The Nili Patera region has enjoyed fairly good media exposure recently, ever since high detail HiRISE (High Resolution Imaging Science Experiment) photographs hit the Internet, showing fabulous sand dunes to the south of the region in all their glory. Just to refresh your memory, or in case these photographs didn't reach you, here's a small preview of these aeolian formations, areology-wise short-lived migratory topological features of the Syrtis Major Planum:
Nili Patera dunes, while indeed lovely, are however hardly an unexplained phenomena.
Looking at results of some Mars Odyssey THEMIS (Thermal Emission Imaging System) observations though, I've noticed something rather peculiar within the impact craters in the otherwise sandless North by Northeastern part of the Nili Patera region. Seemingly regular shaped protrusions, straight or only slightly bent ridges nearly parallel to each other that appear almost as if, tongue-in-cheek, Martians were keeping a hand-drawn score on how many times Voyager 1 was believed to have left the Solar system. Smaller craters have one straight ridge in them, slightly bigger ones maybe two to three, and larger ones - albeit still relatively small craters on global Martian scale - three or more of these ridges. This is what I'm trying to describe:
Cropout of the larger THEMIS image of the Nili Patera dunes. Source: Arizona State University THEMIS data, Image ID V39109006
These within impact crater ripples only appear in the roughly 2 km depressed region of Nili Patera caldera, and the craters on the top plain surrounding this volcanic caldera bed appear clean from this phenomena. My first impression was these are smaller sand dunes, remains of aeolian sedimentation from the dust blowing from the higher plain onto Nili Patera and keeping some of this sand trapped within the even more depressed impact craters, but I later realized THEMIS images are actually recording heat signature, and sand would have to appear on average darker, just like the sand dunes to the South do on the same source image.
I did a bit of digging around to answer my question, and indeed found somewhat related blog post by Emily Lakdawalla, after one of her HiWish requests for HiRISE to focus on one of the Nili Patera butterfly (low-angle impact) elliptical craters has been granted and results came in. Sadly though, that particular crater doesn't show same ridges in it like the ones I'm inquiring about. While there is a central scar in it along the path of the impacting meteor, it is not nearly as seamless as the ones from the THEMIS image appear to be:
HiRISE imaging of the butterfly crater in the Nili Patera region. Credit: NASA / JPL / UA / Emily Lakdawalla
So, failing to find an answer on my own, here's my question:
What is causing these seemingly seamless and regular shaped protrusions within the impact craters on the Nili Patera caldera? What is the story behind their formation, and do we have any physical models that would explain their shape and number?