Bob Carlson wrote:

wrote in message . ..
http://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=8552
Interesting! When you click on the full-res version of the photo the
"blob" looks like an arrowhead or a windows-like pointer. Also, I
wonder what would be cause of the luminescence in the streak?

Bob Carlson

Remember that you're looking at a .jpg image, which (as I understand it)
has been subjected to some sort of lossy compression. The original,
uncompressed image may well look different. This includes the apparent
shape of Mimas, which would present roughly a half-moon appearance with
its terminator at about the same angle as Saturn's. Mimas's big crater
would not have been visible in this picture, so we can't blame the
strange shape of the terminator on it.

There are actually *two* streaks in the image -- if you put a ruler up
to one of them, the other doesn't lie in the same line. These are
caused
by cosmic rays or other charged particles impinging on the camera's CCD
detector during the exposure or during readout. They deposit some of
their energy into the CCD by ionizing some of the silicon atoms. The
electrons thus liberated look no different from the electrons liberated
by the light that hits the chip during the exposure. This happens all
the time -- you can see lots of little white dots all over Saturn --
it's
just that these two happened to come in almost parallel to the chip's
surface (at "grazing incidence") and left long trails as a result.

-- Bill Owen