There is a very large basin where fluid once rested (now called Piri Planitia). The side walls that held the fluid can easily be seen, The outside edges of the walls are littered with jagged craters, yet the craters inside the basin are mostly smoothed over as are the fractures leading into the basin. The multiple craters at the outer edge of the basin help identify its boundary.
There are cracks in the ice indicating how the fluid flowed into the basin.
This was once a lake similar to Sputnik Planum (SP) and was likely created by similar processes. The more we understand the evolution of this basin the more likely we are to understand Sputnik Planitia.
Flow to basin
The horizontal ridge line running east to west forms river channels, They are also indicators of a weakened substructure. There are large walls along the ridge as the land ice has cracked and collapsed down.
The nitrogen flow probably seeped through these cracks and flowed like a slow river to the west filling the basin. These cracks are called Inanna Fossa and Dumuzi Fossa.
Inanna and Dumuzi have left an enormous ice island between them. This is probably what happened to form Sputnik Planitia.
Scientists tend to struggle with my use of the words fluid and flow when describing these scenes.
In their mind everything is frozen and while I agree with this, using words like fluid and flow is more of a description of variations in time frames.
On earth water flows over glacial ices. Glaciers flow over rock and rock flows via subduction under tectonic plates in a recycling process.
The only difference in these flowing processes is rigidity and time. On Pluto some ices flow more easily than others but they do flow and this flow creates erosion.
Piri Planitia is an eroded basin where fluids once flowed else it wouldn't be suspiciously devoid of impact craters as can be seen in this image.
The edges of the fractures called Inanna and Dumuzi Fossa are rounded compared to the cracks running southward out of the spider.
The area north east of Sputnik Planitia has this region with multiple fractures converging into a central distended point.
Piri Planitia also has the distinct signature of a fluid outline.
With this info at hand I think its safe to say Piri Planitia was created by some type of flowing fluid.
Rounded cracks, cracks organized into a river delta formation, smoothed and missing impacts and the signature fluid outline all indicated Piri Planitia was a basin filled with some type of flowing fluid at some point.
NASA scientists suggest Piri Planitia was created by sublimation where ice converts from a solid directly into a gas bypassing the liquid phase. Dry ice on Earth does. If sublimation is the reason for this land feature, how would we explain all the tell tale signs demonstrating the presence of a fluid?
We don't, instead mainstream scientists just ignore the evidence.
This image is showing where the horizontal ridge that used to feed nitrogen flow westward into the old dried river basin, now connects to the vertical land ice ridge that's currently holding back the nitrogen flow from Sputnik Planitia (SP).
Based on this, I'm postulating there was a vertical crack a long time ago (closer to Sputnik Planitia's center than the current wall). The plains to the east of the crack were at a lower elevation than the plain where the dried basin lies. As the horizontal crack in the land ice met the vertical crack the nitrogen flow began to spill into the Sputnik Planitia's vertical crack zone bubbling out from under the surface contributing fluid to SP while emptying the basin.
Methane on Pluto is thickest at the North Pole. The methane is snowing down and slowly migrating from the North pole southward. There are seasons on Pluto, Winter, Spring, Summer and Fall (snow fall).
As the methane snow is carried south it lands on elevated terrain such as mountain peaks and even raised rims of impact sites.
The methane on the edges of Piri Rupes is accumulating and building up more than it is sublimating away. Look at the two large impact sites and how the northern elevated edges have accumulated methane snow as it migrated south.
This is exactly what's happening at Piri Rupes. It's a process of accumulation more than sublimation (evaporation). Additionally, how does methane evaporation account for the missing impacts within the basin. Fluid flowed here and smoothed over the surface.
Here's our Moon
The near side of the moon always faces the earth as it is tidally locked to earth. This has caused the moon to look very different on one side vs the other.
The near side is dark dense and smooth the far side is lighter in color and density.
Why did the moon tidally lock to earth with the more dense side facing earth? The denser side would naturally tend to lock or wobble toward the earth's gravitational pull and stop like a ball that's unevenly weighted with one side heavier than the other.
This is Charon
Charon has evidence of the same processes as Sputnik Planum. Here you can see a smooth petrified lake/sea area with a giant cliff face (rock) that fell off the land ice, This giant cliff face dropped into a fluid then the fluid hardened into ice and froze the scene.
The jagged edge of the land ridge running bottom left to top right matches the jagged edges of the partially sunken rock of ice that fell off the land and into an ocean of fluid.
We see the same process here as at Sputnik Planitia (SP). The land has fractured, allowing softer, warmer fluid to seep around the crack, in turn, eroding the land and breaking it into chunks. Those chunks then migrate with the fluid's movement. Which indicates, the fluid on Charon was moving NE (in this image) since the rock is slightly north of its breakaway position.
Zone 5 Again
Look at these ice chunks that are sometimes referred to as icebergs.
This looks like massively thick but relatively uniform in thickness ice blocks that were broken apart as if you dropped a 2 inch thick 5 foot round sheet of ice on the ground and it fractured into large blocks.
The red surface on these blocks was exposed to the environment for a long period of time as the tholin collected. Then the sheet of ice broke creating white edges that hadn't been exposed to the atmosphere.
This is the remnant of the land ice wall that is being eroded by the nitrogen flow.
Here is the active zone. From this angle the cracks at the bottom leading into the plains of the Sputnik Planitia's runoff, show how there is a series of cracks which are creating the weakened ice (created by the nitrogen flow) which is turning the land soft enough to break under the gravitational strain created by Charon and Pluto?
Near the top, the north to south land wall is holding back the flow while the east/west crack is leading to the western dried basin near the top right of the image.
So I'm proposing, the dried basin on both Charon and Pluto portrays how this process is occurring and Sputnik Planitia's nitrogen flow is hiding the cracks that allowed for its creation.