Scientists studying the data from New Horizons have had an absolute wonderland to play in, but one of the most compelling features right from the start was definitely Pluto’s “heart” — a vast white plain called Sputnik Planum that covers a huge portion of the surface. It’s completely unlike the rest of Pluto’s visible surface, is clearly very young, geologically speaking, and has a curious geometrical coincidence: it’s directly in line with the the Pluto-Charon barycenter. Pluto and Charon are mutually synchronous — that is, due to tidal interactions between the two bodies, they have settled into an arrangement where one rotation of Pluto is the same as one rotation of Charon and also one revolution of Charon. Our own Moon rotates synchronously — it is tidally locked to the Earth, which is why we only ever see one side of it. But the Pluto-Charon system has evolved further: not only is Charon locked to Pluto, but Pluto is locked to Charon. If you stood on Sputnik Planum, you would see Charon fixed immobile in the sky above you, going through its phases as you experience day and night, the stars and Sun and planets wheeling through the sky behind it. It would be a very strange sight to us.
It’s also very suggestive of a positive mass anomaly (a region which is much denser). Charon is quite large relative to Pluto, and orbits fairly close; this means that tidal interactions will inevitably drive the system towards a condition where the densest part of Pluto points towards Charon, and that is exactly what appears to have happened. Planetary scientists believe that Sputnik Planum was created when a very massive asteroid impacted Pluto – not big enough to produce Charon or anything like that, but big enough to liquify a lot of the planet, leaving a huge scar similar to the massive impact basins on the lunar nearside that we call the maria. If there is still an enormous liquid water reservoir below Sputnik Planum, it would explain a lot of the surface features as well as the positive mass anomaly that is believed to exist in that location, because as anybody drinking a glass of ice water can easily see, liquid water is considerably denser than solid water. This weird feature of water could entirely explain Sputnik Planum and its location on Pluto. Doing the math, researchers estimate the ocean below Sputnik Planum could be 100 km deep, and about 30% salinity (comparable to the Dead Sea on Earth, and so not beyond the limits of what we know life can handle).
It also means that Pluto doesn’t just have a heart — Pluto has a broken heart. But out of that broken heart comes beauty that no one had ever dreamed of: