Transcript Unit 03 Slides - Chapter 12

Chapter 12:
Remnants of
Rock & Ice
Asteroids, Comets & Pluto
Small Bodies in the Solar System
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Small bodies, the leftover “scraps” from the
formation of the Solar System, fall into three
distinct groups:
asteroids
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rocky or metallic in composition
most are located between the orbits of Mars and Jupiter
Small Bodies in the Solar System
Kuiper belt comets
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made mostly of ice
orbit the Sun beyond
Neptune
orbit in same direction and
plane as the planets
Oort cloud comets
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made mostly of ice
orbit at the outer fringe of
the Solar System
spherically distributed about
the Sun
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
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
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 NearEarth asteroids
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
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
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
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
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
Meteorite Impact
© 2014 Pearson Education, Inc.
Chicago, March 26, 2003
Types of Meteorites
Based on composition, meteorites fall into two basic categories:
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primitive
• about 4.6 billion years old
• accreted in the Solar nebula
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processed
• younger than 4.6 billion years
• matter has differentiated
• fragments of a larger object
which processed the original
Solar nebula material
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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
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
Comets
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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
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They were taken as omens of good or bad fortune.
Recent
Comets:
Hale-Bopp
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Hyakutake
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1986 Halley’s
Comet
1996 Comet
Hyakutake
1997 Comet HaleBopp
Dozens per year too
dim to be seen by
eye
Comets
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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:
Composition and Structure of Comets
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Comets are “dirty snowballs”…ice mixed with rock and
dust.
• ices are H2O, CO2, CO, NH3, CH4
nucleus
• the “dirty snowball”
• how the comet appears
far from the Sun
coma
• surrounds nucleus when
near the Sun
• sublimated gas & dust
plasma tail
• ionized gas swept back
by Solar wind
dust tail
• dust particles swept
back more slowly by
radiation
Nucleus of Comet
►A
"dirty
snowball"
► Source
of
material for
comet's tail
© 2014 Pearson Education, Inc.
A Comet’s Journey
A Comet’s Journey
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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.
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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
http://sohowww.nascom.nasa.gov/
The Origin of Comets
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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 50,000 AU distant
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Others orbit along the
ecliptic plane in the same
sense as the planets.
• they come from the Kuiper
belt 30-100 AU distant
Pluto and Neptune
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Pluto has an eccentric and inclined orbit.
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
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An orbital resonance between the two planets keeps them
from ever colliding!
Resonance
Neptune
completes three
orbits for every
two orbits that
Pluto makes.
Pluto and Charon
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Pluto’s moon, named Charon, was
discovered in 1978.
• it orbits Pluto every 6.4 days
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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
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Pluto’s density (2 gm/cm3) is larger
than expected for an icy world.
• Charon is less dense (1.6 gm/cm3)
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Explanation similar to Earth/Moon
• Charon formed from large impact
• Pluto lost lower density outer layers
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Reconstructed image of Pluto
Hubble's View of Pluto and Its Moons
© 2014 Pearson Education, Inc.
Planet or Kuiper Belt Comet?
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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
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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
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But…it is smaller than Triton, which presumably once roamed the
Kuiper belt!
This is why Pluto is part of a new class of Dwarf Planets
How big can a comet be?
© 2014 Pearson Education, Inc.
Comet Shoemaker-Levy 9
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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.
Comet Shoemaker-Levy 9
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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
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Such impacts probably occur on Jupiter once every 1,000 years.
This was a reminder to us that impacts still occur in the present!!
Meteor Showers
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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.
Meteor Showers
Meteors appear to shoot
from the point directly
ahead in the direction that
the Earth is moving.
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.
Impacts and Mass Extinctions on
Earth
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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
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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
Iridium Layer
No dinosaur fossils in
upper rock layers
Thin layer containing
the rare element
iridium
Dinosaur fossils in
lower rock layers
© 2014 Pearson Education, Inc.
Impacts and Mass Extinctions on Earth
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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
Could it happen again?
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This chart shows how frequently objects of various sizes will
impact Earth.
The odds of a large impact are small … but not zero!
Influence of the Jovian Planets
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Gravity of a jovian planet (especially Jupiter) can redirect a comet.
Jupiter has directed some comets toward Earth but has ejected
many more into the Oort cloud.
PAST
FUTURE ?
http://impact.arc.nasa.gov
?