I wonder... I've been looking at the other raw images of Rhea, and haven't seen anything like that. However it shows up in the same place in a number of images from the same vantage point, taken by different filters. Does anyone know if these images are taken simultaneously or sequentially?

If it's the former, then my suspicion is that this is actually a small asteroid whizzing by Cassini. It occurs to me that, if this *is* a surface feature, it isn't actually straight (the way a ray from an impact crater would be). If it were straight (on the ground), it would appear to be curving around Rhea's limb. The fact that it is straight from Casinni's vantage, means it would have to be curving *precisely* counter to the spherical foreshortening that we'd otherwise be seeing, which would just be too weird. But this theory only works if the different filter images are shot simultaneously.

If they're shot sequentially, then it really *is* a surface feature -- and my guess is that it's a freshly carved ditch, with little green men with shovels standing nearby.

Cheers,

Nathan