Dear Dr. Wooden,
Thanks for sharing this information.
The following questions come to mind:
1. Could you be more explicit about the location of the dust cloud
reference "Goldstein, D.B. et al. 2008, Amer. Inst. Phys. 1084,
1061"? I am not familiar with a journal called "Amer. Inst. Phys.".
Is it available on-line?
2. Could you be more explicit about the time and location at which the
predicted column density of 1E7 m^-2 applies? I assume this is
referring to the expected density at the lowest height where sunlight
can be found over the target location? Does your statement that "the
brighter part of the ejecta plume reaches about 1.5 to 3 arc sec in
height" mean relative to the starting point on the crater floor or
relative to the point at which it reaches sunlit? Over how many
projected square arc-seconds horizontally and vertically is the
surface brightness shown in the red curve expected? Will this be seen
throughout the interval of 4-20 sec post impact, or only for some
portion of that interval? Finally, does "a = 35 micron" mean
particles of 35 micron radius or diameter?
3. Regarding the ~35x brighter curve for "a = 1 micron" particles
versus "a = 35 micron" particles at 30-90 sec, does this mean an
impact scenario with the ejected mass forming small particles is as
likely as one with it forming larger particles? Also, does it mean
the assumed particle size has no effect on the predicted size or shape
of the cloud other than in number density? And does it apply to the
red 4-20 sec curve as well (that is, would it, too, be 35x brighter at
visible wavelengths if the particles were "a = 1 micron" instead of "a
= 35 micron")?
4. How does the peak radiance of 9E-10 W m^-2 micron^-1 arcsec^-2
compare to the modeled radiance of the solid sunlit surface of the
Moon at "Cabeus_A1" as seen from Earth at the impact phase?
5. Could you be more explicit about how grain column densities were
converted to surface brightness? The fraction of the line of sight
filled by 1E7 particles m^-2 would be only 0.038 if 35 microns is a
diameter or 0.010 if it is a diameter. For visual observers, either
size is much larger than the wavelength of light, so I would assume
they can be regarded as opaque (Lambertian?) spheres lit at the same
phase angle as the Moon. How can such a small number of particles
produce such a high reflectance?
6. Could you be more explicit about how the crater coordinates were
determined? My understanding was that crater coordinates (at least
the longitude and latitude) have always referred to the centroid of
the of the rim (seen as an elliptical shape from Earth). By "common
designation coordinates" do you mean IAU catalog coordinates? Are
your coordinates for the centroid of "Cabeus_A1" based on the USGS
ULCN2005 warped Clementine and/or Lunar Orbiter products, on
independent Kaguya measurements, Earth-based radar maps, or some other
source?
7. The resolution and scale of the Gemini N image is not particularly
impressive by amateur standards. Is this a slit-jaw image of some
sort? What is the native scale in arc-sec per pixel? Is the resolution
of the present image limited by seeing, by focus errors, or was it
reduced in the reproduction? The scale of the images as posted is 0.84
arcsec/pixel and in Clavius the smallest craters detected seem to be
about 2.5-3 km in diameter -- about the same as skilled amateur can
achieve (in good seeing conditions) with a 50 mm aperture:
http://ltvt.wikispaces.com/Crater+Resolution
Does the Gemini-N GMOS spectrograph have a capability to use adaptive
optics to combat bad seeing? Any idea what the odds are of it
detecting kilometer-sized craters at a random moment (as at impact
time)?
Thanks,
Jim
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