LICK OBSERVATORY CROSSLEY 36" REFLECTOR IMPACT SITE IMAGES

This is a mosaic of 12 images obtained on 19 and 20 July UT 
from the Lick Observatory Crossley 36" reflector using a 
high speed CCD camera system. Images are shown that were 
obtained through a Bessel B filter (top row), a 7500 +/- 
100A narrowband filter (middle row), and an 8900 +/- 100A 
narrowband filter (bottom row). Each of the columns depicts 
images at these wavelengths obtained at roughly the same 
time. 

South is up in these images. For scale, the apparent size 
of the Earth is indicated at the bottom of the image. The 
raw data have been processed using a blurred mask filter 
to enhance details. In this filtering scheme, the image is 
gaussian filtered and then the difference between the 
original image and the gaussian filtered version is 
amplified and added to the original image. The resulting 
images have been contrast stretched to bring out the most 
detail around latitudes centered on the impact sites. 

The various impact spots observed are: [1] 19 July 0505 UT: 
G on dawn limb; [2] 19 July 0610 UT: G on central meridian; 
[3] 20 July 0355 UT: A (and E?) on dawn limb, H on central 
meridian, G on evening limb; [4] 20 July 0445 UT: A (and E?) 
to the left of the central meridian, H to the right of the 
central meridian. 

Note that the spots are bright in our 8900A images but dark 
in all of our other filters. This is consistent with HST 
imaging results, but there is some confusion about whether 
the spots are bright or dark in 8900A groundbased images. 
Many groups have reported that the impact sites are NOT 
visible as bright spots using the Spencer et al. 
8930 +/- 300A filter. However, we are using an 
8900 +/- 100A filter and we see the impact sites as 
prominent features comparable in brightness to the GRS. 
We offer the following explanation for this discrepancy: 
the 8930 +/- 300 filters are centered on Jupiter's 
methane band but they also are measuring some of the 
continuum signature on either side of the band (see West, 
Icarus vol 38., p. 14, 1979). Our 8900 +/- 100 filter does 
not allow any of the surrounding continuum to "leak" in 
and thus allows a higher degree of contrast in the images. 
This may also explain why the spots are bright at 8900A 
for HST. 

Also of note is the dark partial ring seen apparently 
surrounding the A site in the 20 July images. This dark 
arc-like structure extends from the south pole up to -20 
degrees S latitude and is centered on the A impact site 
(which was also dark in our B images). The arc has a 
radius of roughly 10 arcsec. The structure is also visible 
in our U band images but is not visible in any of our 
narrowband filters. There are at least two immediately 
obvious possible explanations for this feature: (1) it may 
be the visible expression of an expanding shell (or wave) 
of material from the A impact site: the feature is large 
because the A site is oldest; (2) it may be an artifact 
introduced by a combination of 2-arcsec seeing and the 
fortuitous alignment of the GRS and H impact site along an 
annulus concentric to the A site. This structure was also 
reported to be seen in B images by E. Howell and co-workers 
at Mount Stromlo. 

Lick Observatory Crossley 36" reflector observing team: 

Jim Bell (NASA Ames) 
Ted Dunham (NASA Ames) 
Dominique Toublanc (Obs. Bordeaux) 
Bob Thompson (Univ. Wyoming)