Astronomy 330

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Astronomy 330

• Lecture 23

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Astronomy 330 Comets Overview

• Small, primitive and dark
• Composition is different from the asteroids.
• Contain a substantial amount of ice (especially
  water)
• As a comet approaches the Sun, ice evaporates to
  form a thin atmosphere.
• The cometary atmosphere is blown back away from
  the sun by the solar winda comet tail is formed
  and this is generally what is seen from Earth.

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• The word comet comes from Greek Komitis meaning
  long-haired.
• Comets formed at low temperatures and remained at
  these temperatures for their lifetimes in order
  to have preserved their volatile compounds in a
  solid state.
• Therefore, most comets must be in the outer solar
  system beyond Neptune and only occasionally do
  they approach near to the Sun.

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• Comets are a remnant of the formation of the
  solar system and are related to the outer planets
  and to their satellites and ring systems.

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Astronomy 330 History of Comets

• Comets have been known throughout history since
  every so often, they appear at the Earth.
• Also, they seem to appear at random times and
  then they disappear. Also the visible forms of
  comets are different from one to the next.
• Approximately one comet appears per year which is
  visible to the naked eye.
• The appearance of comets was thought to foretell
  some great or terrible event.

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• It was unknown whether comets occurred in the
  atmosphere or where celestial phenomena until the
  16th century (at least in Europe!).
• Comets generally appear dim and fuzzysmaller
  than the Moon.
• The atmosphere of the comet is usually what is
  seen when viewed from Earth.
• The nucleus of a comet is the solid part of the
  comet from which the atmosphere is released.

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• The nucleus is generally too small to be seen,
  but it is the real comet and is where most of
  the mass is.
• The coma is the atmosphere surrounding the
  nucleus.
• The dust and gas of the coma is swept into a tail
  by the solar wind.

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• Recent cometsHyakutake (1996) and Hale-Bopp
  (1997) and SL 9.
• Comets are named after their discovers.
• Tycho showed that comets do not have parallax and
  therefore are far away. They are definitely not
  in the atmosphere and they are beyond the orbit
  of the Moon.
• With Keplers laws it was realized that comets
  are really on highly elliptical orbits around the
  Sun.

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• Edmund Halley theorized that comets occur on
  closed, elliptical orbits and will occur
  regularly. Halleys comet is named after him and
  has a 76 years orbital period.
• Halleys comet last appeared in 1986.
• Comet Encke is another example of a periodic
  cometperiod of 3.3 years.
• The size of the typical coma of a comet is about
  the same size as the Earth.

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• Since the comae of comets are so large it led
  people to believe that comets might cause plagues
  and collisions with the comet.
• Since stars could be observed through the tails
  and cometary comae, it was realized that comae
  are tenuous.
• Newton first suggested that what was observed was
  a thin atmosphere ejected from a solid object as
  it was heated by the Sun.

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• Public interest in comets was what provided a
  boost for observatory building and Astronomy in
  general.
• In 1910 the Earth actually passed through the
  tail of Halleys comet and it could be easily
  seen.
• The 1986 appearance of Halley was much less
  spectacular (I know, I saw it!).
• However, space missions from the USSR, ESA, and
  Japan were sent to Halley.

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Astronomy 330 Cometary Orbits

• Comets are classified by their orbits.
• Long period comets gt 200 years
• Intermediate period comets 30-200 years (e.g.
  Halley).
• Short period comets lt 30 years.
• Generally the Intermediate comets are grouped
  with the long periods since they have a common
  origin.

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• Long period comets are on very, very eccentric
  orbitsthey fall towards the sun from great
  distances in the outer solar systems and spend
  only a small fraction of their orbit near the
  sun.
• Their orbits are very nearly parabolic and many
  have orbital periods on the order of a million
  years.
• Most comets discovered are long period comets.
  1500 have been discovered since 2001 and 90 of
  those are long period comets.

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• The short period comets have aphelia (distance
  farthest from the sun) which are near the orbit
  of Jupiter.
• Comet orbits are unstable due to their high
  eccentricitythey cross the orbits of the planets
  and are easily influenced by their gravity.
• Therefore, the comets must come from somewhere
  else in the solar systemlong period comets are
  thought to come from the Oort cloud and short
  period comets are thought to come from the Kuiper
  belt just outside the orbit of Neptune.

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• The comets we see from the Earth are only a very
  small fraction of the comets which exist in these
  two places.

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Astronomy 330 Cometary Atmospheres

• The visible part of a comet as viewed from Earth.
• The inner coma is the brightest part of a comet
  and is composed of gas and dust recently ejected
  from the nucleus.
• The inner coma is typically hundreds to thousands
  of kilometers in diameter.
• The inner coma is sometimes symmetric but more
  often is brighter on the side of the comet
  closest to the sun.

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• The inner coma also displays streams or fans of
  more dense gas jetting from the nucleus.
• Once gas is released from the nucleus it is
  quickly broken down by solar UV radiation.
• OH, CH, and NH are observed and are pieces of
  molecules like H2O.
• Also observed, C2 and C3. The presence of such
  molucules of Carbon is hard to understand and
  remains a mystery.
• Chemistry in the inner coma of comets is complex
  and not well understood.

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Astronomy 330 Comet Borrellys nucleus and dust
jets from Deep Space 1
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2186
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• Near the surface of active comets the gas density
  is about 1 millionth that of the Earths
  atmosphere.
• The density further falls off rapidly with
  distance from the nucleus.
• At distances of 10,000 km the gas begins to flow
  smoothly.
• At distances farther than this, chemistry stops.
• Surrounding the rest of atmosphere is glowing,
  ionized hydrogen gas and can extend to 1 million
  km from the nucleus.

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• This cloud of ionized H is larger than the sun in
  some instances (but this only lasts when the
  comet is near the Sun).
• Beyond 100,000 km the gas is fully ionized and is
  swept up in the solar wind.
• Spectroscopy can only be used effectively for the
  inner coma. This information is then used to
  work backwards to determine the composition of
  the parent materials.
• Such studies indicate that water ice is important.

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• Water vapor is released from the nucleus and
  ionized into H2O.
• This is further broken down into OH (hydroxyl)
  and H. OH is commonly observed in cometary
  spectra.
• All of these substances, including neutral water
  vapor have been detected by optical and radio
  observations from Earth as well as spacecraft
  missions.

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• Also observedC2, C3, CO, CO2, CH4(methane), and
  CH3OH (methanol).
• These substances indicate that ices other than
  just water are present in comets.
• These ices consist of hydrocarbons (H and C
  compounds) and also oxides of Carbon.
• So, comets contain both reduced and oxidized
  molecules. This is unusual for objects in the
  solar system as we have seen.
• This indicates, again, that comets formed in very
  cold regions of the solar system. (Chemical
  reactions are inhibited below 40K).

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• Nitrogen compounds have also been detectedN2,
  HCN, NH3, and CH3CN.
• Sulfur compounds have also been detected.
• HCN is also seen (Hydrogen cynanide). Its
  presence caused widespread panic in 1910 when the
  Earth passed through Halleys tail.

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Astronomy 330 Tails

• A comets gases are not gravitationally bound to
  the comets nucleus and freely expand.
• The gases are ionized as they expand are are
  caught up in the solar wind to form a taila
  plasma tail.
• The tail is seen by the flourescence of CO.
• Flourescence is induced when a substance absorbs
  light (UV light from the Sun in this case) and
  reemits it at visible wavelengths.

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• Also, ions of water vapor, CO, and molecular N
  have been observed in cometary tails.
• Plasma tails are straight and point directly away
  from the Sun.
• Plasma tails are made up of discrete, individual
  streamers which are a few thousand kms across but
  the plasma tail itself is typically several
  10-100 millions km long (the distance of the Sun
  to the Earth).

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• The density in the plasma tail is only about a
  few hundred molecules per cm3.
• Plasma tails can be used to study the solar wind
  and shows that typical solar wind velocities are
  400 km/s.
• Plasma tails change there appearance from hour to
  hour due to the fast speed of the solar wind.

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• Most comets have a second kind of taila dust
  tail.
• The plasma tail is ions caught in the solar wind
• The dust tail is composed of dust particles and
  is generally shorted than the plasma tail.
• Dust tails are as bright or brighter than plasma
  tails.
• There color is yellowish and comes from reflected
  sunlight, whereas ion tails are bluish.

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• Dust tails also are curved and not straight.
• The dust grains move under the influence of solar
  gravity and solar radiation pressure (the
  pressure created by photons striking the dust
  grains).
• The dust moved more slowly than the ions in the
  plasma tail and shows where the comet has been
  and is thus curved.
• The mass in the dust tail similar to the that of
  the ion tail. This suggests that the comet
  nucleus is made up of half dust and half ices.

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Astronomy 330 Tails
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID903
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Astronomy 330 The cometary nucleus

• Where most of the mass of comet is.
• Cannot be seen directly from the Earth by even
  the best telescopes.
• Radar was first to directly detect comet nuclei.
• Spacecraft have imaged some nuclei of comets.

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Astronomy 330 Dirty snowballs

• Fred Whipple, 1950, theorized that comets are
  small and composed of equals parts silicates and
  ice.
• Up to this time is was thought that comets were
  composed mostly of rock.
• The dust (silicates) and ice are uniformly mixed
  since this is what one would expect to form in
  the early solar nebula.
• Some comets show most dust than ice (icy dirt
  balls). These differences suggest a different
  origin.

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• Little is know directly about the composition of
  the nucleus, but it is thought water ice is the
  main constituent.
• This was surmised since comets turn on and
  become active at about 3 AU from the Sun, the
  distance where water will begin rapid
  evaporations (a temp. of 210 K).
• Ices of different composition would evaporate at
  different temps. and therefore different
  distances from the Sun.

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• There are exceptionsHale-Bopp became active at 5
  AU indicating the presence of more volatile
  compounds such as CO and N2 than simply water
  ice.
• Direct evidence for the gases evaporating from
  some comets has been collected by spacecraft and
  indicates mostly a water composition but a few
  percent is CO and CO2 and methyl alcohol (CH3OH)
  and traces of hydrocarbons.

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• Dark carbonaceous and silicate dust is also
  detected.
• For comet Halley, carbon and hydrocarbon dust was
  more prevalent than silicate dust.
• The surface of comet Borley was seen to be very
  dark, 3 reflectivity on average and in some
  places is was 1. This is darker than coal.
• The ices must orginate below the surfaces of the
  nuclei if they are covered with such dark dust.

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• As the ices evaporate they also release the dust
  particles.
• Cometary dust is the primary source of meteors
  that we see at night.
• The cometary dust is also interesting since much
  of it might have orginated from the original
  solar nebula as the comet formed and may be very
  similar to interstellar dust grains.

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• Also, almost all of the volatile compounds in
  comets have also been detected in dense,
  interstellar gas clouds (molecular clouds)
  throughout the Galaxy and in other galaxies.
• So, the ices in comets may also preserve a record
  of what the original solar nebula was made out
  of.
• Comets may be a source for carrying volatile
  compounds and other organic molecules throughout
  the solar system.

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• The exact sizes of cometary nuclei have not been
  directly measured (except for Halley and
  Borrelly).
• Radar can be used as well to get a rough picture
  of the nucleus. Used with comet Enckediameter
  of 5 - 10 km.
• Radar also indicates clouds of large particles
  around some cometsboulders?
• Giotto traveled with 500 kms of comet Halley and
  looked directly at the comet nucleus.
• It was irregular and dark, reflectivity of 3-4 .

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• Comet Borrelly was flown by in 2001 by Deep
  Space 1.
• Borrelly is a short period comet and is much less
  active than Halley.
• This indicated rough terrain on the surface and a
  bowling pin shape.
• Masses of the Halley and Borrelly were not
  measured.

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Astronomy 330 Comet Borrellys nucleus
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID2185
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Astronomy 330 Comet Wild 2s nucleus from
Stardust
http//solarsystem.nasa.gov/multimedia/display.cfm
?IM_ID604
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Astronomy 330

• The dark color of comets seems to be due to the
  evaporation of ices from its upper layers,
  leaving behind the dust.
• A similar process blackens the surfaces of
  glaciers on Earth.
• The sizes for the comets which have been directly
  observed is about 10 km in diameters.
• There are fewer small comets on the order of 1-2
  km in size, than for the asteroids.
• Some comets have been estimated to be on the
  order of 100 km to 200 km.

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• Cometary activity refers to the process of
  evaporation of ices and the creation of an
  atmosphere and tail as the comet approaches the
  Sun.
• 10 to 100 million tons of material is typically
  lost during a passage near the sun. This is
  about 0.1 of a typical comets mass.
• It takes about a few thousand passes near the sun
  to loose all its ices.
• If a core is left over after this process,
  evaporated comets could be a source for near
  Earth asteroids.

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• Some comets may simply disappear all together.
• As gases escape from the surface of a comet, they
  do so non-uniformly forming jets of material.
• These jets can induce a rocket effect on the
  comet and influence its orbit around the sun.
• This is a consequence of the comet being a dirty
  snowballdarker surfaces absorb more sunlight,
  heat more and geysers may burst out of such
  locations.

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Astronomy 330 What SL 9 has taught us.

• Comets have very low tensile strength.
• A density estimate was possible1 g/cm3.

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Astronomy 330 Comet Dust

• Comet dust fills the inner solar systemmany
  comets have gone through the inner solar system
  after all (about 1 per year).
• Most of it falls into the Sun or is swept away by
  the solar radiation pressure.
• A small amount strikes the Earth as meteors.
• A typical meteor is no larger than a pea.
• 25 million meteors (bright enough to be seen) hit
  Earth every dayhundreds of tons of material are
  added to the Earth each day.

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• Two classes of meteors
• Sporadic meteors-can come from any direction
• Meteor showers - meteors come from a certain spot
  in the sky, caused by comet debris following the
  regular orbit of a comet.
• Most meteors are associated with showers and thus
  are associated with meteor streams orbiting the
  sun.
• Also, most of the sporadic meteor are probably
  dispersed meteor streams.
• So, probably most meteors have cometary origin.

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• Meteorites, on the other hand (objects which
  actually make it to the surface of the Earth) are
  NOT associated with meteor showers. Meteorites
  are not just shower meteors which happen to make
  it Earththey appear to be a completely different
  type of object.
• Meteorites have a density of 3-7 g/cm3 and shower
  meteors seem to be composed of material with a
  density of 1 g/cm3 (like comets).

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• Also, meteoric particles have been collected from
  the upper atmosphere and have been shown to be
  composed of chemically primitive material and
  some organic matter.
• Also, these materials have very different isotope
  ratios of nitrogen and hydrogen than elsewhere in
  solar system.
• Comets preserve a record from the early history
  of the early solar system.

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Astronomy 330 Origin and Evolution of Comets

• Comets stay in the inner solar system on average
  no more than a few million years since they
  evaporate and their orbits are unstable.
• Therefore, comets have an origin outside the
  inner solar system.
• There are 2 reservoirs for comets The Oort cloud
  and the Kuiper belt.

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Astronomy 330 The Oort cloud.

• Named for Jan Oort.
• In 1950 calculated that all long period comets
  have aphelions near 50,000 AU.
• This is 1000 times farther than Pluto.
• Oort hypothesized a cloud of comet nuclei
  orbiting the Sun as this distance.
• Also, the long period comets show inclinations at
  all angles and are not confined to the ecliptic.

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• It is very cold at these distances and ices in
  the Oort cloud objects will be preserved
  indefinitely.
• Direct detection of them is very difficult, only
  when they get near the sun can they be seen.
• It is believed that occasionally the Oort cloud
  is gravitationally perturbed by a passing star
  and some of them are put on orbits which bring
  them close to the Sun.

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• To account for the observed number of near comets
  we see, Oort calculated that there must be about
  100 billion objects in the Oort cloud.
• More recent calculations suggest 1 trillion
  objects.
• Only about 5 have been lost by being thrown into
  orbits which bring them near the Sun since the
  formation of the solar system.

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Astronomy 330 The Kuiper Belt

• The Oort cloud accounts for most comets, but not
  all.
• Short period comets share the same sense of
  rotation around the Sun as the planets. The long
  period comets come from any direction with any
  sense of revolution about the sun.
• Short period comets also have small orbital
  inclinations.

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• Calculation using celestial mechanics show that
  short period comets must originate and a disk of
  objects, not a sphere as for the Oort cloud.
• Further, these calculations indicate that short
  period comets should come from a region just
  beyond the orbit Pluto and should have many
  potential cometary nucleiotherwise known as icy
  planetesimals.

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• Kuiper, in the 50s, theorized that there was
  such a belt of planetesimals just outside the
  orbit of Neptune based on theorys of the
  formation of the solar system which said that
  Neptune should not consume all of the material in
  the outer solar system during its formation.
• In other words, the Kuiper belt is debris which
  is left over from the solar sytems formation.

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• More than 500 Kuiper belt objects have now been
  telescopically detected.
• Pluto is considered a member of the Kuiper belt.
• Triton is probably a Kuiper belt object which was
  captured by Neptune.
• Kuiper belt-like objects probably accumulated to
  form the cores of Uranus and Neptune.
• The outer edge of the Kuiper belt is at about 50
  AU from the Sun. Some members have orbits which
  reach to 1000 AU.

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• The Kuiper belt objects are clustered around
  resonances with Neptunes orbit. Pluto is in a
  32 resonance with Neptune.

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• Comets in the Kuiper belt and the Oort cloud
  occasionally collide, which changes their orbits.
• These collisions create dust and such dust was
  observed in the Kuiper belt as Voyager crossed
  the Kuiper belt.
• Also, we expect these collisions to create a dust
  ring around the sun. Such dust rings are
  observed around other stars (e.g. Beta Pictoris).

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Astronomy 330 Beta Pictoriss dust ring observed
by HST
http//hubblesite.org/newscenter/newsdesk/archive/
releases/1998/03/
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• Collisions, plus gravitational perturbations put
  KBOs onto orbits which bring them into the inner
  solar system.
• Once a comet or planetesimal is thrown into such
  an orbit it is further perturbed by the influence
  of the gas giant planets.
• Sometimes these perturbations force the KBO into
  an orbit which brings it into an orbit similar to
  an Oort cloud object.
• Other times the KBO can be flung into a short
  period comet orbit.

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• Some comets strike the sun, others simply fade
  away before reaching the sun.
• Still others breakup into pieces for no known
  reason. Comet West was observed to split 3
  different times in 1976.
• Many are either thrown out of the solar system or
  gradually loose their volatiles over time as they
  execute their highly elliptical orbits.

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• What is the origin of the planetesimals in the
  Kuiper belt and the Oort cloud?
• Since the Oort cloud is so far from the sun it is
  unlikely that the solar nebula was dense enough
  for planetesimals to condense.
• The Oort cloud objects probably formed closer to
  where the planets formed.
• The current theory is that comets formed in the
  outer part of the solar nebular, where the Kuiper
  belt and outer planets are since this is where we
  expect temperatures of 30-100kwhere ices of
  volatile compounds can form.

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• If such an icy planetesimal formed closer to the
  sun than the Kuiper belt, it probably was ejected
  into the Oort cloud by gravitational interactions
  with the large planets.
• Others were dispersed throughout the solar system
  and collided with the sun or planets.
• Possible that comets delivered Earths water and
  other compounds to Earth this way.

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Astronomy 330 Deep Impact Mission

• What are comets REALLY made of?
• Will send a 350 kg copper projectile smashing
  into comet 9P/Tempel 1 on July 5 and observe the
  results
• Chief scientist is Dr. M. Ahearn of UMD.
• WEB site http//deepimpact.jpl.nasa.gov/index.htm
  l

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Astronomy 330 Reading

• Read Chapter 6 of Morrison and Owen