Enceladus Volcanoes? Options

[dilo]

Most recent narrow-field images of Enceladus, reported in the "raw images"section of Cassini homepage, are very interesting but also pretty strange...
Images link are reported below and were taken on February 17 using the CL1 and CL2 with increasing exposure time.

http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=32337
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=32338
filtershttp://saturn.jpl.nasa.gov/multimedia/images/raw/raw-images-details.cfm?feiImageID=32339

Looking in particular to first one, sky is bright while Enceladus darkside still black. Apparently, the only possible explaination is that background is the dark side of Saturn atmosphere, but the presence of a star trail below the satellite demonstrate that, incredibly, background is the sky (see first attached image, which is the sun the 3 images associated with RGB channels). I don't know encounter geometry and I will appreciate very much if anyone can help me...
Anyway, the interesting thing is the bright halo near the bottom of Enceladus, clearly showed in the second attached image (an elaboration made strating from first and last image). Could this be the plume produced by some geologic activity (like an iceberg/volcano)?
Regards...

Attached thumbnail(s)