The Kuiper Belt might be the “make-out” point of the Solar System, at least judging by Pluto’s large hickey. The curious region on the dwarf planet was spotted by NASA’s New Horizon spacecraft in its flyby last July but only now have scientists managed to work out the origin of the curious formation.

Astronomers think that Piri Planitia, as the region is informally called, is undergoing a phase change. The methane ice on the surface is sublimating (turning to gas), exposing a hard layer of water ice.

Pluto's average temperature on its surface is -229 °C (-380 °F) and it has a surface pressure 100,000 times less than Earth. Under those conditions, water ice is as hard as rock and methane can only exist in two states, as a solid or as a gas.

^{The region of Piri Planitia seen in high-definition and in false colors. The purple represents methane ice, while the blue is water ice bedrock. NASA/JHUAPL/SwRI}

Piri Planitia is surrounded to the west by a high plateau called the Vega Terra, and the two are separated by the Piri Rupes, a long wall of cliffs that extends for hundreds of kilometers. The cliffs are rich in methane, and the New Horizon team has speculated that they formed from the sublimation of the methane ice. As they eroded away, they formed the Piri Planitia.

The region is very young, at least in geological terms. Piri Planitia shows nearly no sign of meteor impacts (as can be seen in the nearby Vega Terra region), indicating that whatever process has been shaping the region has either stopped very recently or is continuing to the present day.

The same can be said about Tombaugh Regio, the heart-shaped region that dominates Pluto. Its smoothness has perplexed astronomers since the first pictures of Pluto arrived, and it is an indication that the dwarf planet is somehow still geologically active. Dwarf planets are too small to maintain internal heat for long, but there’s currently no alternative explanation for what’s going on inside Pluto.

The New Horizon team is still working on the data from Pluto, which is slowly being downloaded (about 1 image every 42 minutes) and analyzed by the team. Hopefully, more data will help us understand how Piri Planitia formed.

^{The region of Piri Planitia in relation to the rest of the planet seen in high-definition and in false colors. The purple represents methane ice, while the blue is water ice bedrock. NASA/JHUAPL/SwRI}