Quick thoughts...
Great raw site, good work REB, and everyone else with the quick image links and posts. I've found if you crop to the individual frames before enhancing, it helps with the final result.
We might be seeing a large ice cap with internal drainage channels leading to a dry plain. Reminds a bit of the edge of an ice sheet breaking up, too.
The "river" pattern is not purely dendritic, there seems to be some structural control by the eroded material.
Did the probe land in the white area with the "rivers" or did it land in the dark area?
What fun! The overloaded web sites just shows how interested people are in space.