I thought I would move this interesting thread up to the forefront by responding to Peter Ness' post at http://disc.server.com/discussion.cgi?id=131070&article=10483

Peter Ness wrote:
The white line is just a diversion from the really interesting thing in the other photo. It is the link I was looking for - if one looks at alll the other photo this one provides the solution to the spiders. And it is so obvious that it almost begs the question...

Alex writes:
Perhaps you might expand on your model, but...

I'm not sure that I would agree that the white line is an ice intrusion, though I'll admit your first order reasoning is sound. Identifying the basement material, the areal extent and stratigraphy of which is uncertain, is certainly a prime objective in the region. The problem, though, as Nick Hoffman and others (including myself who have extensively studied the polar layered terrain) noted, is interpreting a high resolution image in light of the regional context, in this case Viking-era orbital imagery. My first snap interpretation (at local NA scales) was one of any intrusion dike, but at larger scales the white line corresponds highly to the regional scarp (which is also evident in the MOLA data) that appears to have been exposed by polar cap retreat. The sinousity of the scarp at regional scales appears (at least to me) to argue against a fault zone ice intrusion, especially given that the immediately surrounding areas are *defrosted*. Why would the ice persist as a partially exposed outlier? Recall that in the first image (acquired at the height of southern summer) the remnant cap has retreated further south. At any rate, higher resolution synoptic coverage (perhaps by THEMIS VIS and/or MARCI+ MA) is highly desired.

Alex R. Blackwell
University of Hawaii