The outer solar system should be far too cold to allow liquid water to exist at any point, so how could we find an ocean on Pluto? Astronomers suspect that Pluto and its largest moon Charon came to be in their present forms after an impact. This would have produced a lot of heat that liquified the icy crust of Pluto and gave it an internal ocean.
The frigid temperatures out past Neptune wouldn’t have allowed a surface ocean to persist for long, but the subsurface water could have remained for a relatively long time. This is possible because as water freezes at the surface, salts and ammonia are concentrated in the remaining liquid portion. Higher concentrations of these substances lowers the freezing point of water — it’s like putting salt on the roads during winter.
Astronomers have also suggested this ancient ocean could have given rise to a short-lived period of active plate tectonics on Pluto’s surface. The New Horizons probe might see the ridges and fault lines of massive shifting ice sheets on Pluto as it passes next year. This tectonic activity would have occurred as Pluto and Charon drifted apart after colliding. When they were closer, the increased gravitational pull would have stretched Pluto into more of an egg shape, but the ice sheets on the surface of the ocean would have shifted and cracked as it returned to a round shape. Thus, tectonic faults on the surface.
The probe might not spot any faults when it passes, but that doesn’t mean the ocean never existed. Pluto has an atmosphere that freezes to the surface when it reaches the far end of its orbit. This repeated freeze-thaw cycle could have eroded the surface ridges over billions of years. We’ll find out in a year when humanity finally gets a good look at the first dwarf planet.