Comet dust and nuclei:Clues from meteor studies

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Comet dust and nuclei-Clues from meteor studies

• Iwan Williams
• Rosetta Meeting Budapest
• 17-19 December 07

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Meteor showers

• Meteor Showers represent an increased flux of
  meteors above the normal background (about 6
  meteors per hour)
• Showers appear at the same SIDERIAL time each
  year (indicating an extra-terrestrial origin)
• In showers, all meteors appear to radiate from
  same point on the sky, called the radiant.
• CONCLUSION. They are caused when the Earth
  intersects a stream of small dust grains or
  meteoroids.
• Showers are named after the constellation in
  which the radiant lies.

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• Some showers can be spectacular, for example the
  Leonids every 30 years or so with rates in excess
  of 1 per second.
• The majority are not, they just produce 10-20
  meteors per hour on an annual basis

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What do we get directly from showers, that is
simply from looking at individual meteors and
measuring deceleration and magnitude?

• Brightness is proportional to rate of change of
  kinetic energy ( mass, velocity, deceleration,
  mass loss rate)
• Deceleration depends cross-section, mass and speed

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• In principle, we know velocity, deceleration and
  brightness.
• This leaves two equations and three variables,
• mass, cross section and mass loss rate.
• So need an other equation. This is usually done
  by relating mass loss rate to surface temperature
  or by approximating( eg ,mass loss small, ok for
  big things but not for all then the first
  equation gives mass)

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Mass range

• Interest is in the large end. There are quite
  large bodies associated with well-known showers.
• Five Perseids with masses of 3,10,16,16 and 600g
  have been measured.The largest of these could be
  as much as 4cm in radius.

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Mass distribution function

• dn CM-s dM, ie dn is the number with mass
  between m and mdm.
• Note if slt2 mass is dominated by large meteoroids
• And if sgt2 by the small end
• There is some indication that s is larger for
  meteors with mass above .08g, say s 1.7 below
  this and 2.3 above. If true total mass is finite
  even for 0 to infinity range. (cumulative 3)
• There is some variation from shower to shower eq
  Lyrids about 1.7 and Orionids at 2.1
• This number is same ball-park as is obtained by
  theoretical collisional fragmentation models

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Density (value depends on whether fragmentation
in the atmosphere is assumed- sudden
fragmentation increase the surface area to mass
ratio, slow continuous sputtering affects mass
loss rate)
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Meteor Streams

• For a given meteor it position is known- that of
  the Earth.
• Its Geocentric speed is determined, hence its
  energy, semi-major axis and period are easily
  obtained.
• Its path (giving the radiant)
• This is sufficient to determine its orbital
  elements.
• It has been recognized for a long time that
  stream orbital elements are very similar to those
  of specific comets, eg Orionids and Halley,
  Perseids and Swift-Tuttle or the Leonids and
  Tempel-Tuttle.

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Stream Formation

• Meteoroid are carried away from the nucleus by
  the normal gas production process. (Called large
  cometary dust grains)
• They thus have a slightly different heliocentric
  velocity to that of the comet.
• This implies a different orbital energy and thus
  a different semi-major axis (and Period)

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• Hence, the difference in semi-major axis gives us
  the ejection velocity?
• Unfortunately planetary perturbations change
  orbits, so its not quite that simple.
• This process is repeated at every perihelion
  passage, giving rise to a broad stream with many
  strands in it since the orbit of the comet will
  have changed, mainly due to planetary
  perturbations.

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• As the stream ages, these strands may merge
  together, giving a broad stream with the shower
  having a broad activity curve
• The most recent activity may still show up (eg
  Perseids) or there may be resonance effects
  maintaining the strands (eg Leonids)

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• Detailed modeling of the Perseids or Quadrantids
• can however give a strong indication of the value
  of the ejection velocity
• This usually turns out at 10s m/sec (Not 100s)

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Is there an other way of forming meteor showers?

• Some asteroids have orbits that are very similar
  to stream orbits and of course, collisions can
  cause dust to be ejected from the asteroid.

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• Such a process can not really insert enough dust
  in orbit to form a strong meteor shower, it is a
  one-off event.
• A collision between two big bodies, forming an
  asteroid family might, but all know events
  happened a long time ago, so the dust will have
  dispersed

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Some comets break up, for example Comet Biela

• Pre 1833 a normal comet.
• 1839 Not observe.
• 1846 Two comets present
• 1852 Both comets still present but fainter.
• 1872 a strong meteor shower seen at the time the
  Earth passed close to where comet Biela would
  have been.

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Shoemaker-Levy 9
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SW3 in 2006

• Such a break-up releases vast amounts of dust and
  this is taking place now.
• SW3s orbit does not intersect the Earths orbit
  so we are unlikely to observe a meteor shower
  from it.

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The Quadrantids may have a complex history with
all activities possible

• The Quadrantid parent had been around for several
  thousand years ejecting meteoroids in the normal
  way
• Comet Maccholtz may well represent a fragment of
  this.

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• Comet of 1491 may also be a sighting of some
  fragment.
• Sometime later (about 1800) a major fragmentation
  took place, leaving a dormant fragment 2003EH1and
  a large new shower ( the narrow strong Quadrantid
  peak)

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Is this distinction in stream formation processes
important?

• Yes. Normal stream formation essentially produces
  data only on near surface grains.
• Comet break-up produces data on interior grains
• Quadrantids are essentially no different to
  others shower grains so perhaps the surface
  grains are good representatives of all. BUT 2003
  EH1 is a big bit (several kilometers) so there
  may be big bits inside

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Conclusions

• Fragility from Deep Impact (No large grains
  observed)
• However large grains (over cm size) are initially
  there.
• Density is the real problem. Is the high or the
  low values the correct ones?
• For the cm sized, the high is more likely
  correct??