The rate at which pictures are being released by the project are pathetic. I don't want to hear how low the data rate is from Pluto. Paranoid scientists are witholding dozens or hundreds of them to keep from being scoped. Spunik Planum is a glacier. Of what material, who knows? CO, N2, or some weird mixture. You can see that the region covers the darker terrain by the many embayment relationships at the meeting of terrains at its edge. The patterned ground is also a giveaway, but who can explain the large size of the patterns? What is the dark stuff? Organic gunk, not unlike light crude oil or LNG at fantastically low temperature. In processed approach images you can see the path that gas sublimating from the glacier takes as it sublimates and migrates to the pluto dark side from the sub solar latitude. It faintly stains the terrain. My guess is that glaciers like Sputnik Planum accumulate on the dark side during Pluto's pronounced winters, as the erode from the light side. They seem to be 100's of meters if not kilometers thick. The complex seasons produced by Pluto's elliptical orbit and high inclination surely disrupt the surface asymmetrically. Pluto is in southern summer and just passed perihelion. The south cap should be near its minimum extent. Pluto is a freaky place. Release more pictures greedy scientists!