Hi Dan,

Sorry to disagree with you here, but because computer rendering has earned been paying my wages since I was 16, I'm kinda picky when it comes to discussions of lighting effects. Hope you don't mind! :-)

Since you drew a parallel between this phenomina (seeing through a particle-cloud) and reflectivity, I feel like I should give a quick explanation of what reflectivity is, and why that parallel, in this case, doesn't work.

There are two types of reflections -- diffuse reflections and specular reflections. Diffuse reflections are what come off of any kind of rough object -- wood, dirt, soft rubber, and so forth. When the light strikes the object, it is reflected in an essentially stochiastic fashion, with the angle of reflection only slightly biased towards the angle of incedence. This produces a smooth brightness curve with a roughly sine-wave falloff. You can indeed draw an inverse correlation between this type of reflection and the appearence of a particle system.

The second type of reflectivity is specular reflectivity, which is possessed by any kind of very smooth object -- billiard balls are always used as the classic example. When light strikes the object, it is reflected in a perfectly coherent fashion, with the angle of reflection exactly equalling angle of reflection. This produces a mirror-like image of the light-source, which is why you get a bright spot on lightward side of the cue ball, and bright streaks on the corners of your car (unless it's as dirty as mine).

But while your analogy works for the first type of reflectivity, it does not work for the second kind. If you observe the a shell-like translucent sphere (which is a reasonable comparison), you will see that it is indeed gradually more insubstantial at the center than at the edges -- just like the inverse of a diffuse reflection -- you will not see any distinct "dark spot" that parallels the specular highlight. If you don't believe me, try it. Likewise, you will not see any abrupt dark streak on a comet's tail.

Also, while the spreading of the dark streak is consistant with the overall spread of the comet's tail, it's also consistant with the behavior of an umbra shadow. And furthermore, my understanding of comets is that the nucleus tends to outgas in all directions, so there would not a signifigant vaccuum "wake" in any particular direction. And yet the dark streak obviously penetrates well into the coma, and to my eye anyways, seems to touch the nucleus itself. That would be an area of (relatively) high concentrations of dust and gas, and there would be no reason for a dark streak to appear unless some of that material was failing to reflect any light. Ie, in shadow.

So, I'm sticking with my original conclusion. It's either a shadow, or (much less likely), an actual physical gap in the tail, perhaps caused by some stopped-up vent on the nucleus. Since Ikeya-Zheng has already demonstrated some fairly extreme variability in its outgassing, I'll also entertain this as a possibility.

Cheers,

Nathan.