As we can probably guess, Pluto is a cold place. Though it's frozen, it is far from static. Chemicals that would normally be in a gas or liquid state here on Earth become solid layers of ice on Pluto. When the world receives even the tiniest amount of heating, volatiles - such as methane or ammonia ice - sublimate.
Sublimation occurs when the atmospheric pressure or temperature (or both) are very low. Water ice, for example, at sea level pressures on Earth will melt into a liquid and then boil into a vapor (or gas) when heated. On Pluto, because it's so cold and the pressures are a near vacuum, liquid water is not possible and heated water ice will sublimate directly from a solid into a gas. And it appears that's what's happening with the methane ice in Piri Planitia; it's sublimating into Pluto's thin atmosphere, likely contributing to the dwarf planet's atmospheric cycles of surface ices and exposing water ice-rich layers below.
NEWS: Pluto's Weirdly Young Surface Doesn't Make Sense
It's almost as if Pluto is getting a rejuvenating facial scrub; over millions of years, as its surface is gently heated by our distant sun, the surface layers of ice are sublimated away, creating young-looking regions curiously free of ancient impact craters.