NASA scientist Bill McKinnon lectured on Sputnik Planum at LPSC 47 calling it an impact site (his lecture starts at 1hr 56 min if you want to skip to it.
The below image was used by Bill to compare Sputnik Planitia (SP) to the crater on Mars called Hellas.
Bill stated, "This is evidence that SP is an impact site".
Bill continued to explain how the object impacted at the center traveling southward as is indicated by the circle and arrow in the below image presented by Bill.
This is what a more accurate representation of the SP basin would look like.
Drawing a big circle in the middle of SP manipulates your eyes to see only what you are expected to see, a big circle but that's not what SP is.
See what is actually there and you will notice it doesn't look like a big impact basin.
By drawing this big circled area and associating it with Hellas Planitia on Mars, your mind imprints this image as a round deep impact site.
Even if an impactor came in at a 5 degree angle there would still have been ejecta material off to the sides as seen in this image the blue is where the impact took place and the red spray pattern is the ejecta. Ejecta still sprays out to the sides with a 5 degree hit.
When this fact was pointed out to Bill he simply said "That's why we need to go back to Pluto".
In addition to the above, consider the difference between the two sides of SP.
To the west there is a ridge running from north to south along which we see bulging land masses, stretch marks, fold mountains, cryovolcanoes and turbulent looking deep fluid welling up.
To the east in the uplands, the surface is as youthful as is seen at SP. Yet the terrain is totally different. Would an impact cause this variance on each side of SP?
SP is a geologically active zone that is creating fold mountains, fluid falls, cryovolcanoes, foggy wetlands and an atmosphere.
This is how I visualize what I see taking place at SP. Basically its a tear or a rip in Pluto running mostly Northwest to Southeast.
Place your two hands together like you are praying then while keeping your pinkies together open your hands at your thumbs.
To me this is what the western edge of SP is doing, its prying apart in a long line or underground chasm.
The right image sorta gives a simplified vision of what I see occurring on Pluto.
The Green line is where the subsurface pressure is focused this is where Pluto is cracked open.
The Pink arrows are the general direction the nitrogen is migrating out of the subsurface crack.
The Blue lines are the compression ridge that is created by the land ice resisting the upwelling fluids, in turn, forcing the flow eastwards.
The Yellow lines are the nearby bulge created by the subsurface nitrogen pressure. The further it extends westwards it has frozen and collapsed creating Peri Planitia, whereas the subsurface fluid creating the bulge at Viking Tera is still in a pressurized state this area is distended like it wants to burp.
The real problem for the impact theory is that Sputnik Planitia is geologically active.
The nitrogen fluid rising up from below is forming the landscape. Pushing, squeezing fracturing compressing and expanding the land ice surface.
Impact sites look like holes punched into the surface because that's what they are, whereas, SP is an active live nitrogen lava like caldera similar to what we see in the mouth of volcanoes on earth.
NASA scientists describe the surface of SP as having convection cells.
Caldera's exhibit convection cells which move on shorter time scales than one would expect to see at SP with its sluggish pasty nitrogen ices but the process is very similar with the variance primarily being temperature and time.
To understand this geological movement we'll take a close look at Venera Terra and travel SE down into Sputnik Planitia. Seeing this process helps you understand how SP is a geological feature and part of a larger process not an impact.
The nitrogen fluid that was once under Venera Terra migrated underground towards SP or the surface ice crust slipped over a hot spot.
Along with breaking off large ice blocks, the flowing fluid has also created river delta looking patterns with waves frozen into the nitrogen mix contained within SP. In this one image below there are eddy's, whirlpools, waves, river deltas and dislodged eroded ice blocks migrating into the ocean of nitrogen.
All this subsurface broiling and churning has created these cryovolcanoes along the Western North edge of SP.
These current's push massive icebergs southward into a land zone that is crater free and soft like thick cold mud. The two mile high icebergs are compressing the semi frozen mud into 2 mile high mounds as the fluid creeps inland like a slow tsunami.
The fold mountains have mounds with mouths spilling fluids out of them like cryovolcanoes creating tributaries rivers and lakes of gray fluid.
To the south eastern side of SP fluid flows off the uplands or wetlands back into SP creating a scene that looks similar to Niagara Falls.
To the south of SP at the night day terminator line we can see fog which is created by SP. The moisture (CH4, N2, CO) in this fog is wrung out like a wash cloth as the fog climbs up the uplands cools condenses then flows back into SP.
The name "Tartarus" as in Tartarus Dorsa is the name of the deep abyss (hell) that is used as a dungeon of torment and suffering for the wicked and as the prison for the Titans. Tartarus is the place where, according to Plato in Gorgias (c. 400 BC), souls were judged after death and where the wicked received divine punishment.
In the heart of Siberia's boreal forest gapes a monstrous chasm local Yakutians call a "gateway to the underworld," connecting this life to the next.
Below is the Batagaika crater. This crater began forming in the 1960's and is believed to have developed by methane deposits in the permafrost escaping collapsing the land into a crater. Perhaps there is a similar process occurring at Piri Planitia and SP. Methane gasses as they warm on Pluto as on Earth may be causing surface land features to collapse into depressions.
The Batagaika crater is what scientists are now calling a "megaslump": an immense void, or "thermokarst," in the geomorphology of a permafrost landscape. These sudden rifts appear when permafrost is allowed to rapidly thaw, causing scar zones to sink into the "saturated slurry." They can remain active for decades at a time. And while understandably terrifying, thaw slumps are a pretty typical feature in Arctic environments like Siberia.
Batagaika crater was formed by subsurface erosion processes suggesting Piri Planitia & SP may as well.
Piri Planitia is a depression in the land ice surface created by erosion processes eating away and evaporating methane and nitrogen from below the surface much like we see at Batagaika.
NW of Sputnik Planitia is Venera Terra which in past years was in a softened nitrogen fluid state from the warm underground nitrogen. Some of these recent impact craters look like they punched completely through the ice crust sinking into the fluid below? This along with a lack of impact craters suggest the crust here is thin and relatively young.
The fluid continued to pour into the SP basin/caldera from the NW and up from below. There are many polygonal convection cells lined by lower troughs on the surface of SP similar to lava lakes in a caldera on Earth. SP is a collapsed crustal basin 2 miles deep slowly writhing with warmed nitrogen.
Along the North Western edge of SP, fluids flow south and east creating river deltas and wave patterns in the soft nitrogen ice.
Just south of the volcanoes along the Western South edge of SP fluid is bubbling up and outward from the land ice breaking off ice blocks which are carried south by the primary current down to the pit creating the iceberg mountains at Norgay Montes.
The pressure being applied to this land mass is creating a ridge line of fold mountains further inland.
Moving east from SP the uplands aka wetlands begin to wring moisture out of the foggy wet atmosphere flowing it back into SP. This process dries the air creating long penitente icicles (aka Tartarus Dorsa) further inland to the east.
This land surface east of SP has no impact craters so it is renewed quickly and must be soft yet icy rigid.
Tartarus Dorsa is bulging upward from subsurface pressures with expansion fractures lining the mounds of mountains leading north to the mouth of the spider. Tall ice spikes called penitentes lean southward in this area from arid south blowing winds.
At the center of the spider is another cryovolcano which was created by underground pressures pushing the land up creating extensional fractures leading to this central point of subsurface pressures.
It is safe to conclude that Sputnik Planitia is not an impact site simply from the fact that there is no ejecta blanket nor is there a circular ring pattern around its edges but when we add in the fact that SP is an active caldera creating erosion, fold mountains, cryovolcanoes, expansion fractures, atmospheric fog, lakes, river deltas, eddy's, waves and whirlpools it becomes ridiculous to think SP is an impact site.
There are indicators at Viking Terra with it's distended bulge and in the area south of Piri Planitia with its corrosive looking landscape that suggest there are subsurface processes taking place.
Take note of the crinkled puckered surface feature to the south of both Piri Planitia and Viking Terra.
It suggests the land ice is being heated and eroded from below.
Viking Terra bulges while Piri is collapsed similar to Batagaika and to the immediate south the terrain is crinkled.
Sputnik Planitia is the heart of Pluto's heat source welling up from below creating all this geological activity.
I hope you clearly see how SP is an active geological site not an impact site.