Chapter 12 Remnants of Rock and Ice

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Chapter 12Remnants of Rock and Ice

• Asteroids, Comets, and the Kuiper Belt

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12.1 Asteroids and Meteorites

• Our goals for learning
• What are asteroids like?
• Why is there an asteroid belt?
• Where do meteorites come from?

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What are asteroids like?
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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 wouldnt
  add up to even a small terrestrial planet.

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Asteroids are cratered and not round
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Asteroids with Moons

• Some large asteroids have their own moon
• Asteroid Ida has a tiny moon named Dactyl

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Density of Asteroids

• Measuring orbit of asteroids moon tells us
  asteroids mass
• Mass and size tell us asteroids density
• Some asteroids are solid rock others just piles
  of rubble

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Asteroid Orbits

• Most asteroids orbit in a belt between Mars and
  Jupiter
• Trojan asteroids follow Jupiters orbit
• Orbits of near-Earth asteroids cross Earths orbit

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Thought QuestionWhy are there very few asteroids
beyond Jupiters orbit?

• There was no rocky material beyond Jupiters
  orbit.
• The heaviest rocks sank towards the center of the
  solar system.
• Ice could form in the outer solar system.
• A passing star probably stripped away all of
  those asteroids, even if they were there at one
  time.

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Thought QuestionWhy are there very few asteroids
beyond Jupiters orbit?

• There was no rocky material beyond Jupiters
  orbit.
• The heaviest rocks sank towards the center of the
  solar system.
• Ice could form in the outer solar system.
• A passing star probably stripped away all of
  those asteroids, even if they were there at one
  time.

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Why is there an asteroid belt?
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Thought QuestionWhich explanation for the belt
seems the most plausible?

• The belt is where all the asteroids happened to
  form.
• The belt is the remnant of a large terrestrial
  planet that used to be between Mars and Jupiter.
• The belt is where all the asteroids happened to
  survive.

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Thought QuestionWhich explanation for the belt
seems the most plausible?

• The belt is where all the asteroids happened to
  form.
• The belt is the remnant of a large terrestrial
  planet that used to be between Mars and Jupiter.
• The belt is where all the asteroids happened to
  survive.

But WHY didnt they form a little planet?
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Orbital Resonances

• Asteroids in orbital resonance with Jupiter
  experience periodic nudges
• Eventually those nudges move asteroids out of
  resonant orbits, leaving gaps in belt

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Origin of Asteroid Belt

• Rocky planetesimals between Mars and Jupiter did
  not accrete into a planet.
• Jupiters gravity, through influence of orbital
  resonances, stirred up asteroid orbits and
  prevented their accretion into a planet.

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Where do meteorites come from?
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Meteor Terminology

• Meteorite A rock from space that falls through
  Earths atmosphere
• Meteor The bright trail left by a meteorite

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Meteorite Impact
Chicago, March 26, 2003
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Meteorite Types

• Primitive Unchanged in composition since they
  first formed 4.6 billion years ago.
• Processed Younger, have experienced processes
  like volcanism or differentiation.

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Primitive Meteorites
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Processed Meteorites
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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.

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What have we learned?

• What are asteroids like?
• They are rocky leftovers from the era of planet
  formation
• Why is there an asteroid belt?
• Orbital resonances with Jupiter prevented
  planetesimals between Jupiter and Mars from
  forming a planet

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What have we learned?

• Where do meteorites come from?
• Primitive meteorites are remnants from solar
  nebula
• Processed meteorites are fragments of larger
  bodies than underwent differentiation

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12.2 Comets

• Our goals for learning
• What are comets like?
• Where do comets come from?

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What are comets like?
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Comet Facts

• Formed beyond the frost line, comets are icy
  counterparts to asteroids.
• Nucleus of comet a dirty snowball
• Most comets do not have tails.
• Most comets remain perpetually frozen in the
  outer solar system.
• Only comets that enter the inner solar system
  grow tails.

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Sun-grazing Comet
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Nucleus of Comet

• A dirty snowball
• Source of material for comets tail

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Deep Impact

• Mission to study nucleus of Comet Tempel 1
• Projectile hit surface on July 4. 2005
• Many telescopes studied aftermath of impact

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Anatomy of a Comet

• Coma is atmosphere that comes from heated nucleus
• Plasma tail is gas escaping from coma, pushed by
  solar wind
• Dust tail is pushed by photons

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Growth of Tail
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Comets eject small particles that follow the
comet around in its orbit and cause meteor
showers when Earth crosses the comets orbit.
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Meteors in a shower appear to emanate from the
same area of sky because of Earths motion
through space
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Where do comets come from?
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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
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How did they get there?

• Kuiper belt comets formed in the Kuiper belt
  flat plane, aligned with the plane of planetary
  orbits, orbiting in the same direction as the
  planets.
• Oort cloud comets were once closer to the Sun,
  but they were kicked out there by gravitational
  interactions with jovian planets spherical
  distribution, orbits in any direction.

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What have we learned?

• What are comets like?
• Comets are like dirty snowballs
• Most are far from Sun and do not have tails
• Tails grow when comet nears Sun and nucleus heats
  up
• Where do comets come from?
• Comets in plane of solar system come from Kuiper
  Belt
• Comets on random orbits come from Oort cloud

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12.3 Pluto Lone Dog No More

• Our goals for learning
• How big can a comet be?
• What are the large objects of the Kuiper belt
  like?
• Is Planet X a planet?

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How big can a comet be?
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Plutos Orbit

• Pluto will never hit Neptune, even though their
  orbits cross, because of 32 orbital resonance
• Neptune orbits three times during the time Pluto
  orbits twice

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Is Pluto a Planet?

• By far the smallest planet.
• Not a gas giant like other outer planets.
• Has an icy composition like a comet.
• Has a very elliptical, inclined orbit.
• Pluto has more in common with comets than with
  the eight major planets

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Other Icy Bodies

• There are many icy objects like Pluto on
  elliptical, inclined orbits beyond Neptune.
• The largest of these, Planet X was discovered
  in summer 2005, is even larger than Pluto

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Kuiper Belt Objects

• These large, icy objects have orbits similar to
  the smaller objects in the Kuiper Belt that
  become short period comets
• So are they very large comets or very small
  planets?

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What are the large objects of the Kuiper belt
like?
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What is Pluto like?

• Its moon Charon is nearly as large as Pluto
  itself (probably made by a major impact)
• Pluto is very cold (40 K)
• Pluto has a thin nitrogen atmosphere that will
  refreeze onto the surface as Plutos orbit takes
  it farther from the Sun.

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HSTs view of Pluto Charon
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Other Kuiper Belt Objects

• Most have been discovered very recently so little
  is known about them.
• NASAs New Horizons mission will study Pluto and
  a few other Kuiper Belt object in a planned
  flyby.

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Is Planet X a planet?
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Pluto and Planet X

• Plutos size was overestimated after its
  discovery in 1930
• It was considered a planet, and nothing of
  similar size was discovered for several decades
• Now other large objects have been discovered in
  Kuiper Belt, including Planet X
• Some scientists consider all of those objects
  planets others consider none of them planets.

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What have we learned?

• How big can a comet be?
• The Kuiper belt from which comets come contains
  objects as large as Pluto.
• What are the large objects of the Kuiper belt
  like?
• Large objects in the Kuiper belt have orbits and
  icy compositions like those of comets.
• Is Planet X a planet?
• It remains a matter of opinion because scientists
  have not yet settled on a definition of the
  minimum size of a planet.

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12.4 Cosmic Collisions Small Bodies Versus 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 media hype?
• How do the jovian planets affect impact rates and
  life on Earth?

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Have we ever witnessed a major impact?
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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 a previous encounter with Jupiter
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This crater chain on Callisto probably came from
another comet that tidal forces tore to pieces
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Impact plume from a fragment of comet SL9 rises
high above Jupiters surface
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Dusty debris at an impact site
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Artists conception of SL9 impact
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Several impact sites
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Impact sites in infrared light
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Did an impact kill the dinosaurs?
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Mass Extinctions

• Fossil record shows occasional large dips in the
  diversity of species mass extinctions.
• Most recent was 65 million years ago, ending the
  reign of the dinosaurs.

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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, probably by a meteorite impact.
• Dinosaur fossils all lie below this layer

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Iridium Layer
No dinosaur fossils in upper rock layers
Thin layer containing the rare element iridium
Dinosaur fossils in lower rock layers
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Consequences of an Impact

• Meteorite 10 km in size would send large amounts
  of debris into atmosphere.
• Debris would reduce sunlight reaching Earths
  surface.
• Resulting climate change may have caused mass
  exinction.

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Likely Impact Site

• Geologists have found a large subsurface crater
  about 65 million years old in Mexico

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Comet or asteroid about 10 km in diameter
approaches Earth
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Is the impact threat a real danger or media hype?
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Facts about Impacts

• Asteroids and comets have hit the Earth.
• A major impact is only a matter of time not IF
  but WHEN.
• Major impacts are very rare.
• Extinction level events millions of years.
• Major damage tens to hundreds of years.

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Tunguska, Siberia June 30, 1908 A 40 meter
object disintegrated and exploded in the
atmosphere
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Meteor Crater, Arizona 50,000 years ago (50
meter object)
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Frequency of Impacts

• Small impact happen almost daily.
• Impacts large enough to cause mass extinctions
  are many millions of years apart

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The asteroid with our name on it

• We havent 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

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What are we doing about it?

• Stay tuned to
• http//impact.arc.nasa.gov

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How do the jovian planets affect impact rates and
life on Earth?
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Influence of Jovian Planets
Gravity of a jovian planet (especially Jupiter)
can redirect a comet
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Influence of Jovian Planets
Jupiter has directed some comets toward Earth but
has ejected many more into the Oort cloud.
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Was Jupiter necessary for life on Earth?
Impacts can extinguish life. But were they
necessary for life as we know it?
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What have we learned?

• Have we ever witnessed a major impact?
• The most recent major impact happened in 1994,
  when fragments of comet SL9 hit Jupiter.
• Did an impact kill the dinosaurs?
• Iridium layer just above dinosaur fossils
  suggests that an impact caused mass extinction 65
  million years ago.
• A large crater of that age has been found in
  Mexico

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What have we learned?

• Is the impact threat a real danger or media hype?
• Large impacts do happen, but they are rare.
• They can cause major extinctions about every 100
  million years
• How do the jovian planets affect impact rates and
  life on Earth?
• Jovian planets sometimes deflect comets toward
  Earth but send many more out to Oort cloud