On July 14, 2015, NASA’s New Horizons flew by Pluto.
At a resolution of only 80 meters (260 feet) per pixel, Pluto was revealed at resolutions thousands of times better than Hubble.
Near the poles, we found cratered highlands: an old, level, icy surface.
That terrain gives way, towards the equator, to hilly, ice-covered regions with scarred markings.
Hills transition into mountains of ice, some of which rise more than a mile (1600 meters) high.
These mountains aren’t static and stable, but rather are temporary water-ice mountains atop a volatile, nitrogen sea.
The evidence for this comes from multiple independent observations.
The mountains only appear between the hilly highlands, after the edge of a basin rim, and young plains with flowing canals.
These young plains occur in Pluto’s heart-shaped lobe, which itself was caused by an enormous impact crater.
Only a subsurface, liquid water ocean beneath the crust could cause the uplift we then see, leaving the nitrogen to fill it in.
The observed gravitational anomaly under Sputnik Planitia further indicates a sub-surface ocean.
Over time, this crater loads with volatile ices, eventually causing the whole world to tip over.
The most frozen, distant known worlds are still active today.