Why Study Cometary Comae?

The coma of a comet is the unbound atmosphere of gases which have
sublimed from the nucleus ices and have evolved via chemistry.
Why study the coma?

-- From the ground, the nucleus of a comet is invisible to the observer
   because it is shrouded by the coma.  

-- The vast majority of our current information about the nature and
   composition of comets comes from observations of the coma.
   --> Studies of the coma with CONTOUR are necessary to tie to the
   vast extant databases.

-- Observations of the chemical composition of the coma lead to a probe
   of the composition of the nucleus.  The NGIMS will be able to
   obtain much more detailed information than an imager about the
   composition of the coma BUT ONLY IN A SPECIFIC LOCATION.
   --> Images are necessary to extend our knowledge from those in situ
   measurements to the total context of the area surrounding the nucleus.

-- The coma is made up of dust and gas.  It is believed that some
   of the volatiles come directly from the dust.  Only coma imaging
   will be able to make the connection.

-- At the poor scales of ground based images, jet-like features are
   apparent.  However, it is impossible at those scales to trace
   the jets to their source or to determine if the "jets" are
   collimated.  CONTOUR will be able to do precisely this.
   Imaging in narrowband filters are necessary to differentiate dust
   from gas.




                        The CONTOUR Coma experiment

The filters on the CONTOUR images were chosen with our current 
understanding of the spectra of comets in mind. 

-- We have three narrow-band filters which isolate molecular emission
   bands we expect to see in comets.
   1) OH  
      The dominant constituent of ice in the nucleus is H2O.
      H2O is undetectable with CONTOUR.
      However, 90% of the H2O photodissociates to form OH
         H2O + h nu  --> H + OH
[NOTE TO ED, this is the greek letter nu]
      Resonance fluoresence forms the strongest emission band at
      308 nm.  Our filter isolates this band.
      --> We can use this filter to trace the quantity of H2O gas.
      This feature is extensively studied from the ground and
      with IUE and HST by two members of the CONTOUR team (Feldman
      and Cochran) but at much lower spatial resolution than CONTOUR
      can achieve.  Also, ground-based observations are hampered
      by the atmosphere and by optical cutoffs.

   2) CN
      The CN is a dissociative daughter product of trace species.
      However, it is of interest for two reasons.  
      a) It is the strongest feature in the optical with the exception
         of the OH described above and is the most extensively
         studied cometary emission
      b) Evidence exists (mostly from Halley) that indicates some of
         the source of the CN must be the dust. 
      Previous observations of CN have lacked the spatial resolution
      to be able to determine how admixed the dust and gas is
      and how much dust gets converted to gas.

   3) C2
      C2 is also well studied from the ground.  It is of interest to
      the CONTOUR team for two reasons:
      a) The C2 does not seem to be as strongly linked to the dust as
         the CN.  Thus, observations of C2 will serve as an independent
         tracer of the gas.
      b) Ground-based studies of ensembles of comets (A'Hearn et al 1995;
         Cochran et al (1992) have shown that the ratio of
         CN/OH is generally approximately constant in comets but
         that there are some comets which show a depletion of
         C2 with respect to OH and CN.
         The comets which are depleted are thought to be Kuiper belt
         comets and generally Jupiter family comets such as we
         will be observing. 

-- In addition, appropriate narrowband continuum filters have been
   included to demarcate the dust in the coma.




[NOTE to Ed:
In addition to this text, I will send you two gif files (uuencoded)
of cometary images.  '

The first image is of comet Encke.  We need to add the following
information

Comet Encke
This is an image of short-period comet Encke obtained by Jim Scotti on
1994 January 5.09 while using the 0.91-meter Spacewatch Telescope on 
Kitt Peak.  The image is 9.18 arcminutes square with north on the right
and east at top.  The integration time is 150 seconds.

At time of this image, Ecnek was at a geocentric distance of 0.88 AU
and a heliocentric distance of 0.90 AU.  The scale is 636 km/arcsec.
Therefore, not only can the nucleus not be seen but the densest
part of the coma, the collisional zone, cannot be seen.

Comet Encke is the first CONTOUR target.



The second image is an HST image of Hale-Bopp.   It needs the following


Hale-Bopp
Hubble Space Telescope observations of the region around
the nucleus of Hale-Bopp, taken on eight different dates since September 1995.
(Credit: Harold Weaver (Johns Hopkins University) and NASA)

All images are processed at the same spatial scale of 470 km per pixel,
so the solid nucleus, no larger than 25 miles across, is far
below Hubble's resolution.

Note the changing jet-like features.  With the spatial scale of the
HST, it is difficult to follow the features to their sources.

Hale-Bopp is an example of the type of newly discovered comet
to which CONTOUR could be retargeted.