Shocked quartz, found at Meteor Crater, forms in impacts.
Rock droplets would form from molten rock “rain.”
Forest fires would ensue from this hot rain.
All this evidence would imply that Earth was struck by an
asteroid 65 million years ago.
• In 1991, a 65 million year old
impact crater was found on the
coast of Mexico.
• 200 km in diameter
• implies an asteroid size of about
10 km across
• called the Chicxulub crater
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Impacts and Mass Extinctions on Earth
• We have a plausible scenario of how the impact led to mass extinction.
• debris in atmosphere blocks sunlight; plant die…animals starve
• poisonous gases form in atmosphere
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Could it happen again?
• This chart shows how frequently objects of various sizes will
impact Earth.
• The odds of a large impact are small … but not zero!
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What have we learned?
• What are the three major groups of small bodies in the
Solar System?
• Asteroids, comets of the Kuiper belt, comets of the Oort cloud.
The groups are distinguished by their orbits.
• Describe asteroid sizes, shapes, and orbits.
• Most asteroids are small, potato-shaped, and orbit in the asteroid
belt. Trojan asteroids share Jupiter’s orbit. Near-Earth asteroids
have orbits that pass near Earth’s orbit.
• Why didn’t a planet form in the region of the asteroid belt?
• Orbital resonances with Jupiter disrupted orbits of planetesimals,
preventing them from accreting into a planet. Resonances also
cause the gaps in the asteroid belt today.
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What have we learned?
• How do we measure asteroid properties?
• Orbits from observations of asteroid motion and
application of the law of gravity. Sizes by comparing
infrared and visible brightness, combined with distance.
Masses from gravitational effects on moons or passing
spacecraft. Densities from size and mass. Shapes from
brightness changes with rotation or radar. Composition
from spectroscopy.
• How is a meteor different from a meteorite?
• meteor: a flash of light caused by a small particle
entering our atmosphere. meteorite: a rock that survives
the plunge from space to reach the ground.
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What have we learned?
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How are meteorites categorized?
• Two major categories: primitive meteorites are remnants from the
Solar nebula and processed meteorites are fragments of larger
objects that underwent differentiation. Primitive meteorites may
be either stony or carbon-rich. Processed meteorites may be
similar in composition either to Earth’s crust or to its mantle or
core.
Why do meteorites differ from one another?
• Whether a primitive meteorite is stony or carbon-rich depends on
where it formed. Carbon-rich meteorites formed farther from the
Sun (beyond about 3 AU) where it was cool enough for carbon
compounds to condense. Processed meteorites made of crust-like
material are chips from the surface of larger objects. Processed
meteorites made of mantle- or core-like material are fragments
from the interior of a shattered asteroid.
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What have we learned?
• What are comets made of?
• Ice mixed with rocky dust, giving them a “dirty
snowball” composition.
• What happens to a comet as it approaches the Sun?
• The nucleus – all there is when the comet is far away
and frozen – heats up and gases begin to sublime from
its surface. Escaping gases carry some dust along. The
gas and dust form a coma and tails: a plasma tail of
ionized gas and a dust tail.
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What have we learned?
• How do we know that vast numbers of comets reside in the
Oort cloud and Kuiper belt?
• Analysis of orbits shows where comets in the inner
Solar System have come from. Based on the number of
comets seen in the inner Solar System and the relatively
short times during which comets can survive in the inner
Solar System, we conclude that the Oort cloud and
Kuiper belt must contain enormous numbers of comets.
• Why don’t Pluto and Neptune collide?
• A stable orbital resonance ensures that Pluto always
remains a safe distance from Neptune, even though it is
sometimes closer to the Sun than Neptune.
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What have we learned?
• What is surprising about Pluto’s density, and how might it
have come to be?
• It is slightly higher than expected for material that condensed in
the outer Solar System. It may be the result of a giant impact the
blasted away Pluto’s low-density outer layers and led to the
formation of its moon, Charon.
• Why do we think Pluto is a Kuiper belt comet?
• Both its composition and orbit are more similar to Kuiper belt
comets than to other planets. Even its size is not that much bigger
than other known Kuiper belt comets, and it is smaller than one
object that almost certainly once roamed the Kuiper belt –
Neptune’s moon Triton.
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What have we learned?
• What happened to Jupiter in 1994?
• It was struck by a string of nuclei that were all
fragments of comet Shoemaker-Levy 9. Such impacts
probably occur on Jupiter only once every 1,000 years,
on average.
• How often do small particles impact Earth?
• Constantly – an estimated 25 million small particles
create meteors each day. Even more hit the Earth during
meteor showers.
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What have we learned?
• Why do we think the dinosaurs were driven extinct by an
impact?
• Careful analysis of sediments from the time of the
dinosaurs’ demise shows evidence of a major impact.
An impact crater has been found, and we have a
plausible scenario to describe how the impact led to the
mass extinction.
• Do future impacts pose a real threat to our civilization?
• The probability of a major impact in our lifetimes is
very low, but not zero. The threat is still being assessed.
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