Re: My bet on what this is likely to be:
HH Kiefer at USGS/Flagstaff would probably agree with you. He wrote a paper for Mars Polar Science 2000 titled "Annual Punctuated CO2 Slab-Ice and Jets on Mars". I have a copy of the abstract but I can't find a link to the conference site for the full paper.
In case you were wondering where Greg got the term "Martian spiders", Kiefer mentions that that's the moniker the MOC team gave these features when they first spotted them. Morphology, not a hint of biology....
--Robert