It isn't water.
I think you'll find, in (professional academic) discussions of the flow, that nobody is suggesting that it is water. It's definately a granular flow that could be enhanced by some volatile subliming in response to an initial purely granular flow. These figures, which Nick PhD. may wish to annotate in a reply, are representative of some work he's done on the matter.
It's been pointed out elsewhere that this is not the result of a referred paper in a journal, but a press release of two pictures showing a presumed difference.